ORCID Profile
0000-0001-5898-7635
Current Organisation
Australian National University
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Research Topics
In Research Link Australia (RLA), "Research Topics" refer to ANZSRC FOR and SEO codes. These topics are either sourced from ANZSRC FOR and SEO codes listed in researchers' related grants or generated by a large language model (LLM) based on their publications.
Top 5 Research Topics
ANZSRC Field of Research (FoR)
Oceanography | Physical Oceanography | Climate Change Processes | Atmospheric Sciences | Climatology (Incl. Palaeoclimatology) | Physical Oceanography | Fluid Physics | Atmospheric Dynamics | Meteorology | Physical oceanography | Geophysical Fluid Dynamics | Theoretical and Computational Chemistry not elsewhere classified | Optical Properties of Materials | Resources Engineering and Extractive Metallurgy not elsewhere classified | Geophysical and environmental fluid flows | Evolutionary Biology not elsewhere classified | Cloud Physics | Numerical Computation | Condensed Matter Physics not elsewhere classified | Climate change processes | Climatology | Composite and Hybrid Materials | Photonics, Optoelectronics and Optical Communications | Galactic Astronomy | Structural Engineering | Oceanography | Atmospheric sciences | Enzymes | Meteorology | Computational Fluid Dynamics | Atmospheric dynamics | Atmospheric Sciences not elsewhere classified | Bioinformatics | Particle Physics | Tectonics
ANZSRC Socio-Economic Objective (SEO)
Climate Change Models | Climate variability | Oceanic processes (excl. climate related) | Climate change | Physical and Chemical Conditions of Water in Marine Environments | Climate Variability (excl. Social Impacts) | Atmospheric Processes and Dynamics | Effects of Climate Change and Variability on Antarctic and Sub-Antarctic Environments (excl. Social Impacts) | Effects of Climate Change and Variability on Australia (excl. Social Impacts) | Expanding Knowledge in the Chemical Sciences | Expanding Knowledge in the Physical Sciences | Expanding Knowledge in the Information and Computing Sciences | Civil Construction Design | Antarctic and Sub-Antarctic Oceanography | Oil and Gas Extraction | Cardiovascular System and Diseases | Aerospace Transport not elsewhere classified | Expanding Knowledge in Technology | Global Effects of Climate Change and Variability (excl. Australia, New Zealand, Antarctica and the South Pacific) (excl. Social Impacts) | Expanding Knowledge in the Environmental Sciences | Expanding Knowledge in the Earth Sciences | Expanding Knowledge in Engineering | Expanding Knowledge in the Agricultural and Veterinary Sciences | Expanding Knowledge in the Biological Sciences |
Related Links
Publications
Quantifying the influence of sub-mesoscale dynamics on the supply of iron to Southern Ocean phytoplankton blooms
Publisher: Elsevier BV
Date: 09-2016
Establishment of momentum balance by form stress in a wind-driven channel
Publisher: Elsevier BV
Date: 2011
Kinetic Energy of Eddy‐Like Features From Sea Surface Altimetry
Publisher: American Geophysical Union (AGU)
Date: 10-2019
DOI: 10.1029/2019MS001769
Asymmetric internal tide generation in the presence of a steady flow
Publisher: Wiley
Date: 28-06-2020
Vigorous deep-sea currents cause global anomaly in sediment accumulation in the Southern Ocean
Publisher: Geological Society of America
Date: 08-07-2016
DOI: 10.1130/G38143.1
SEASTAR: A Mission to Study Ocean Submesoscale Dynamics and Small-Scale Atmosphere-Ocean Processes in Coastal, Shelf and Polar Seas
Publisher: Frontiers Media SA
Date: 13-08-2019
Supplementary material to "ACCESS-OM2: A Global Ocean-Sea Ice Model at Three Resolutions"
Publisher: Copernicus GmbH
Date: 30-04-2019
Thermal Responses to Antarctic Ice Shelf Melt in an Eddy-Rich Global Ocean–Sea Ice Model
Publisher: American Meteorological Society
Date: 08-2020
Abstract: The response of near-Antarctic waters to freshening by increased glacial melt is investigated using a high-resolution (0.1°) global ocean–sea ice model with realistic Antarctic water-mass properties. Two meltwater perturbation experiments are conducted where the ocean model is forced with constant elevated glacial melt rates of 1.5 and 2.8 times the control rate. Within 10 years of the onset of enhanced meltwater forcing, the generation of Antarctic Bottom Water from Dense Shelf Water ceases, as shelf waters become increasingly buoyant. Increased ocean stratification triggers subsurface warming in Dense Shelf Water source regions, suggesting a localized positive feedback to melt. In a parallel response, meltwater forcing enhances the subsurface lateral density gradients of the Antarctic Slope Front that modulate the transport of warm Circumpolar Deep Water across the continental slope toward ice shelf grounding lines. Consequently, coastal freshening acts to isolate the Antarctic Ice Sheet from open ocean heat, suggesting a cooling response to melt that counteracts warming associated with stratification. Further, these strengthening density gradients accelerate westward geostrophic currents along the coast and shelf break, homogenizing shelf waters and lifying remote feedbacks. The net effect on the continental shelf is transient warming, followed by cooling in both experiments however, this signal is the aggregate of a complex pattern of regional warming and cooling responses. These results suggest coastal freshening by meltwater may alter the thermal forcing of the Antarctic ice sheet in ways that both accelerate and inhibit ice shelf melt at different locations along the Antarctic coastline.
Response of the Southern Ocean overturning circulation to extreme Southern Annular Mode conditions
Publisher: Wiley
Date: 28-06-2020
An Antarctic Circumpolar Current driven by surface buoyancy forcing
Publisher: American Geophysical Union (AGU)
Date: 12-2010
DOI: 10.1029/2010GL044777
Climate-driven changes to ocean circulation and their inferred impacts on marine dispersal patterns
Publisher: Wiley
Date: 02-05-2016
DOI: 10.1111/GEB.12456
The impact of lee waves on the Southern Ocean circulation
Publisher: American Meteorological Society
Date: 13-07-2021
Abstract: Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee wave parameterization that includes both lee wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.
A New Open Source Implementation of Lagrangian Filtering: A Method to Identify Internal Waves in High‐Resolution Simulations
Publisher: American Geophysical Union (AGU)
Date: 10-2021
DOI: 10.1029/2021MS002616
Abstract: Identifying internal waves in complex flow fields is a long‐standing problem in fluid dynamics, oceanography and atmospheric science, owing to the overlap of internal waves temporal and spatial scales with other flow regimes. Lagrangian filtering—that is, temporal filtering in a frame of reference moving with the flow—is one proposed methodology for performing this separation. Here we (a) describe an improved implementation of the Lagrangian filtering methodology and (b) introduce a new freely available, parallelized Python package that applies the method. We show that the package can be used to directly filter output from a variety of common ocean models including MITgcm, Regional Ocean Modeling System and MOM5 for both regional and global domains at high resolution. The Lagrangian filtering is shown to be a useful tool to both identify (and thereby quantify) internal waves, and to remove internal waves to isolate the non‐wave flow field.
Seasonal and interannual variability of the Weddell Gyre from a high-resolution global ocean-sea ice simulation during 1958 - 2018
Publisher: Wiley
Date: 16-11-2021
High-resolution model simulation of the interannual and seasonal variability of the Weddell Gyre during 1958 - 2018
Publisher: Wiley
Date: 16-06-2021
Seasonal and Interannual Variability of the Subtropical Front in the New Zealand Region
Publisher: American Geophysical Union (AGU)
Date: 02-2021
DOI: 10.1029/2020JC016412
Abstract: The meridional variability of the Subtropical Front (STF) and the drivers of variability on interannual time scales in the New Zealand region are analyzed using a multi‐decadal eddy‐resolving ocean hindcast model, in comparison with Argo data. The STF marks the water mass boundary between subtropical waters and subantarctic waters, and is defined as the southern‐most location of the 11°C isotherm and 34.8 psu isohaline between 100 and 500 m. The STF shifts up to 650 km (6°) meridionally on seasonal timescales. In addition to seasonal variability, shifts of around 200 km (2°) occur on interannual time scales. These shifts are connected to regional wind stress curl anomalies in the eastern Tasman Sea and east of New Zealand, which trigger Ekman convergence/ ergence and Rossby waves and result in meridional transport of heat and salt into/out of the Tasman Sea. The net transports across the northern boundary of the Tasman Sea show the largest sensitivity to these wind stress curl anomalies. During periods of positive wind stress curl anomalies and Ekman convergence, the heat and salt content increases shifting the position of the STF southward. The opposite tendency occurs during periods of negative wind stress curl anomalies. The migration of the STF does not appear to be directly linked to regional climate oscillations.
Contrasting trends in short-lived and long-lived mesoscale eddies in the Southern Ocean since the 1990s
Publisher: Authorea, Inc.
Date: 08-12-2022
DOI: 10.22541/ESSOAR.167048300.05862577/V1
Abstract: Mesoscale eddies play an important role in both momentum and heat balances in the Southern Ocean. Previous studies have documented an increasing intensity of the Southern Ocean eddy field during recent decades however, it is still unclear whether the mesoscale eddies with different lifetimes have different temporal variations. Using satellite altimeter observations from 1993 to 2020, we find that the increasing trend in the intensity of eddies is dominated by long-lived eddies (with lifetimes ≥ 90 days), whose litude has increased at a rate of ~2.8% per decade the increase is concentrated downstream of topography. In contrast, short-lived eddies (with lifetimes 90 days) do not appear to have a significant trend in their litudes since the early 1990s. An energy conversion analysis indicates that the increased baroclinic instabilities of the mean flows associated with topography are responsible for the litude increase of the long-lived eddies.
Antarctica’s ecological isolation will be broken by storm-driven dispersal and warming
Publisher: Springer Science and Business Media LLC
Date: 16-07-2018
Southern Ocean heat and momentum uptake are sensitive to the vertical resolution at the ocean surface
Publisher: Elsevier BV
Date: 11-2019
Sequential changes in ocean circulation and biological export productivity during the last glacial cycle: a model-data study
Publisher: Copernicus GmbH
Date: 13-12-2019
DOI: 10.5194/CP-2019-146
Abstract: Abstract. We conduct a model-data analysis of the ocean, atmosphere and terrestrial carbon system to understand their effects on atmospheric CO2 during the last glacial cycle. We use a carbon cycle box model SCP-M, combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and biological productivity across marine isotope stages spanning 130 thousand years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea ice cover and shallow water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimised in each marine isotope stage, against proxy data for atmospheric CO2, δ13C and ∆14C and deep ocean δ13C, ∆14C and carbonate ion. Our model-data results suggest that global overturning circulation weakened at marine isotope stage 5d, coincident with a ∼ 25 ppm fall in atmospheric CO2 from the penultimate interglacial level. This change was followed by a further slowdown in Atlantic meridional overturning circulation and enhanced Southern Ocean biological export productivity at marine isotope stage 4 (∼−30 ppm). There was also a transient slowdown in Atlantic meridional overturning circulation at MIS 5b. In this model, the last glacial maximum was characterised by relatively weak global ocean and Atlantic meridional overturning circulation, and increased Southern Ocean biological export productivity (∼−15–20 ppm during MIS 2–4). Ocean circulation and Southern Ocean biology rebounded to modern values by the Holocene period. The terrestrial biosphere decreased by ∼ 500 Pg C in the lead up to the last glacial maximum, followed by a period of intense regrowth during the Holocene (∼ 750 Pg C). Slowing ocean circulation, a cooler ocean and, to a lesser extent, shallow carbonate dissolution, contributed ∼−75 ppm to atmospheric CO2 in the ∼ 100 thousand-year lead-up to the last glacial maximum, with a further ∼−10 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological productivity was one of the ingredients required to achieve the last glacial maximum atmospheric CO2 level. The incorporation of longer-timescale data into quantitative ocean transport models, provides useful insights into the timing of changes in ocean processes, enhancing our understanding of the last glacial maximum and Holocene carbon cycle transition.
Projected Changes of Surface Winds Over the Antarctic Continental Margin
Publisher: American Geophysical Union (AGU)
Date: 25-08-2022
DOI: 10.1029/2022GL098820
Abstract: Surface winds around the Antarctic continent control coupled ocean‐ice processes that influence the climate system, including bottom water production, heat transport onto the continental shelf and sea ice coverage. However, few studies have examined projected changes in these winds, even though it would aid in the interpretation and understanding of the ocean's response to climate change. Using Coupled Model Intercomparison Project Phase 6 and reanalysis data, we show a significant reduction in the near‐Antarctic surface winds throughout the historical period that continues until the end of the twenty‐first century, amounting to 23% and 7% for the easterly and southerly wind components respectively under the high emission scenario. The most intense weakening happens during the summer season. We find that the weakening is coherent with the trend toward a positive Southern Annular Mode and a reduction of the pole‐to‐coast meridional pressure gradient, which we term Antarctic Annular Index.
Eddy response to Southern Ocean climate modes
Publisher: American Geophysical Union (AGU)
Date: 10-2010
DOI: 10.1029/2009JC005894
Abstract: Interannual variations in Southern Ocean eddy kinetic energy (EKE) are investigated using 16 years of altimetric data. Circumpolar averages show a peak in EKE from 2000 to 2002, 2–3 years after the peak in the Southern Annular Mode (SAM) index. Although the SAM forcing is in phase around the circumpolar band, we find the EKE response varies regionally. The strongest EKE is in the Pacific, with energy peaks occurring progressively later toward the east. We suggest that this is due to the presence of two climate modes: SAM and ENSO. When strong positive SAM events coincide with La Niña periods, as in 1999, anomalous meridional wind forcing is enhanced in the South Pacific Ocean, contributing to the observed increase in EKE 2–3 years later. When positive SAM events coincide with El Niño periods, as in 1993, the climate modes are in opposition in the South Pacific, leading to a weak EKE response during the mid‐1990s. Numerical modeling supports these observations. By applying different combinations of SAM and ENSO, we can reproduce both the elevated Pacific EKE response to SAM as well as an additional lification/suppression of EKE during La Niña/El Niño. In general, we find that the EKE response depends on the interplay between wind forcing, topography, and mean flow and produces a strongly heterogeneous distribution in the Southern Ocean.
High‐Frequency Fluctuations in Antarctic Bottom Water Transport Driven by Southern Ocean Winds
Publisher: American Geophysical Union (AGU)
Date: 08-09-2021
DOI: 10.1029/2021GL094569
Abstract: Northward flow of Antarctic Bottom Water (AABW) across the Southern Ocean comprises a key component of the global overturning circulation. Yet AABW transport remains poorly constrained by observations and state estimates, and there is presently no means of directly monitoring any component of the Southern Ocean overturning. However, AABW flow is dynamically linked to Southern Ocean surface circulation via the zonal momentum balance, offering potential routes to indirect monitoring of the transport. Exploiting this dynamical link, this study shows that wind stress (WS) fluctuations drive large AABW transport fluctuations on time scales shorter than 2 years, which comprise almost all of the transport variance. This connection occurs due to differing time scales on which topographic and interfacial form stresses respond to wind variability, likely associated with differences in barotropic versus baroclinic Rossby wave propagation. These findings imply that AABW transport variability can largely be reconstructed from the surface WS alone.
The transient response of southern ocean circulation to geothermal heating in a global climate model
Publisher: American Meteorological Society
Date: 27-07-2016
Abstract: Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Water. Here two distinct geothermal heat flux datasets are tested under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. The regional analysis herein reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. While geothermal heating enhances Southern Ocean abyssal overturning circulation by 20%–50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3–5 Sv (1 Sv = 106 m3 s−1). The transient responses in regional bottom temperature increases exceed 0.1°C. The large-scale features that are shown to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.
Change in Dense Shelf Water and Adélie Land Bottom Water Precipitated by Iceberg Calving
Publisher: American Geophysical Union (AGU)
Date: 08-03-2018
DOI: 10.1002/2017GL076195
Response of the Southern Ocean overturning circulation to extreme Southern Annular Mode conditions
Publisher: Wiley
Date: 28-10-2020
The Dynamics of Mixed Layer Deepening during Open-Ocean Convection
Publisher: American Meteorological Society
Date: 06-2020
Abstract: Open-ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high-latitude oceans. However, many climate model simulations overestimate mixed layer depth during open-ocean convection, resulting in excessive formation of dense water in some regions. The physical processes controlling transient mixed layer depth during open-ocean convection are examined using two different numerical models: a high-resolution, turbulence-resolving nonhydrostatic model and a large-scale hydrostatic ocean model. An isolated destabilizing buoyancy flux is imposed at the surface of both models and a quasi-equilibrium flow is allowed to develop. Mixed layer depth in the turbulence-resolving and large-scale models closely aligns with existing scaling theories. However, the large-scale model has an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias is a consequence of the lack of resolved turbulent convection in the model, which delays the onset of baroclinic instability. These findings suggest that in order to reduce mixed layer biases in ocean simulations, parameterizations of the connection between baroclinic instability and convection need to be addressed.
On the Relationship between Southern Ocean Overturning and ACC Transport
Publisher: American Meteorological Society
Date: 2013
Abstract: The eddy field in the Southern Ocean offsets the impact of strengthening winds on the meridional overturning circulation and Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. Here, an idealized numerical model is employed to investigate the response of the large-scale circulation in the Southern Ocean to wind stress perturbations at eddy-permitting to eddy-resolving scales. Significant differences are observed between the sensitivities and the resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modeled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesized that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports.
Available potential energy gain from mixing due to the nonlinearity of the equation of state in a global ocean model
Publisher: American Geophysical Union (AGU)
Date: 28-05-2013
DOI: 10.1002/GRL.50508
Interhemispheric asymmetry in transient global warming: The role of Drake Passage
Publisher: American Geophysical Union (AGU)
Date: 28-04-2013
DOI: 10.1002/GRL.50341
The Geography of Numerical Mixing in a Suite of Global Ocean Models
Publisher: American Geophysical Union (AGU)
Date: 07-2021
DOI: 10.1029/2020MS002333
Abstract: Numerical mixing, defined here as the physically spurious tracer diffusion due to the numerical discretization of advection, is known to contribute to biases in ocean models. However, quantifying numerical mixing is nontrivial, with most studies utilizing targeted experiments in idealized settings. Here, we present a water mass transformation‐based method for quantifying numerical mixing that can be applied to any conserved variable in general circulation models. Furthermore, the method can be applied within in idual fluid columns to provide spatial information. We apply the method to a suite of global ocean model simulations with differing grid spacings and subgrid‐scale parameterizations. In all configurations numerical mixing drives diathermal heat transport of comparable magnitude to that associated with explicit parameterizations. Numerical mixing is prominent in the tropical thermocline, where it is sensitive to the vertical diffusivity and resolution. At colder temperatures numerical mixing is sensitive to the presence of explicit neutral diffusion, suggesting that it may act as a proxy for neutral diffusion when it is explicitly absent. Comparison of otherwise equivalent 1/4° and 1/10° configurations with grid‐scale dependent horizontal viscosity shows only a modest enhancement in numerical mixing at 1/4°. However, if the lateral viscosity is maintained while resolution is increased then numerical mixing is reduced by almost 35 % . This result suggests that the common practice of reducing viscosity in order to maximize permitted variability must be considered carefully. Our results provide a detailed view of numerical mixing in ocean models and pave the way for improvements in parameter choices and numerical methods.
Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater
Publisher: Springer Science and Business Media LLC
Date: 29-03-2023
Reshaping the Antarctic Circumpolar Current via Antarctic Bottom Water Export
Publisher: American Meteorological Society
Date: 10-2017
Abstract: Zonal momentum input into the Antarctic Circumpolar Current (ACC) by westerly winds is ultimately removed via topographic form stress induced by large bathymetric features that obstruct the path of the current. These bathymetric features also support the export of Antarctic Bottom Water (AABW) across the ACC via deep, geostrophically balanced, northward flows. These deep geostrophic currents modify the topographic form stress, implying that changes in AABW export will alter the ocean bottom pressure and require a rearrangement of the ACC in order to preserve its zonal momentum balance. A conceptual model of the ACC momentum balance is used to derive a relationship between the volume export of AABW and the shape of the sea surface across the ACC’s standing meanders. This prediction is tested using an idealized eddy-resolving ACC/Antarctic shelf channel model that includes both the upper and lower cells of the Southern Ocean meridional overturning circulation, using two different topographic configurations to obstruct the flow of the ACC. Eliminating AABW production leads to a shallowing of the sea surface elevation within the standing meander. To quantify this response, the authors introduce the “surface-induced topographic form stress,” the topographic form stress that would result from the shape of the sea surface if the ocean were barotropic. Eliminating AABW production also reduces the magnitude of the eddy kinetic energy generated downstream of the meander and the surface speed of the ACC within the meander. These findings raise the possibility that ongoing changes in AABW export may be detectable via satellite altimetry.
Interbasin Differences in Ocean Ventilation in Response to Variations in the Southern Annular Mode
Publisher: American Geophysical Union (AGU)
Date: 03-2021
DOI: 10.1029/2020JC016540
Abstract: The response of the ventilation of mode and intermediate waters to abrupt changes in the Southern Annular Mode (SAM) is examined by analyzing the ideal age in a global ocean‐sea ice model. The age response is shown to differ between the central Pacific Ocean and other basins. In the central Pacific there are large decreases in the age of subtropical mode and intermediate waters associated with a more positive SAM, contrasting only small age changes in the Atlantic and Indian Oceans, except near where intermediate water density surfaces outcrop. These interbasin differences hold for simulations at different horizontal resolutions, and can be explained by the zonal variations in wind stress changes associated with the SAM. These results suggest that the carbon and heat uptake associated with the SAM will likely vary between ocean basins.
Appreciation of 2017 GRL Peer Reviewers
Publisher: American Geophysical Union (AGU)
Date: 15-05-2018
DOI: 10.1029/2018GL078303
On the Momentum Flux of Internal Tides
Publisher: American Meteorological Society
Date: 04-2019
Abstract: The action of the barotropic tide over seafloor topography is the major source of internal waves at the bottom of the ocean. This internal tide has long been recognized to play an important role in ocean mixing. Here it is shown that the internal tide is also associated with a net (domain integrated) momentum flux. The net flux occurs as a result of the Doppler shifting of the internal tide at the point of generation by near-bottom mean flows. Linear theory is presented that predicts the litude of the wave momentum flux. The net flux scales with the bottom flow speed and the topographic wavenumber to the fourth power and is directed opposite to the bottom flow. For realistic topography, the predicted peak momentum flux occurs at scales of order 10 km and smaller, with magnitudes of order 10 −3 –10 −2 N m −2 . The theory is verified by comparison with a suite of idealized internal wave-resolving simulations. The simulations show that, for the topography considered, the wave momentum flux radiates away from the bottom and enhances mean and eddying flow when the tidal waves dissipate in the upper ocean. Our results suggest that internal tides may play an important role in forcing the upper ocean.
Eddy Saturation of the Southern Ocean: A Baroclinic Versus Barotropic Perspective
Publisher: American Geophysical Union (AGU)
Date: 09-11-2019
DOI: 10.1029/2019GL084117
Abstract: “Eddy saturation” is the regime in which the total time‐mean volume transport of an oceanic current is relatively insensitive to the wind stress forcing and is often invoked as a dynamical description of Southern Ocean circulation. We revisit the problem of eddy saturation using a primitive equations model in an idealized channel setup with bathymetry. We apply only mechanical wind stress forcing there is no diapycnal mixing or surface buoyancy forcing. Our main aim is to assess the relative importance of two mechanisms for producing eddy‐saturated states: (i) the commonly invoked baroclinic mechanism that involves the competition of sloping isopycnals and restratification by production of baroclinic eddies and (ii) the barotropic mechanism that involves production of eddies through lateral shear instabilities or through the interaction of the barotropic current with bathymetric features. Our results suggest that the barotropic flow component plays a crucial role in determining the total volume transport.
Global changes in oceanic mesoscale currents over the satellite altimetry record
Publisher: Springer Science and Business Media LLC
Date: 22-04-2021
Weddell Sea Control of Ocean Temperature Variability on the Western Antarctic Peninsula
Publisher: American Geophysical Union (AGU)
Date: 31-07-2023
DOI: 10.1029/2023GL103018
Abstract: Recent ice loss on the western Antarctic Peninsula has been driven by warming ocean waters on the continental shelf. However, due to the short observational record, our understanding of the dynamics and variability in this region remains poor. High‐resolution ocean model simulations show that the temperature variability along the western Antarctic Peninsula is controlled by the rate of dense water formation in the Weddell Sea. Passive tracer advection reveals connectivity between the Weddell Sea and the coastline of the western Antarctic Peninsula and Bellingshausen Sea. During multi‐year periods of weak Weddell dense water formation, dense overflow transport in the Weddell Sea decreases, while the transport of cold water around the tip of the Antarctic Peninsula strengthens, driving a temperature decrease of 0.4°C along the western Antarctic Peninsula. This mechanism implies that western Antarctic Peninsula coastal ocean temperature may cool in the future if Weddell Dense Shelf Water production slows down.
Spontaneous surface generation and interior amplification of internal waves in a regional-scale ocean model
Publisher: American Meteorological Society
Date: 04-2017
Abstract: Recent theories, models, and observations have suggested the presence of significant spontaneous internal wave generation at density fronts near the ocean surface. Spontaneous generation is the emission of waves by unbalanced, large Rossby number flows in the absence of direct forcing. Here, spontaneous generation is investigated in a zonally reentrant channel model using parameter values typical of the Southern Ocean. The model is carefully equilibrated to obtain a steady-state wave field for which a closed energy budget is formulated. There are two main results: First, waves are spontaneously generated at sharp fronts in the top 50 m of the model. The magnitude of the energy flux to the wave field at these fronts is comparable to that from other mechanisms of wave generation. Second, the surface-generated wave field is lified in the model interior through interaction with horizontal density gradients within the main zonal current. The magnitude of the mean-to-wave conversion in the model interior is comparable to recent observational estimates and is the dominant source of wave energy in the model, exceeding the initial spontaneous generation. This second result suggests that internal lification of the wave field may contribute to the ocean’s internal wave energy budget at a rate commensurate with known generation mechanisms.
Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds
Publisher: American Geophysical Union (AGU)
Date: 12-07-2014
DOI: 10.1002/2014GL060613
JRA55-do-based repeat year forcing datasets for driving ocean–sea-ice models
Publisher: Elsevier BV
Date: 03-2020
Glacial cycles and carbon dioxide: A conceptual model
Publisher: American Geophysical Union (AGU)
Date: 2008
DOI: 10.1029/2007GL032071
On Cabbeling and Thermobaricity in the Surface Mixed Layer
Publisher: American Meteorological Society
Date: 07-2017
Abstract: The surface mixed layer (ML) governs atmosphere–ocean fluxes, and thereby affects Earth’s climate. Accurate representation of ML processes in ocean models remains a challenge, however. The O (100) m deep ML exhibits substantial horizontal thermohaline gradients, despite being near-homogenous vertically, making it an ideal location for processes that result from the nonlinearity of the equation of state, such as cabbeling and thermobaricity. Traditional approaches to investigate these processes focus on their roles in interior water-mass transformation and are ill suited to examine their influence on the ML. However, given the climatic significance of the ML, quantifying the extent to which cabbeling and thermobaricity influence the ML density field offers insight into improving ML representations in ocean models. A recent simplified equation of state of seawater allows the local effects of cabbeling and thermobaric processes in the ML to be expressed analytically as functions of the local temperature gradient and ML depth. These simplified expressions are used to estimate the extent to which cabbeling and thermobaricity contribute to local ML density differences. These estimates compare well with values calculated directly using the complete nonlinear equation of state. Cabbeling and thermobaricity predominantly influence the ML density field poleward of 30°. Mixed layer thermobaricity is basin-scale and winter intensified, while ML cabbeling is perennial and localized to intense, zonally coherent regions associated with strong temperature fronts, such as the Antarctic Circumpolar Current and the Kuroshio and Gulf Stream Extensions. For latitudes between 40° and 50° in both hemispheres, the zonally averaged effects of ML cabbeling and ML thermobaricity can contribute on the order of 10% of the local ML density difference.
Interhemispheric Asymmetry of Warming in an Eddy-Permitting Coupled Sector Model
Publisher: American Meteorological Society
Date: 11-09-2015
Abstract: Climate model projections and observations show a faster rate of warming in the Northern Hemisphere (NH) than the Southern Hemisphere (SH). This asymmetry is partly due to faster rates of warming over the land than the ocean, and partly due to the ocean circulation redistributing heat toward the NH. This study examines the interhemispheric warming asymmetry in an intermediate complexity coupled climate model with eddy-permitting (0.25°) ocean resolution, and results are compared with a similar model with coarse (1°) ocean resolution. The models use a pole-to-pole 60° wide sector domain in the ocean and a 120° wide sector in the atmosphere, with Atlantic-like bathymetry and a simple land model. There is a larger high-latitude ocean temperature asymmetry in the 0.25° model compared with the 1° model, both in equilibrated control runs and in response to greenhouse warming. The larger warming asymmetry is caused by greater melting of NH sea ice in the 0.25° model, associated with faster, less viscous boundary currents transporting heat northward. The SH sea ice and heat transport response is relatively insensitive to the resolution change, since the eddy heat transport differences between the models are small compared with the mean flow heat transport. When a wind shift and intensification is applied in these warming scenarios, the warming asymmetry is further enhanced, with greater upwelling of cool water in the Southern Ocean and enhanced warming in the NH. Surface air temperatures show a substantial but lesser degree of high-latitude warming asymmetry, reflecting the sea surface warming patterns over the ocean but warming more symmetrically over the land regions.
Sensitivity of Marine Heatwave Metrics to Ocean Model Resolution
Publisher: American Geophysical Union (AGU)
Date: 26-12-2019
DOI: 10.1029/2019GL084928
The [simple carbon project] model v1.0
Publisher: Copernicus GmbH
Date: 18-04-2019
Abstract: Abstract. We construct a carbon cycle box model to process observed or inferred geochemical evidence from modern and paleo settings. The [simple carbon project] model v1.0 (SCP-M) combines a modern understanding of the ocean circulation regime with the Earth's carbon cycle. SCP-M estimates the concentrations of a range of elements within the carbon cycle by simulating ocean circulation, biological, chemical, atmospheric and terrestrial carbon cycle processes. The model is capable of reproducing both paleo and modern observations and aligns with CMIP5 model projections. SCP-M's fast run time, simplified layout and matrix structure render it a flexible and easy-to-use tool for paleo and modern carbon cycle simulations. The ease of data integration also enables model–data optimisations. Limitations of the model include the prescription of many fluxes and an ocean-basin-averaged topology, which may not be applicable to more detailed simulations. In this paper we demonstrate SCP-M's application primarily with an analysis of the carbon cycle transition from the Last Glacial Maximum (LGM) to the Holocene and also with the modern carbon cycle under the influence of anthropogenic CO2 emissions. We conduct an atmospheric and ocean multi-proxy model–data parameter optimisation for the LGM and late Holocene periods using the growing pool of published paleo atmosphere and ocean data for CO2, δ13C, Δ14C and the carbonate ion proxy. The results provide strong evidence for an ocean-wide physical mechanism to deliver the LGM-to-Holocene carbon cycle transition. Alongside ancillary changes in ocean temperature, volume, salinity, sea-ice cover and atmospheric radiocarbon production rate, changes in global overturning circulation and, to a lesser extent, Atlantic meridional overturning circulation can drive the observed LGM and late Holocene signals in atmospheric CO2, δ13C, Δ14C, and the oceanic distribution of δ13C, Δ14C and the carbonate ion proxy. Further work is needed on the analysis and processing of ocean proxy data to improve confidence in these modelling results.
Sill-controlled salinity contrasts followed post-Messinian flooding of the Mediterranean
Publisher: Springer Science and Business Media LLC
Date: 11-08-2022
Shear flow and viscosity in single-layer hydraulics
Publisher: Cambridge University Press (CUP)
Date: 02-2006
Interbasin differences in ocean ventilation in response to variations in the Southern Annular Mode
Publisher: Wiley
Date: 29-06-2020
Baroclinic Control of Southern Ocean Eddy Upwelling Near Topography
Publisher: American Geophysical Union (AGU)
Date: 06-04-2022
DOI: 10.1029/2021GL097491
Abstract: In the Southern Ocean, mesoscale eddies contribute to the upwelling of deep waters along sloping isopycnals, helping to close the upper branch of the meridional overturning circulation. Eddy energy (EE) is not uniformly distributed along the Antarctic Circumpolar Current (ACC). Instead, “hotspots” of EE that are associated with enhanced eddy‐induced upwelling exist downstream of topographic features. This study shows that, in idealized eddy‐resolved simulations, a topographic feature in the ACC path can enhance and localize eddy‐induced upwelling. However, the upwelling systematically occurs in regions where eddies grow through baroclinic instability, rather than in regions where EE is large. Across a range of parameters, along‐stream eddy growth rate is a more reliable indicator of eddy upwelling than traditional parameterizations such as eddy kinetic energy, eddy potential energy, or isopycnal slope. Ocean eddy parameterizations should consider metrics specific to the growth of baroclinic instability to accurately model eddy upwelling near topography.
Ice Algae Model Intercomparison Project phase 2 (IAMIP2)
Publisher: Copernicus GmbH
Date: 11-11-2021
Abstract: Abstract. Ice algae play a fundamental role in shaping sea-ice-associated ecosystems and biogeochemistry. This role can be investigated by field observations however the influence of ice algae at the regional and global scales remains unclear due to limited spatial and temporal coverage of observations and because ice algae are typically not included in current Earth system models. To address this knowledge gap, we introduce a new model intercomparison project (MIP), referred to here as the Ice Algae Model Intercomparison Project phase 2 (IAMIP2). IAMIP2 is built upon the experience from its previous phase and expands its scope to global coverage (both Arctic and Antarctic) and centennial timescales (spanning the mid-20th century to the end of the 21st century). Participating models are three-dimensional regional and global coupled sea-ice–ocean models that incorporate sea-ice ecosystem components. These models are driven by the same initial conditions and atmospheric forcing datasets by incorporating and expanding the protocols of the Ocean Model Intercomparison Project, an endorsed MIP of the Coupled Model Intercomparison Project phase 6 (CMIP6). Doing so provides more robust estimates of model bias and uncertainty and consequently advances the science of polar marine ecosystems and biogeochemistry. A diagnostic protocol is designed to enhance the reusability of the model data products of IAMIP2. Lastly, the limitations and strengths of IAMIP2 are discussed in the context of prospective research outcomes.
Global changes in oceanic mesoscale currents over the satellite altimetry record
Publisher: Research Square Platform LLC
Date: 20-10-2020
DOI: 10.21203/RS.3.RS-88932/V1
Abstract: Oceanic eddies play a profound role in mixing tracers such as heat, carbon, and nutrients, thereby regulating regional and global climate. Yet, it remains unclear how global oceanic eddy kinetic energy has evolved over the past few decades. Furthermore, coupled climate model predictions generally fail to resolve oceanic mesoscale dynamics, which could limit their accuracy in simulating future climate change. Here we show a global statistically significant increase of the eddy activity using two independent observational datasets of mesoscale variability, one directly measuring currents and the other from sea surface temperature. Regions characterized by different dynamical processes show distinct evolution in the eddy field. For ex le, eddy-rich regions such as boundary current extensions and the Antarctic Circumpolar Current show a significant increase of 2% and 5% per decade in eddy activity, respectively. In contrast, most of the regions of observed decrease are found in the tropical oceans. Because eddies play a fundamental role in the ocean transport of heat, momentum, and carbon, our results have far-reaching implications for ocean circulation and climate, and the modelling platforms we use to study future climate change.
Experiments with mixing in stratified flow over a topographic ridge
Publisher: American Geophysical Union (AGU)
Date: 09-2016
DOI: 10.1002/2016JC011990
The life cycle of Spontaneously generated internal waves
Publisher: American Meteorological Society
Date: 02-2018
Abstract: Recent numerical modeling studies have suggested significant spontaneous internal wave generation near the ocean surface and energy transfers to and from these waves in the ocean interior. Spontaneous generation is the emission of waves by unbalanced, large Rossby number flows in the absence of direct forcing. Here, the authors’ previous work is extended to investigate where and how these waves exchange energy with the nonwave (mean) flow. A novel double-filtering technique is adopted to separate first the wave and nonwave fields, then the in idual upward- and downward-propagating wave fields, and thereby identify the pathways of energy transfer. These energy transfers are dominated by the interaction of the waves with the vertical shear in the mean flow. Spontaneously generated waves are found to be oriented such that the downward-propagating wave is lified by the mean shear. The internal waves propagate through the entire model depth while dissipating energy and reflect back upward. The now-upward-propagating waves have the opposite sign interaction with the mean shear and decay, losing most of their energy to the nonwave flow in the upper 500 m. Overall, in the simulations described here, approximately 30% of the wave energy is dissipated, and 70% is returned to the mean flow. The apparent preferential orientation of spontaneous generation suggests a potentially unique role for these waves in the ocean energy budget in uniformly drawing net energy from mean flow in the upper-ocean interior and transporting it to depth.
An analytical model of the response of the meridional overturning circulation to changes in wind and buoyancy forcing
Publisher: American Meteorological Society
Date: 08-2012
Abstract: An analytical model of the full-depth ocean stratification and meridional overturning circulation for an idealized Atlantic basin with a circumpolar channel is presented. The model explicitly describes the ocean response to both Southern Ocean winds and the global pattern and strength of prescribed surface buoyancy fluxes. The construction of three layers, defined by the two isopycnals of overturning extrema, allows the description of circulation and stratification in both the upper and abyssal ocean. The system is fully solved in the adiabatic limit to yield scales for the surface layer thickness, buoyancies of each layer, and overturning magnitudes. The analytical model also allows scaling of the Antarctic Circumpolar Current (ACC) transport. The veracity of the three-layer framework and derived scales is confirmed by applying the analytical model to an idealized geometry, eddy-permitting ocean general circulation model. Consistent with previous results, the abyssal overturning is found to scale inversely with wind stress, whereas the North Atlantic overturning and surface-layer thickness scale linearly with wind stress. In terms of the prescribed surface buoyancy fluxes, increased negative fluxes (buoyancy removal) in the North Atlantic increase the North Atlantic overturning and surface-layer thickness, whereas increased positive fluxes in the middle and low latitudes lead to a decrease in both parameters. Increased negative surface buoyancy fluxes to the south of Drake Passage increase the abyssal overturning and reduce the abyssal buoyancy. The ACC transport scales to first order with the sum of the Ekman transport and the abyssal overturning and thus increases with both wind stress and southern surface buoyancy flux magnitude.
Understanding variability of the Southern Ocean overturning circulation in CORE-II models
Publisher: Elsevier BV
Date: 03-2018
Kelvin wave hydraulic control induced by interactions between vortices and topography
Publisher: Cambridge University Press (CUP)
Date: 11-10-2011
DOI: 10.1017/JFM.2011.344
Abstract: The interaction of a dipolar vortex with topography is examined using a combination of analytical solutions and idealized numerical models. It is shown that an anticyclonic vortex may generate along-topography flow with sufficient speeds to excite hydraulic control with respect to local Kelvin waves. A critical condition for Kelvin wave hydraulic control is found for the simplest case of a 1.5-layer shallow water model. It is proposed that in the continuously stratified case this mechanism may allow an interaction between low mode vortices and higher mode Kelvin waves, thereby generating rapidly converging isopycnals and hydraulic jumps. Thus, Kelvin wave hydraulic control may contribute to the flux of energy from mesoscale to smaller, unbalanced, scales of motion in the ocean.
Impact of high resolution on Arctic Ocean simulations in Ocean Model Intercomparison Project phase 2 (OMIP-2)
Publisher: Copernicus GmbH
Date: 26-06-2023
DOI: 10.5194/GMD-2023-123
Abstract: Abstract. This study evaluates the impact of increasing resolution on Arctic Ocean simulations using five pairs of matched low- and high-resolution models within the OMIP-2 framework. The primary objective is to assess whether higher resolution can mitigate typical biases observed in low-resolution models and improve the representation of key climate-relevant variables. We reveal that increasing horizontal resolution contributes to a reduction in biases in mean temperature and salinity, and improves the simulation of the Atlantic Water layer and its decadal warming events. Higher resolution also leads to improved agreement with observed surface mixed layer depth, cold halocline base depth and Arctic gateway transports. However, the simulation of the mean state and temporal changes in Arctic freshwater content does not show improvement with increased resolution. While the use of higher resolution demonstrates positive outcomes for certain variables, it is crucial to recognize that model numerics and parameterizations also play a significant role in achieving faithful simulations. Overall, higher resolution shows promise in improving the simulation of key Arctic Ocean features and processes, but comprehensive model development is required to achieve more accurate representations across all climate-relevant variables.
Decoupling of the Surface and Bottom‐Intensified Antarctic Slope Current in Regions of Dense Shelf Water Export
Publisher: American Geophysical Union (AGU)
Date: 26-08-2023
DOI: 10.1029/2023GL104834
Abstract: The Antarctic Slope Current is guided by the topographic gradient of the Antarctic continental slope and creates a dynamical barrier between the continental shelf and the open ocean. The current's vertical structure varies around the continent affecting cross‐slope water mass exchange with consequences for Antarctic mass loss, ventilation of the deep ocean, and carbon uptake. The Antarctic Slope Current is surface‐intensified in many regions but bottom‐intensified in regions of dense overflows. This study investigates the role of dense overflows in modifying the dynamics of the bottom‐intensified flow using a 0.1° global ocean‐sea ice model. The occurrence of bottom‐intensification is tightly linked with dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along‐slope bottom water flows are coincidentally co‐located with the Antarctic Slope Current, rather than dynamically a part of the current.
Vertical transport in the ocean due to sub-mesoscale structures: Impacts in the Kerguelen region
Publisher: Elsevier BV
Date: 08-2014
Spatial and temporal variability of the Antarctic Slope Current in an eddying ocean-sea ice model
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-13947
Abstract: & & & span& & span& The basal melt rate of Antarctica's ice shelves is largely controlled by heat delivered from the Southern Ocean to the Antarctic continental shelf. The Antarctic Slope Current (ASC) is an almost circumpolar feature that encircles Antarctica along the continental shelf break in an anti-clockwise direction. Because the circulation is to first order oriented along the topographic slope, it inhibits exchange of water masses between the Southern Ocean and the Antarctic continental shelf and thereby impacts cross-slope heat supply. Direct observations of the ASC system are sparse, but indicate a highly variable flow field both in time and space. Given the importance of the circulation near the shelf break for cross-shelf exchange of heat, it is timely to further improve our knowledge of the ASC system. This study makes use of the global ocean-sea ice model ACCESS-OM2-01 with a 1/10 degree horizontal resolution and describes the spatial and temporal variability of the velocity field. We categorise the modelled ASC into three different regimes, similar to previous works for the associated Antarctic Slope Front: (i) A surface-intensified current found predominantly in East Antarctica, (ii) a bottom-intensified current found downstream of the dense shelf water formation sit& /span& & span& e& /span& & span& s in the Ross, Weddell, and Prydz Bay Seas, and (iii) a reversed current found in West Antarctica where the eastward flowing Antarctic Circumpolar Current impinges onto the continental shelf break. We find that the temporal variability of the Antarctic Slope Current varies between the regimes. In the bottom-intensified regions, the variability is set by the timing of the dense shelf water overflows, whereas the surface-intensified flow responds to the sub-monthly variability in the wind field.& /span& & /span& & &
Importance of Background Vorticity Effect and Doppler Shift in Defining Near‐Inertial Internal Waves
Publisher: American Geophysical Union (AGU)
Date: 21-11-2022
DOI: 10.1029/2022GL099498
Abstract: Near‐inertial waves contain a significant fraction of the ocean's internal wave energy and can propagate long distances from their source before dissipating. However, a varying background flow velocity can alter the wave propagation in two ways. The background vorticity modifies the lower bound of the wave frequency bandwidth while Doppler shifts alter the wave intrinsic frequency. Both effects complicate the identification of the waves and the quantification of their energy content. This study analyses the output of a realistic simulation of the North Pacific using adaptive frequency filters to isolate the effects of vorticity and Doppler shift on the apparent wave energy. Spectral filters neglecting background vorticity effects results in apparent near‐inertial energy being underestimated by 40% in anticyclonic structures and overestimated by 100% in cyclonic structures. The asymmetry in energy bias is reinforced when both background vorticity and Doppler shift effects are omitted from the frequency filters.
Localized rapid warming of West Antarctic subsurface waters by remote winds
Publisher: Springer Science and Business Media LLC
Date: 17-07-2017
DOI: 10.1038/NCLIMATE3335
ACCESS-OM2: A Global Ocean-Sea Ice Model at Three Resolutions
Publisher: Copernicus GmbH
Date: 30-04-2019
DOI: 10.5194/GMD-2019-106
Abstract: Abstract. We introduce a new version of the ocean-sea ice implementation of the Australian Community Climate and Earth System Simulator, ACCESS-OM2. The model has been developed with the aim of being aligned as closely as possible with the fully coupled (atmosphere-land-ocean-sea ice) ACCESS-CM2. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1° horizontal grid spacing), an eddy-permitting resolution (nominally 0.25°) and an eddy-rich resolution (0.1° with 75 vertical levels), where the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow testing at lower resolutions and to permit comparison across resolutions. In this manuscript, the model is introduced and the in idual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean-sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents and the abyssal overturning cell, but that there is scope for improvements in sub-grid scale parameterisations at the highest resolution.
Energy Loss from Transient Eddies due to Lee Wave Generation in the Southern Ocean
Publisher: American Meteorological Society
Date: 12-2018
Abstract: Observations suggest that enhanced turbulent dissipation and mixing over rough topography are modulated by the transient eddy field through the generation and breaking of lee waves in the Southern Ocean. Idealized simulations also suggest that lee waves are important in the energy pathway from eddies to turbulence. However, the energy loss from eddies due to lee wave generation remains poorly estimated. This study quantifies the relative energy loss from the time-mean and transient eddy flow in the Southern Ocean due to lee wave generation using an eddy-resolving global ocean model and three independent topographic datasets. The authors find that the energy loss from the transient eddy flow (0.12 TW 1 TW = 10 12 W) is larger than that from the time-mean flow (0.04 TW) due to lee wave generation lee wave generation makes a larger contribution (0.12 TW) to the energy loss from the transient eddy flow than the dissipation in turbulent bottom boundary layer (0.05 TW). This study also shows that the energy loss from the time-mean flow is regulated by the transient eddy flow, and energy loss from the transient eddy flow is sensitive to the representation of anisotropy in small-scale topography. It is implied that lee waves should be parameterized in eddy-resolving global ocean models to improve the energetics of resolved flow.
Energetics of Multidecadal Atlantic Ocean Variability
Publisher: American Meteorological Society
Date: 07-10-2014
DOI: 10.1175/JCLI-D-12-00801.1
Abstract: Oscillatory behavior of the Atlantic meridional overturning circulation (MOC) is thought to underlie Atlantic multidecadal climate variability. While the energy sources and sinks driving the mean MOC have received intense scrutiny over the last decade, the governing energetics of the modes of variability of the MOC have not been addressed to the same degree. This paper examines the energy conversion processes associated with this variability in an idealized North Atlantic Ocean model. In this model, the multidecadal variability arises through an instability associated with a so-called thermal Rossby mode, which involves westward propagation of temperature anomalies. Applying the available potential energy (APE) framework from stratified turbulence to the idealized ocean model simulations, the authors study the multidecadal variability from an energetics viewpoint. The analysis explains how the propagation of the temperature anomalies leads to changes in APE, which are subsequently converted into the kinetic energy changes associated with variations in the MOC. Thus, changes in the rate of generation of APE by surface buoyancy forcing provide the kinetic energy to sustain the multidecadal mode of variability.
Wave drag in oscillatory mean flows
Publisher: Copernicus GmbH
Date: 03-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-1911
Abstract: & & In both the atmosphere and ocean, large-scale (mean) flows over topography generate internal waves. A longstanding question in both fields is what forces & #8211 often known as & #8216 wave drag& #8217 & #8211 are exerted on the mean flow in this process, as such forces must be parameterized in non-wave-resolving numerical models. For a time-invariant mean flow, it is well known that lee waves are generated which extract momentum from the solid earth and deposit it where they break and dissipate at height. Here, I address the equivalent problem for a time-periodic mean flow (e.g. the ocean tide) using theory and numerical simulations. In this situation, the waves influence the litude and phase of the periodic mean flow near the topography regardless of where they dissipate. Dissipation plays a role in terms of controlling the magnitude of wave reflections from an upper boundary (e.g. the ocean surface) which modifies the forces acting near the topography. Our results form a framework for parameterizing tidal internal wave drag in global ocean models.& &
Controls on circulation, cross-shelf exchange, and dense water formation in an Antarctic polynya
Publisher: American Geophysical Union (AGU)
Date: 08-07-2016
DOI: 10.1002/2016GL069479
Transient Response of the Southern Ocean to Idealized Wind and Thermal Forcing across Different Model Resolutions
Publisher: American Meteorological Society
Date: 07-2021
Abstract: The Southern Ocean has undergone significant climate-related changes over recent decades, including intensified westerly winds and increased radiative heating. The interplay between wind-driven cooling and radiative warming of the ocean is complex and remains unresolved. In this study, idealized wind and thermal perturbations are analyzed in a global ocean–sea ice model at two horizontal resolutions: nominally, 1° and 0.1°. The sea surface temperature (SST) response shows a clear transition from a wind-driven cooling phase to a warming phase. This warming transition is largely attributed to meridional and vertical Ekman heat advection, which are both sensitive to model resolution due to the model-dependent components of temperature gradients. At higher model resolution, due to a more accurate representation of near-surface vertical temperature inversion and upward Ekman heat advection around Antarctica, the anomalous SST warming is stronger and develops earlier. The mixed layer depth at midlatitudes initially increases due to a wind-driven increase in Ekman transport of cold dense surface water northward, but then decreases when the thermal forcing drives enhanced surface stratification both responses are more sensitive at lower model resolution. With the wind intensification, the residual overturning circulation increases less in the 0.1° case because of the adequately resolved eddy compensation. Ocean heat subduction penetrates along more tilted isopycnals in the 1° case, but it orients to follow isopycnal layers in the 0.1° case. These findings have implications for understanding the ocean response to the combined effects of Southern Hemisphere westerly wind changes and anthropogenic warming.
Are ocean gyres driven by wind stress, surface buoyancy fluxes, or both?
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-14617
Abstract: The large-scale ocean circulation is fuelled by a combination of winds and buoyancy (or heat) fluxes acting on the ocean& #8217 s surface. Gyres are central features of large-scale ocean circulation and are involved in the transport of many tracers like heat, nutrients, carbon-dioxide and so on within and across ocean basins. Traditionally, the gyre circulation is explained by the relationship between meridional transport and wind stress curl, known as the Sverdrup balance. However, it has been proposed that surface buoyancy fluxes may also contribute to the formation of gyres, although such a theoretical relationship is lacking in oceanographic literature. Through a series of eddy-permitting global ocean model simulations, we aspire to better understand the relative contribution of wind stress and surface buoyancy fluxes on large-scale ocean circulation. We perturb the atmospheric forcing by spatially varying the wind stresses and/or surface buoyancy fluxes, while minimising the associated changes in mixed layer dynamics. We compare perturbed forcing simulations with a control simulation in an attempt to decompose the large- scale ocean circulation into buoyancy and wind-driven components.
On Energy Cascades in General Flows
Publisher: Wiley
Date: 30-09-2020
Response of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode Conditions
Publisher: American Geophysical Union (AGU)
Date: 11-11-2020
DOI: 10.1029/2020GL091103
Abstract: The positive trend of the Southern Annular Mode (SAM) will impact the Southern Ocean's role in Earth's climate however, the details of the Southern Ocean's response remain uncertain. We introduce a methodology to examine the influence of SAM on the Southern Ocean and apply this method to a global ocean‐sea ice model run at three resolutions (1°, (1/4)°, and (1/10)°). Our methodology drives perturbation simulations with realistic atmospheric forcing of extreme SAM conditions. The thermal response agrees with previous studies positive SAM perturbations warm the upper ocean north of the wind speed maximum and cool it to the south, with the opposite response for negative SAM. The overturning circulation exhibits a rapid response that increases/decreases for positive/negative SAM perturbations and is insensitive to model resolution. The longer‐term adjustment of the overturning circulation, however, depends on the representation of eddies, and is faster at higher resolutions.
Does the sensitivity of Southern Ocean circulation depend upon bathymetric details?
Publisher: The Royal Society
Date: 13-07-2014
Abstract: The response of the major ocean currents to changes in wind stress forcing is investigated with a series of idealized, but eddy-permitting, model simulations. Previously, ostensibly similar models have shown considerable variation in the oceanic response to changing wind stress forcing. Here, it is shown that a major reason for these differences in model sensitivity is subtle modification of the idealized bathymetry. The key bathymetric parameter is the extent to which the strong eddy field generated in the circumpolar current can interact with the bottom water formation process. The addition of an embayment, which insulates bottom water formation from meridional eddy fluxes, acts to stabilize the deep ocean density and enhances the sensitivity of the circumpolar current. The degree of interaction between Southern Ocean eddies and Antarctic shelf processes may thereby control the sensitivity of the Southern Ocean to change.
The Drag on the Barotropic Tide due to the Generation of Baroclinic Motion
Publisher: American Meteorological Society
Date: 12-2020
Abstract: The interaction of a barotropic flow with topography generates baroclinic motion that exerts a stress on the barotropic flow. Here, explicit solutions are calculated for the spatial-mean flow (i.e., the barotropic tide) resulting from a spatially uniform but time-varying body force (i.e., astronomical forcing) acting over rough topography. This approach of prescribing the force contrasts with that of previous authors who have prescribed the barotropic flow. It is found that the topographic stress, and thus the impact on the spatial-mean flow, depend on the nature of the baroclinic motion that is generated. Two types of stress are identified: (i) a “wave drag” force associated with propagating wave motion, which extracts energy from the spatial-mean flow, and (ii) a topographic “spring” force associated with standing motion at the seafloor, including bottom-trapped internal tides and propagating low-mode internal tides, which significantly d s the time-mean kinetic energy of the spatial-mean flow but extracts no energy in the time-mean. The topographic spring force is shown to be analogous to the force exerted by a mechanical spring in a forced-dissipative harmonic oscillator. Expressions for the topographic stresses appropriate for implementation as baroclinic drag parameterizations in global models are presented.
Control of the glacial carbon budget by topographically induced mixing
Publisher: American Geophysical Union (AGU)
Date: 18-06-2014
DOI: 10.1002/2014GL059963
Supplementary material to Impact of high resolution on Arctic Ocean simulations in Ocean Model Intercomparison Project phase 2 (OMIP-2)
Publisher: Copernicus GmbH
Date: 26-06-2023
Recent trends in the Southern Ocean eddy field
Publisher: American Geophysical Union (AGU)
Date: 2015
DOI: 10.1002/2014JC010470
Ocean gyres driven by surface buoyancy forcing
Publisher: Wiley
Date: 19-05-2020
The Mixed Layer Depth in the Ocean Model Intercomparison Project (OMIP): Impact of Resolving Mesoscale Eddies
Publisher: Copernicus GmbH
Date: 28-02-2023
DOI: 10.5194/EGUSPHERE-2023-310
Abstract: Abstract. The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed layer depth (MLD) in six pairs of non-eddying (1° resolution) and eddy-rich (up to 1/16°) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model validation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal s ling, which motivates new recommendations for MLD computation in future model intercomparison projects.
Response of Southern Ocean Convection and Abyssal Overturning to Surface Buoyancy Perturbations
Publisher: American Meteorological Society
Date: 12-05-2015
DOI: 10.1175/JCLI-D-14-00110.1
Abstract: This study explores how buoyancy-driven modulations in the abyssal overturning circulation affect Southern Ocean temperature and salinity in an eddy-permitting ocean model. Consistent with previous studies, the modeled surface ocean south of 50°S cools and freshens in response to enhanced surface freshwater fluxes. Paradoxically, upper-ocean cooling also occurs for small increases in the surface relaxation temperature. In both cases, the surface cooling and freshening trends are linked to reduced convection and a slowing of the abyssal overturning circulation, with associated changes in oceanic transport of heat and salt. For small perturbations, convective shutdown does not begin immediately, but instead develops via a slow feedback between the weakened overturning circulation and buoyancy anomalies. Two distinct phases of surface cooling are found: an initial smaller trend associated with the advective (overturning) adjustment of up to ~60 yr, followed by more rapid surface cooling during the convective shutdown period. The duration of the first advective phase decreases for larger forcing perturbations. As may be expected during the convective shutdown phase, the deep ocean warms and salinifies for both types of buoyancy perturbation. However, during the advective phase, the deep ocean freshens in response to freshwater perturbations but salinifies in the surface warming perturbations. The magnitudes of the modeled surface and abyssal trends during the advective phase are comparable to the recent observed multidecadal Southern Ocean temperature and salinity changes.
Circumpolar Variations in the Chaotic Nature of Southern Ocean Eddy Dynamics
Publisher: American Geophysical Union (AGU)
Date: 05-2022
DOI: 10.1029/2022JC018440
Abstract: Circulation in the Southern Ocean is unique. The strong wind stress forcing and buoyancy fluxes, in concert with the lack of continental boundaries, conspire to drive the Antarctic Circumpolar Current replete with an intense eddy field. The effect of Southern Ocean eddies on the ocean circulation is significant—they modulate the momentum balance of the zonal flow, and the meridional transport of tracers and mass. The strength of the eddy field is controlled by a combination of forcing (primarily thought to be wind stress) and intrinsic, chaotic, variability associated with the turbulent flow field itself. Here, we present results from an eddy‐permitting ensemble of ocean model simulations to investigate the relative contribution of forced and intrinsic processes in governing the variability of Southern Ocean eddy kinetic energy. We find that variations of the eddy field are mostly random, even on longer (interannual) timescales. Where correlations between the wind stress forcing and the eddy field exist, these interactions are dominated by two distinct timescales—a fast baroclinic instability response and a multi‐year process owing to feedback between bathymetry and the mean flow. These results suggest that understanding Southern Ocean eddy dynamics and its larger‐scale impacts requires an ensemble approach to eliminate intrinsic variability, and therefore may not yield robust conclusions from observations alone.
Dissipating and reflecting internal waves
Publisher: American Meteorological Society
Date: 21-05-2021
Abstract: Internal waves generated at the seafloor propagate through the interior of the ocean, driving mixing where they break and dissipate. However, existing theories only describe these waves in two limiting cases. In one limit, the presence of an upper boundary permits bottom-generated waves to reflect from the ocean surface back to the seafloor, and all the energy flux is at discrete wavenumbers corresponding to resonant modes. In the other limit, waves are strongly dissipated such that they do not interact with the upper boundary and the energy flux is continuous over wavenumber. Here, a novel linear theory is developed for internal tides and lee waves that spans the parameter space in between these two limits. The linear theory is compared with a set of numerical simulations of internal tide and lee wave generation at realistic abyssal hill topography. The linear theory is able to replicate the spatially-averaged kinetic energy and dissipation of even highly non-linear wave fields in the numerical simulations via an appropriate choice of the linear dissipation operator, which represents turbulent wave breaking processes.
Machine Learning‐Derived Inference of the Meridional Overturning Circulation From Satellite‐Observable Variables in an Ocean State Estimate
Publisher: American Geophysical Union (AGU)
Date: 04-2023
DOI: 10.1029/2022MS003370
Abstract: The oceanic Meridional Overturning Circulation (MOC) plays a key role in the climate system, and monitoring its evolution is a scientific priority. Monitoring arrays have been established at several latitudes in the Atlantic Ocean, but other latitudes and oceans remain unmonitored for logistical reasons. This study explores the possibility of inferring the MOC from globally‐available satellite measurements via machine learning (ML) techniques, using the ECCOV4 state estimate as a test bed. The methodological advantages of the present approach include the use purely of available satellite data, its applicability to multiple basins within a single ML framework, and the ML model simplicity (a feed‐forward fully connected neural network (NN) with small number of neurons). The ML model exhibits high skill in MOC reconstruction in the Atlantic, Indo‐Pacific, and Southern Oceans. The approach achieves a higher skill in predicting the model Southern Ocean abyssal MOC than has previously been achieved via a dynamically‐based approach. The skill of the model is quantified as a function of latitude in each ocean basin, and of the time scale of MOC variability. We find that ocean bottom pressure generally has the highest reconstruction skill potential, followed by zonal wind stress. We additionally test which combinations of variables are optimal. Furthermore, ML interpretability techniques are used to show that high reconstruction skill in the Southern Ocean is mainly due to (NN processing of) bottom pressure variability at a few prominent bathymetric ridges. Finally, the potential for reconstructing MOC strength estimates from real satellite measurements is discussed.
On the mechanisms of late 20th century sea-surface temperature trends over the Antarctic Circumpolar Current
Publisher: American Geophysical Union (AGU)
Date: 11-2011
DOI: 10.1029/2011JC007473
Abstract: The Antarctic Circumpolar Current (ACC), with its associated three‐dimensional circulation, plays an important role in global climate. This study concentrates on surface signatures of recent climate change in the ACC region and on mechanisms that control this change. Examination of climate model simulations shows that they match the observed late 20th century sea‐surface temperature (SST) trends averaged over this region quite well, despite underestimating the observed surface‐wind increases. Such wind increases, however, are expected to lead to significant cooling of the region, contradicting the observed SST trends. Motivated by recent theories of the ACC response to variable wind and radiative forcing, the authors used two idealized models to assess contributions of various dynamical processes to the SST evolution in the region. In particular, a high‐resolution channel model of the ACC responds to increasing winds by net surface ACC warming due to enhanced mesoscale turbulence and associated heat transports in the mixed layer. These fluxes, modeled, in a highly idealized fashion, via increased lateral surface mixing in a coarse‐resolution hybrid climate model, substantially offset zonally non‐uniform surface cooling due to air‐sea flux and Ekman‐transport anomalies. These results suggest that the combination of these opposing effects must be accounted for when estimating climate response to any external forcing in the ACC region.
The Sensitivity of the Antarctic Ice Sheet to a Changing Climate: Past, Present, and Future
Publisher: American Geophysical Union (AGU)
Date: 11-11-2020
DOI: 10.1029/2019RG000663
Abstract: The Antarctic Ice Sheet (AIS) is out of equilibrium with the current anthropogenic‐enhanced climate forcing. Paleoenvironmental records and ice sheet models reveal that the AIS has been tightly coupled to the climate system during the past and indicate the potential for accelerated and sustained Antarctic ice mass loss into the future. Modern observations by contrast suggest that the AIS has only just started to respond to climate change in recent decades. The maximum projected sea level contribution from Antarctica to 2100 has increased significantly since the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report, although estimates continue to evolve with new observational and theoretical advances. This review brings together recent literature highlighting the progress made on the known processes and feedbacks that influence the stability of the AIS. Reducing the uncertainty in the magnitude and timing of the future sea level response to AIS change requires a multidisciplinary approach that integrates knowledge of the interactions between the ice sheet, solid Earth, atmosphere, and ocean systems and across time scales of days to millennia. We start by reviewing the processes affecting AIS mass change, from atmospheric and oceanic processes acting on short time scales (days to decades), through to ice processes acting on intermediate time scales (decades to centuries) and the response to solid Earth interactions over longer time scales (decades to millennia). We then review the evidence of AIS changes from the Pliocene to the present and consider the projections of global sea level rise and their consequences. We highlight priority research areas required to improve our understanding of the processes and feedbacks governing AIS change.
Warm Circumpolar Deep Water transport toward Antarctica driven by local dense water export in canyons
Publisher: American Association for the Advancement of Science (AAAS)
Date: 05-2020
Abstract: Mid-depth heat transport toward Antarctica is focused in “cold” regions, mechanically forced by overflowing dense water.
Drivers of Atmospheric and Oceanic Surface Temperature Variance: A Frequency Domain Approach
Publisher: American Meteorological Society
Date: 05-2021
Abstract: Ocean–atmosphere coupling modifies the variability of Earth’s climate over a wide range of time scales. However, attribution of the processes that generate this variability remains an outstanding problem. In this article, air–sea coupling is investigated in an eddy-resolving, medium-complexity, idealized ocean–atmosphere model. The model is run in three configurations: fully coupled, partially coupled (where the effect of the ocean geostrophic velocity on the sea surface temperature field is minimal), and atmosphere-only. A surface boundary layer temperature variance budget analysis computed in the frequency domain is shown to be a powerful tool for studying air–sea interactions, as it differentiates the relative contributions to the variability in the temperature field from each process across a range of time scales (from daily to multidecadal). This method compares terms in the ocean and atmosphere across the different model configurations to infer the underlying mechanisms driving temperature variability. Horizontal advection plays a dominant role in driving temperature variance in both the ocean and the atmosphere, particularly at time scales shorter than annual. At longer time scales, the temperature variance is dominated by strong coupling between atmosphere and ocean. Furthermore, the Ekman transport contribution to the ocean’s horizontal advection is found to underlie the low-frequency behavior in the atmosphere. The ocean geostrophic eddy field is an important driver of ocean variability across all frequencies and is reflected in the atmospheric variability in the western boundary current separation region at longer time scales.
Attribution of horizontal and vertical contributions to spurious mixing in an Arbitrary Lagrangian – Eulerian ocean model
Publisher: Elsevier BV
Date: 11-2017
The mixed layer depth in Ocean Model Intercomparison Project (OMIP) high resolution models
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-8162
Abstract: The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. Numerical models used in climate studies should therefore have a good representation of the mixed layer, especially its depth (MLD). Here we use simulations from the Ocean Model Intercomparison Project (OMIP), which have been forced by a common atmospheric state, to assess the realism of the simulated MLDs. For model validation, an updated MLD dataset has been computed from observations using the fixed density threshold recommended by the OMIP protocol. We evaluate the influence of horizontal resolution by using six pairs of simulations, non-eddying (typically 1& #176 resolution) and eddy-rich (1/10& #176 to 1/16& #176 resolution). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere, where the eddy-rich models produce a more robust MLD and deep biases are reduced. The Southern Ocean offers a more contrasted view, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to an hypothesis that the deepening of the MLD in anticyclones would make the MLD deeper globally, eddy-rich models tend to have a shallower MLD in the zonal mean. In summer, a deep MLD bias is found in all the non-eddying models north of the equator this bias is greatly reduced at high resolution. In addition, our study highlights the sensitivity of the MLD computation to choice of a reference level and the spatio-temporal s ling, which motivates new recommendations for MLD computation in future model intercomparison projects.
Lee Waves Break Eddy Saturation of the Antarctic Circumpolar Current
Publisher: American Geophysical Union (AGU)
Date: 26-05-2023
DOI: 10.1029/2023GL103866
Abstract: Eddy‐resolving ocean models suggest that the transport of the Antarctic Circumpolar Current (ACC) may be insensitive to increasing wind. This insensitivity is due to eddies that flatten the isopycnals and compensate for their wind‐driven steepening. However, the eddy‐resolving models do not accurately represent the eddy dissipation processes that occur at scales smaller than the model resolution, including lee wave generation at rough topography. Using a lee wave parameterization in an idealized model of the Southern Ocean, we show that the ACC transport becomes more sensitive to wind when the lee wave drag is included. The sensitivity arises from the dependence of the lee wave drag on the bottom stratification. When the bottom stratification increases in response to wind, it increases the lee wave generation, and hence the eddy dissipation, at rough topography. As a result, the ACC shear (baroclinic transport) increases to drive stronger eddy generation to compensate.
Southern Ocean circulation and eddy compensation in CMIP5 models
Publisher: American Meteorological Society
Date: 09-09-2013
DOI: 10.1175/JCLI-D-12-00504.1
Abstract: Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows—fundamental players in the global distribution of heat, gases, and nutrients—respond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO2 reaches 1370 ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000 m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (~20%), the projected ACC transports lie within ±15% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells.
Circumpolar response of Southern Ocean eddy activity to a change in the Southern Annular Mode
Publisher: American Geophysical Union (AGU)
Date: 2006
DOI: 10.1029/2006GL026499
The mixed-layer depth in the Ocean Model Intercomparison Project (OMIP): impact of resolving mesoscale eddies
Publisher: Copernicus GmbH
Date: 12-07-2023
Abstract: Abstract. The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed-layer depth (MLD) in six pairs of non-eddying (1∘ grid spacing) and eddy-rich (up to 1/16∘) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model evaluation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low-resolution models exhibit large biases in the deep-water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode-water formation regions of the Northern Hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability in MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal s ling, which motivates new recommendations for MLD computation in future model intercomparison projects.
Sensitivity of the Southern Ocean overturning circulation to surface buoyancy forcing
Publisher: American Geophysical Union (AGU)
Date: 07-2011
DOI: 10.1029/2011GL048031
Sea level changes forced by Southern Ocean winds
Publisher: American Geophysical Union (AGU)
Date: 05-11-2013
DOI: 10.1002/2013GL058104
Super Residual Circulation: a new perspective on ocean vertical heat transport
Publisher: American Meteorological Society
Date: 21-05-2021
Abstract: Ocean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities to diagnose circulation. Using quasi-steady state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature-depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of Super-Residual Transport (SRT), which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies, but excludes small-scale three dimensional turbulent mixing effect, to construct a new overturning circulation – the ‘Super Residual Circulation’ (SRC). We find that in the coarse resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11 Sv circulation which transports heat upward. The SRC’s upward heat transport is ~2 times larger in a finer horizontal resolution (0.1°) version of ACCESS, suggesting a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into super-residual processes, as the SRC elucidates the pathways in temperature and depth space along which watermass transformation occurs.
On the application of no-slip lateral boundary conditions to 'coarsely' resolved ocean models
Publisher: Elsevier BV
Date: 2011
Asymmetric Internal Tide Generation in the Presence of a Steady Flow
Publisher: American Geophysical Union (AGU)
Date: 10-2020
DOI: 10.1029/2020JC016503
Thank You to Our 2018 Peer Reviewers
Publisher: American Geophysical Union (AGU)
Date: 06-11-2019
DOI: 10.1029/2019GL084031
Abstract: On behalf of the journal, AGU, and the scientific community, the Editors would like to sincerely thank those who reviewed manuscripts for Geophysical Research Letters in 2018. The hours reading and commenting on manuscripts not only improves the manuscripts but also increases the scientific rigor of future research in the field. We particularly appreciate the timely reviews, in light of the demands imposed by the rapid review process at Geophysical Research Letters . With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics. In total, 4,484 referees contributed to 7,557 in idual reviews in journal. Thank you again. We look forward to the coming year of exciting advances in the field and communicating those advances to our community and to the broader public.
The Energetics of a Collapsing Meridional Overturning Circulation
Publisher: American Meteorological Society
Date: 07-2013
Abstract: A well-studied ex le of natural climate variability is the impact of large freshwater input to the polar oceans, simulating glacial melt release or an lification of the hydrological cycle. Such forcing can reduce, or entirely eliminate, the formation of deep water in the polar latitudes and thereby weaken the Atlantic meridional overturning circulation (MOC). This study uses a series of idealized, eddy-permitting numerical simulations to analyze the energetic constraints on the Atlantic Ocean's response to anomalous freshwater forcing. In this model, the changes in MOC are not correlated with the global input of mechanical energy: both kinetic energy and available potential energy (APE) increase with northern freshwater forcing, while the MOC decreases. However, a regional analysis of APE density supports the notion that local maxima in APE density control the response of the MOC to freshwater forcing perturbations. A coupling between APE input and changes in local density anomalies accounts for the difference in time scales between the recovery and collapse of the MOC.
The Ekman Streamfunction and the Eulerian and Residual Overturning Circulations of the Southern Ocean
Publisher: American Geophysical Union (AGU)
Date: 09-2021
DOI: 10.1029/2021GL093438
Abstract: The Ekman streamfunction is a wind‐derived metric that can be used to infer the Southern Ocean overturning circulations (SOOCs) in both latitude‐depth and latitude‐potential density spaces. The Ekman streamfunction integrates the Ekman pumping zonally and northwards from Antarctica, either to a given latitude or potential density. Here, we evaluate the relationship between the Ekman streamfunction and SOOCs in a global 0. ocean‐sea‐ice model driven by interannual forcing (1958–2018). In certain regions of the Southern Ocean, strong correlations ( ) exist between the Ekman streamfunction and the Eulerian and residual SOOCs on monthly and annual timescales. Regression analysis identifies regions where Ekman streamfunction variability coincides with Sv changes in the overturning one such location is where the wind stress curl changes sign and the Ekman pumping is highly variable.
Eddy Cancellation of the Ekman Cell in Subtropical Gyres
Publisher: American Meteorological Society
Date: 10-2016
Abstract: The presence of large-scale Ekman pumping associated with the climatological wind stress curl is the textbook explanation for low biological activity in the subtropical gyres. Using an idealized, eddy-resolving model, it is shown that Eulerian-mean Ekman pumping may be opposed by an eddy-driven circulation, analogous to the way in which the atmospheric Ferrel cell and the Southern Ocean Deacon cell are opposed by eddy-driven circulations. Lagrangian particle tracking, potential vorticity fluxes, and depth–density streamfunctions are used to show that, in the model, the rectified effect of eddies acts to largely cancel the Eulerian-mean Ekman downwelling. To distinguish this effect from eddy compensation, it is proposed that the suppression of Eulerian-mean downwelling by eddies be called “eddy cancellation.”
Multidecadal variability in the climate system: phenomena and mechanisms
Publisher: Springer Science and Business Media LLC
Date: 06-2020
DOI: 10.1140/EPJP/S13360-020-00515-4
Abstract: An open and fundamental issue in climate dynamics is the origin of multidecadal variability in the climate system. Resolving this issue is essential for adequate attribution of human-induced climate change. The purpose of this paper is to provide a perspective on multidecadal variability from the analysis of observations and results from model simulations. Data from the instrumental record indicate the existence of large-scale coherent patterns of multidecadal variability in sea surface temperature. Combined with long time series of proxy data, these results provide le evidence for the existence of multidecadal sea surface temperature variations. Results of a hierarchy of climate models have provided several mechanisms of this variability, ranging from pure atmospheric forcing, via internal ocean processes to coupled ocean-atmosphere interactions. An important problem is that current state-of-the-art climate models underestimate multidecadal variability. We argue that these models miss important processes in their representation of ocean eddies and focus on a robust mechanism of multidecadal variability which is found in multi-century simulations with climate models having a strongly eddying ocean component.
Thank You to Our 2019 Peer Reviewers
Publisher: American Geophysical Union (AGU)
Date: 02-09-2020
DOI: 10.1029/2020GL088048
Abstract: On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2019. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters . With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. With the advent of AGU's data policy, many reviewers have helped immensely to evaluate the accessibility and availability of data associated with the papers they have reviewed, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics.
The Ekman Streamfunction: a wind-derived metric to quantify the Southern Ocean overturning circulation
Publisher: Wiley
Date: 22-03-2021
Climatology, Seasonality, and Trends of Spatially Coherent Ocean Eddies
Publisher: American Geophysical Union (AGU)
Date: 07-2022
DOI: 10.1029/2021JC017453
Abstract: Ocean eddies influence regional and global climate by mixing and transporting heat, carbon, nutrients, and other properties. One of the most recognizable and ubiquitous features of oceanic variability are mesoscale eddies (vortices and meanders/jets) with spatial scales of tens to hundreds of kilometers. Vortices resemble Gaussian‐like features in sea surface height which locally affect near‐surface wind, cloud properties, and rainfall patterns we refer to these Gaussian‐like eddies as spatially coherent eddies. Although spatially coherent eddies are ubiquitous, their climatology, seasonality, and long‐term temporal evolution is yet to be explored. Here, we examine the kinetic energy (KE) contained in spatially coherent eddies and present their temporal variability using satellite observations between 1993 and 2020. Around half of the transient KE contained by eddies corresponds to spatially coherent eddies. A strong seasonal cycle is observed in the spatially coherent eddy field, and correlates with the wind forcing at time‐lags of 3–6 months. The seasonal correlation between wind forcing and spatially coherent eddies with diameters smaller than 120 km is faster (∼3 months), than that of coherent eddies with larger diameters (∼6 months). These lags between different spatially coherent eddy scales are consistent with the transfer of energy from small to large scales (inverse energy cascade). Our analysis highlights the relative importance of the spatially coherent eddy field in the KE budget, revealing a lagged response between the spatially coherent eddy and forcing at different time‐scales, and showcases the seasonality, and multidecadal trends of coherent eddy properties over the global ocean.
SUSTAINED MONITORING OF THE SOUTHERN OCEAN AT DRAKE PASSAGE: PAST ACHIEVEMENTS AND FUTURE PRIORITIES
Publisher: American Geophysical Union (AGU)
Date: 02-12-2011
DOI: 10.1029/2010RG000348
The Impact of Abyssal Hill Roughness on the Benthic Tide
Publisher: American Geophysical Union (AGU)
Date: 05-2021
DOI: 10.1029/2020MS002376
Abstract: The flow of tides over rough bathymetry in the deep ocean generates baroclinic motion including internal waves and bottom‐trapped tides. The stresses generated by this motion feedback on the litude and phase of the large‐scale tide. Quantifying the stresses associated with tidal flow over abyssal hills is especially important, as this scale of bathymetry is often unresolved in global baroclinic tide models, and the stresses must therefore be parameterized. Here, we extend the previous theoretical work of the authors to determine the litude, phasing, and vertical location of the stresses exerted on a flow driven by a time‐periodic body force when it encounters rough bathymetry. The theory compares favorably with a suite of fully nonlinear numerical simulations. It is shown that all topographic stresses are applied directly above the bathymetry, leading to a two‐layer baroclinic flow, with the near‐bottom spatial‐mean flow (the benthic tide) strongly modified by topographic stresses, and the flow at height unperturbed by the presence of topography. Our results provide a framework to improve baroclinic tide models by (i) providing a simple parameterization for the subinertial stress which is currently not included in any models, (ii) establishing that parameterized stresses should be applied in the diffusive boundary layer directly above the topography, independent of where internal tides may dissipate, and (iii) identifying a minimum resolution of ∼10 km for baroclinic tidal models to adequately capture wave resonance effects that can significantly impact the magnitude of the benthic tide.
The [simple carbon project] model v1.0
Publisher: Copernicus GmbH
Date: 18-07-2018
DOI: 10.5194/GMD-2018-176
Abstract: Abstract. We construct a carbon cycle box model to process observed or inferred geochemical evidence from modern and paleo settings. The [simple carbon project] model v1.0 ("SCP-M") combines a modern understanding of the ocean circulation regime with the earth's carbon cycle. SCP-M estimates the concentrations of a range of elements within the carbon cycle for use in paleo reconstructions or future projections, by simulating ocean circulation, biological, chemical and atmospheric and terrestrial carbon cycle processes. In this paper we demonstrate the model's application primarily with analysis of the Last Glacial Maximum (LGM) to Holocene carbon cycle transition, and also with the modern carbon cycle under the influence of anthropogenic emissions. The model is shown to be capable of reproducing both paleo and modern observations, and aligns with CMIP5 model projections. We conduct an atmospheric and ocean multi-proxy data-model parameter optimisation for the LGM and late Holocene periods, using the growing pool of published paleo atmosphere and ocean data for CO2, δ13C, Δ14C and carbonate ion proxy. The results provide strong evidence for an ocean-wide physical mechanism to deliver the LGM to Holocene carbon cycle transition. Alongside ancillary changes in ocean temperature, volume, salinity, sea ice cover and atmospheric radiocarbon production rate, changes in global overturning circulation, and, to a lesser extent Atlantic meridional overturning circulation, can drive the observed LGM and late Holocene signals in atmospheric CO2, δ13C, Δ14C, and the oceanic distribution of δ13C, Δ14C and carbonate ion proxy. Further work is needed on analysis and processing of ocean proxy data to improve confidence in these modelling results, but this preliminary use of SCP-M suggests that a solution to the LGM-Holocene dilemma is close at hand.
Jet Jumping: Low-Frequency Variability in the Southern Ocean
Publisher: American Meteorological Society
Date: 05-2013
Abstract: The authors study intrinsic variability in the position of jets in a β-plane channel ocean with simple topography using a quasigeostrophic numerical model. This study links the variability in jet position with abyssal anticyclones that form as a result of interaction of mesoscale eddies and subsurface topography, reminiscent of such flows as the Zapiola anticyclone. A simple dynamical framework explaining this behavior is developed. In this framework, this study shows that the topographic anticyclones form closed regions of homogenized yet time-varying potential vorticity. Neighboring topographic anticyclones are coupled by eddy fluxes. Interaction of a baroclinic jet with these two (or more) anticyclones can drive variability in local jet strength. Predictions of the dynamical framework are then compared with the results of the numerical model, and it is demonstrated that this model has merit in explaining the observed model variability. This study argues that this simple mode of variability has relevance for the ocean.
The viscous lee wave problem and its implications for ocean modelling
Publisher: Elsevier BV
Date: 05-2017
Projected changes of surface winds over the Antarctic continental margin
Publisher: Wiley
Date: 29-03-2022
On Energy Cascades in General Flows: A Lagrangian Application
Publisher: American Geophysical Union (AGU)
Date: 12-2020
DOI: 10.1029/2020MS002090
Abstract: An important characteristic of geophysically turbulent flows is the transfer of energy between scales. Balanced flows pass energy from smaller to larger scales as part of the well‐known upscale cascade, while submesoscale and smaller scale flows can transfer energy eventually to smaller, dissipative scales. Much effort has been put into quantifying these transfers, but a complicating factor in realistic settings is that the underlying flows are often strongly spatially heterogeneous and anisotropic. Furthermore, the flows may be embedded in irregularly shaped domains that can be multiply connected. As a result, straightforward approaches like computing Fourier spatial spectra of nonlinear terms suffer from a number of conceptual issues. In this paper, we develop a method to compute cross‐scale energy transfers in general settings, allowing for arbitrary flow structure, anisotropy, and inhomogeneity. We employ Green's function approach to the kinetic energy equation to relate kinetic energy at a point to its Lagrangian history. A spatial filtering of the resulting equation naturally decomposes kinetic energy into length‐scale‐dependent contributions and describes how the transfer of energy between those scalestakes place. The method is applied to a doubly periodic simulation of vortex merger, resulting in the demonstration of the expected upscale energy cascade. Somewhat novel results are that the energy transfers are dominated by pressure work, rather than kinetic energy exchange, and dissipation is a noticeable influence on the larger scale energy budgets. We also describe, but do not employ here, a technique for developing filters to use in complex domains.
Weddell Sea control of ocean temperature variability on the western Antarctic Peninsula
Publisher: Authorea, Inc.
Date: 09-02-2023
DOI: 10.22541/ESSOAR.167591098.80596021/V1
Abstract: Recent ice loss on the western Antarctic Peninsula has been driven by warming ocean waters on the continental shelf. However, due to the short observational record, our understanding of the dynamics and variability in this region remains poor. High-resolution ocean model simulations show that the temperature variability along the western Antarctic Peninsula is controlled by the rate of dense water formation in the Weddell Sea. Passive tracer advection reveals connectivity between the Weddell Sea and the coastline of the western Antarctic Peninsula and Bellingshausen Sea. During multi-year periods of weak Weddell dense water formation, dense overflow transport in the Weddell Sea decreases, while the transport of cold water around the tip of the Antarctic Peninsula strengthens, driving a temperature decrease of 0.4C along the western Antarctic Peninsula. This mechanism implies that western Antarctic Peninsula coastal ocean temperature may cool in the future if Weddell Dense Shelf Water production slows down.
The Moist Quasi-Geostrophic Coupled Model: MQ-GCM 2.0
Publisher: Copernicus GmbH
Date: 07-10-2022
Abstract: Abstract. This paper contains a description of recent changes to the formulation and numerical implementation of the Quasi-Geostrophic Coupled Model (Q-GCM), which constitute a major update of the previous version of the model (Hogg et al., 2014). The Q-GCM model has been designed to provide an efficient numerical tool to study the dynamics of multi-scale midlatitude air–sea interactions and their climatic impacts. The present additions/alterations were motivated by an inquiry into the dynamics of mesoscale ocean–atmosphere coupling and, in particular, by an apparent lack of the Q-GCM atmosphere's sensitivity to mesoscale sea-surface temperature (SST) anomalies, even at high (mesoscale) atmospheric resolutions, contrary to le theoretical and observational evidence otherwise. Major modifications aimed at alleviating this problem include an improved radiative-convective scheme resulting in a more realistic model mean state and associated model parameters a new formulation of entrainment in the atmosphere, which prompts more efficient communication between the atmospheric mixed layer and free troposphere and an addition of a temperature-dependent wind component in the atmospheric mixed layer and the resulting mesoscale feedbacks. The most drastic change is, however, the inclusion of moist dynamics in the model, which may be key to midlatitude ocean–atmosphere coupling. Accordingly, this version of the model is to be referred to as the MQ-GCM model. Overall, the MQ-GCM model is shown to exhibit a rich spectrum of behaviors reminiscent of many of the observed properties of the Earth's climate system. It remains to be seen whether the added processes are able to affect in fundamental ways the simulated dynamics of the midlatitude ocean–atmosphere system's coupled decadal variability.
Aftermath of catastrophic flooding of a desiccated ocean basin
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-9593
Abstract: & & & strong& .33 Million years ago, a mile-high marine cascade terminated the Messinian Salinity Crisis& /strong& & strong& due to partial collapse of the Gibraltar arc/sill that isolated a largely desiccated Mediterranean from the Atlantic Ocean. Atlantic waters may have refilled the basin within 2 years& /strong& & strong& . Prevailing hypotheses suggest that normal marine conditions were established across the Mediterranean immediately after the catastrophic flooding& /strong& & strong& . Here we use new proxy data and modelling to show that normal conditions were likely for the western Mediterranean (wMed), but that flooding caused massive wMed salt transfer to the eastern Mediterranean (eMed), which became a hyper-salinity-stratified basin. Hyper-stratification inhibited deep-water ventilation, causing anomalously long-lasting organic-rich (sapropel) sediment deposition. Model:data agreement indicates that hyper-stratification breakdown required 26,000 years. Testing an alternative hypothesis& #8212 reconnection of a largely refilled Mediterranean& /strong& & strong& & #8212 reveals hyper-stratification in both the wMed and eMed, which would have left sapropels in both basins, in disagreement with observations. Our findings offer novel insight into the processes involved in re-establishing normal marine conditions following abrupt refilling of a previously desiccated ocean basin.& /strong& & &
Seasonal and interannual variability of the Subtropical Front in the New Zealand region
Publisher: Wiley
Date: 19-05-2020
Baroclinic control of Southern Ocean eddy upwelling near topography
Publisher: Wiley
Date: 24-12-2021
Sensitivity of the Overturning Circulation in the Southern Ocean to Decadal Changes in Wind Forcing
Publisher: American Meteorological Society
Date: 2012
Abstract: The sensitivity of the overturning circulation in the Southern Ocean to the recent decadal strengthening of the overlying winds is being discussed intensely, with some works attributing an inferred saturation of the Southern Ocean CO2 sink to an intensification of the overturning circulation, while others have argued that this circulation is insensitive to changes in winds. Fundamental to reconciling these erse views is to understand properly the role of eddies in counteracting the directly wind-forced changes in overturning. Here, the authors use novel theoretical considerations and fine-resolution ocean models to develop a new scaling for the sensitivity of eddy-induced mixing to changes in winds, and they demonstrate that changes in Southern Ocean overturning in response to recent and future changes in wind stress forcing are likely to be substantial, even in the presence of a decadally varying eddy field. This result has significant implications for the ocean’s role in the carbon cycle, and hence global climate.
Unidirectional stratified flow through a non-rectangular channel
Publisher: Cambridge University Press (CUP)
Date: 25-06-2004
Ice Algae Model Intercomparison Project phase 2 (IAMIP2)
Publisher: Copernicus GmbH
Date: 17-11-2020
DOI: 10.5194/GMD-2020-305
Abstract: Abstract. Ice algae play a fundamental role in shaping polar marine ecosystems and biogeochemistry. This role can be investigated by field observations, however the influence of ice algae at the regional and global scales remains unclear due to limited spatial and temporal coverage of observations, and because ice algae are typically not included in current Earth System Models. To address this knowledge gap, we introduce a new model intercomparison project (MIP), referred to here as the Ice Algae Model Intercomparison Project phase 2 (IAMIP2). IAMIP2 is built upon the experience from its previous phase, and expands its scope to global coverage (both Arctic and Antarctic) and centennial timescales (spanning the mid-twentieth century to the end of the twenty-first century). Participating models are three-dimensional regional and global coupled sea ice–ocean models that incorporate sea-ice ecosystem components. These models are driven by the same initial conditions and atmospheric forcing datasets by incorporating and expanding the protocols of the Ocean Model Intercomparison Project, an endorsed MIP of the Coupled Model Intercomparison Project phase 6 (CMIP6). Doing so provides more robust estimates of model bias and uncertainty, and consequently advances the science of polar marine ecosystems and biogeochemistry. A diagnostic protocol is designed to enhance the reusability of the model data products of IAMIP2. Lastly, the limitations and strengths of IAMIP2 are discussed in the context of prospective research outcomes.
Frequency-Domain Analysis of the Energy Budget in an Idealized Coupled Ocean–Atmosphere Model
Publisher: American Meteorological Society
Date: 15-01-2020
Abstract: Climate variability is investigated by identifying the energy sources and sinks in an idealized, coupled, ocean–atmosphere model, tuned to mimic the North Atlantic region. The spectral energy budget is calculated in the frequency domain to determine the processes that either deposit energy into or extract energy from each fluid, over time scales from one day up to 100 years. Nonlinear advection of kinetic energy is found to be the dominant source of low-frequency variability in both the ocean and the atmosphere, albeit in differing layers in each fluid. To understand the spatial patterns of the spectral energy budget, spatial maps of certain terms in the spectral energy budget are plotted, averaged over various frequency bands. These maps reveal three dynamically distinct regions: along the western boundary, the western boundary current separation, and the remainder of the domain. The western boundary current separation is found to be a preferred region to energize oceanic variability across a broad range of time scales (from monthly to decadal), while the western boundary itself acts as the dominant sink of energy in the domain at time scales longer than 50 days. This study paves the way for future work, using the same spectral methods, to address the question of forced versus intrinsic variability in a coupled climate system.
Thank You to Our 2020 Peer Reviewers
Publisher: American Geophysical Union (AGU)
Date: 06-05-2021
DOI: 10.1029/2021GL093126
ACCESS-OM2 v1.0: a global ocean–sea ice model at three resolutions
Publisher: Copernicus GmbH
Date: 05-02-2020
Abstract: Abstract. We introduce ACCESS-OM2, a new version of the ocean–sea ice model of the Australian Community Climate and Earth System Simulator. ACCESS-OM2 is driven by a prescribed atmosphere (JRA55-do) but has been designed to form the ocean–sea ice component of the fully coupled (atmosphere–land–ocean–sea ice) ACCESS-CM2 model. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1∘ horizontal grid spacing), an eddy-permitting resolution (nominally 0.25∘), and an eddy-rich resolution (0.1∘ with 75 vertical levels) the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow for testing at lower resolutions and to permit comparison across resolutions. In this paper, the model is introduced and the in idual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean–sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents, and the abyssal overturning cell but that there is scope for improvements in sub-grid-scale parameterizations at the highest resolution.
The role of vertical eddy flux in Southern Ocean heat uptake
Publisher: American Geophysical Union (AGU)
Date: 17-10-2013
DOI: 10.1002/2013GL057706
Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes
Publisher: American Meteorological Society
Date: 31-12-2016
Abstract: The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S. Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses.
Seasonal and Interannual Variability of the Weddell Gyre From a High‐Resolution Global Ocean‐Sea Ice Simulation During 1958–2018
Publisher: American Geophysical Union (AGU)
Date: 30-10-2021
DOI: 10.1029/2021JC017662
Abstract: The Weddell Gyre's variability on seasonal and interannual timescales is investigated using an ocean‐sea ice model at three different horizontal resolutions. The model is evaluated against available observations to demonstrate that the highest resolution configuration (0. in the horizontal) best reproduces observed features of the region. The simulations suggest that the gyre is subject to large variability in its circulation that is not captured by summer‐biased or short‐term observations. The Weddell Gyre's seasonal cycle consists of a summer minimum and a winter maximum and accounts for changes that are between one third and a half of its mean transport. On interannual time scales we find that the gyre's strength is correlated with the local Antarctic easterlies and that extreme events of gyre circulation are associated with changes in the characteristics of the warm inflow at the eastern boundary, that in turn drives changes in sea ice concentration.
Ventilation of the Southern Ocean Pycnocline
Publisher: Annual Reviews
Date: 03-01-2022
DOI: 10.1146/ANNUREV-MARINE-010419-011012
Abstract: Ocean ventilation is the transfer of tracers and young water from the surface down into the ocean interior. The tracers that can be transported to depth include anthropogenic heat and carbon, both of which are critical to understanding future climate trajectories. Ventilation occurs in both high- and midlatitude regions, but it is the southern midlatitudes that are responsible for the largest fraction of anthropogenic heat and carbon uptake such Southern Ocean ventilation is the focus of this review. Southern Ocean ventilation occurs through a chain of interconnected mechanisms, including the zonally averaged meridional overturning circulation, localized subduction, eddy-driven mixing along isopycnals, and lateral transport by subtropical gyres. To unravel the complex pathways of ventilation and reconcile conflicting results, here we assess the relative contribution of each of thesemechanisms, emphasizing the three-dimensional and temporally varying nature of the ventilation of the Southern Ocean pycnocline. We conclude that Southern Ocean ventilation depends on multiple processes and that simplified frameworks that explain ventilation changes through a single process are insufficient.
Vertical resolution of baroclinic modes in global ocean models
Publisher: Elsevier BV
Date: 05-2017
Response of Southern Ocean Ventilation to Changes in Midlatitude Westerly Winds
Publisher: American Meteorological Society
Date: 09-2019
Abstract: Changes in ventilation of the Southern Hemisphere oceans in response to changes in midlatitude westerly winds are examined by analyzing the ideal age tracer from global eddy-permitting ocean–ice model simulations in which there is an abrupt increase and/or a meridional shift in the winds. The age response in mode and intermediate waters is found to be close to linear the response of a combined increase and shift of peak winds is similar to the sum of the in idual responses to an increase and a shift. Further, a barotropic response, following Sverdrup balance, can explain much of the age response to the changes in wind stress. There are similar peak decreases (of around 50 years) in the ideal age for a 40% increase or 2.5° poleward shift in the wind stress. However, while the age decreases throughout the thermocline for an increase in the winds, for a poleward shift in the winds the age increases in the north part of the thermocline and there are decreases in age only south of 35°S. As a consequence, the change in the volume of young water differs, with a 15% increase in the volume of water with ages younger than 50 years for a 40% increase in the winds but essentially no change in this volume for a 2.5° shift. As ventilation plays a critical role in the uptake of carbon and heat, these results suggest that the storage of anthropogenic carbon and heat in mode and intermediate waters will likely increase with a strengthening of the winds, but will be much less sensitive to a meridional shift in the peak wind stress.
The geography of numerical mixing in a suite of global ocean models
Publisher: Wiley
Date: 29-09-2020
The Energetics of Southern Ocean Upwelling
Publisher: American Meteorological Society
Date: 2017
Abstract: The ocean’s meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean–sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of available potential energy in the Southern Hemisphere, associated with stronger upwelling of deep water. Poleward shifts in the Southern Ocean westerlies lead to a complicated transient response, with the formation of bottom water induced by increased polynya activity in the Weddell Sea and a weakening of the upper overturning cell in the Northern Hemisphere. The energetic consequences of the upper overturning cell response indicate an interhemispheric connection to the input of available potential energy in the Northern Hemisphere.
A near-global climatology of oceanic coherent eddies
Publisher: Wiley
Date: 23-04-2021
Lagrangian Filtering: A novel method for separating internal waves from non-wave flows in high-resolution simulations
Publisher: Wiley
Date: 12-05-2021
Circumpolar variations in the chaotic nature of Southern Ocean eddy dynamics
Publisher: Wiley
Date: 17-01-2022
How Does Antarctic Bottom Water Cross the Southern Ocean?
Publisher: American Geophysical Union (AGU)
Date: 09-04-2022
DOI: 10.1029/2021GL097211
Abstract: Antarctic Bottom Water (AABW), which fills the global ocean abyss, is derived from dense water that forms in several distinct Antarctic shelf regions. Previous modeling studies have reached conflicting conclusions regarding export pathways of AABW across the Southern Ocean and the degree to which AABW originating from distinct source regions are blended during their export. This study addresses these questions using passive tracer deployments in a 61‐year global high‐resolution (0.1°) ocean/sea‐ice simulation. Two distinct export “conduits” are identified: Weddell Sea‐ and Prydz Bay‐sourced AABW are blended together and exported mainly to the Atlantic and Indian Oceans, while Ross Sea‐ and Adelie Land‐sourced AABW are exported mainly to the Pacific Ocean. Northward transport of each tracer occurs almost exclusively ( %) within a single conduit. These findings imply that regional changes in AABW production may impact the three‐dimensional structure of the global overturning circulation.
Eddy saturation of the Southern Ocean: a baroclinic versus barotropic perspective
Publisher: Wiley
Date: 11-02-2020
The Impact of Turbulence and Convection on Transport in the Southern Ocean
Publisher: American Geophysical Union (AGU)
Date: 06-2019
DOI: 10.1029/2018JC014883
The Dynamics of Southern Ocean Storm Tracks
Publisher: American Meteorological Society
Date: 03-2015
Abstract: The mechanisms that initiate and maintain oceanic “storm tracks” (regions of anomalously high eddy kinetic energy) are studied in a wind-driven, isopycnal, primitive equation model with idealized bottom topography. Storm tracks are found downstream of the topography in regions strongly influenced by a large-scale stationary meander that is generated by the interaction between the background mean flow and the topography. In oceanic storm tracks the length scale of the stationary meander differs from that of the transient eddies, a point of distinction from the atmospheric storm tracks. When the zonal length and height of the topography are varied, the storm-track intensity is largely unchanged and the downstream storm-track length varies only weakly. The dynamics of the storm track in this idealized configuration are investigated using a wave activity flux (related to the Eliassen–Palm flux and eddy energy budgets). It is found that vertical fluxes of wave activity (which correspond to eddy growth by baroclinic conversion) are localized to the region influenced by the standing meander. Farther downstream, organized horizontal wave activity fluxes (which indicate eddy energy fluxes) are found. A mechanism for the development of oceanic storm tracks is proposed: the standing meander initiates localized conversion of energy from the mean field to the eddy field, while the storm track develops downstream of the initial baroclinic growth through the ageostrophic flux of Montgomery potential. Finally, the implications of this analysis for the parameterization and prediction of storm tracks in ocean models are discussed.
The Injection of Zonal Momentum by Buoyancy Forcing in a Southern Ocean Model
Publisher: American Meteorological Society
Date: 2015
Abstract: An overturning circulation, driven by prescribed buoyancy forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and buoyancy forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing. The zonal momentum budget integrated over each layer is considered in the buoyancy-forced, wind-forced, and combined forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the buoyancy-forced case, the forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom friction to act as the initial sources of eastward momentum, with bottom friction acting over a longer time frame. In the wind-forced and combined forcing cases, the surface wind stress dominates the initial momentum budget, and the time to reach equilibration is shorter in the combined forcing simulation. These results imply that future changes in the rate of formation of Antarctic Bottom Water may alter the volume transport of the Antarctic Circumpolar Current.
Jet–Topography Interactions Affect Energy Pathways to the Deep Southern Ocean
Publisher: American Meteorological Society
Date: 07-2017
Abstract: In the Southern Ocean, strong eastward ocean jets interact with large topographic features, generating eddies that feed back onto the mean flow. Deep-reaching eddies interact with topography, where turbulent dissipation and generation of internal lee waves play an important role in the ocean’s energy budget. However, eddy effects in the deep ocean are difficult to observe and poorly characterized. This study investigates the energy contained in eddies at depth, when an ocean jet encounters topography. This study uses a two-layer ocean model in which an imposed unstable jet encounters a topographic obstacle (either a seamount or a meridional ridge) in a configuration relevant to an Antarctic Circumpolar Current frontal jet. The authors find that the presence of topography increases the eddy kinetic energy (EKE) at depth but that the dominant processes generating this deep EKE depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography. In cases with high topography, horizontal shear instability is the dominant source of deep EKE, while a flat bottom or a strongly sheared inflow leads to deep EKE being generated primarily through baroclinic instability. These results suggest that the deep EKE is set by an interplay between the inflowing jet properties and topography and imply that the response of deep EKE to changes in the Southern Ocean circulation is likely to vary across locations depending on the topography characteristics.
A Moist Quasi-Geostrophic Coupled Model: MQ-GCM2.0
Publisher: Copernicus GmbH
Date: 18-06-2021
DOI: 10.5194/GMD-2021-160
Abstract: Abstract. This paper contains a description of recent changes to the formulation and numerical implementation of the Quasi-Geostrophic Coupled Model (Q-GCM), which constitute a major update of the previous version of the model (Hogg et al., 2014). The Q-GCM model has been designed to provide an efficient numerical tool to study the dynamics of multi-scale mid-latitude air–sea interactions and their climatic impacts. The present additions/alterations were motivated by an inquiry into the dynamics of mesoscale ocean–atmosphere coupling and, in particular, by an apparent lack of Q-GCM atmosphere’s sensitivity to mesoscale sea-surface temperature (SST) anomalies, even at high (mesoscale) atmospheric resolutions, contrary to le theoretical and observational evidence otherwise. Major modifications aimed at alleviating this problem include an improved radiative-convective scheme resulting in a more realistic model mean state and associated model parameters, a new formulation of entrainment in the atmosphere, which prompts more efficient communication between the atmospheric mixed layer and free troposphere, as well as an addition of temperature-dependent wind component in the atmospheric mixed layer and the resulting mesoscale feedbacks. The most drastic change is, however, the inclusion of moist dynamics in the model, which may be key to midlatitude ocean–atmosphere coupling. Accordingly, this version of the model is to be referred to as the MQ-GCM model. Overall, the MQ-GCM model is shown to exhibit a rich spectrum of behaviours reminiscent of many of the observed properties of the Earth’s climate system. It remains to be seen whether the added processes are able to affect in fundamental ways the simulated dynamics of the mid-latitude ocean–atmosphere system’s coupled decadal variability.
Ocean Gyres Driven by Surface Buoyancy Forcing
Publisher: American Geophysical Union (AGU)
Date: 11-08-2020
DOI: 10.1029/2020GL088539
Abstract: Midlatitude gyres in the ocean are large‐scale horizontal circulations that are intensified on the western boundary of the ocean, giving rise to currents such as the Gulf Stream. The physical mechanism underlying gyres is widely recognized to involve the curl of the wind stress, which injects potential vorticity into the upper ocean. However, model results have highlighted the role of surface buoyancy fluxes (principally heating and cooling of the ocean surface) in driving circulation and enhancing gyre variability. Here we present two numerical simulations—one in the fully turbulent regime and the second an eddy‐permitting ocean model—which show that gyre‐like circulation can be driven by surface buoyancy fluxes alone. We explore this phenomenon through a combination of modeling and linear theory to highlight that the transport of ocean gyres depends upon surface buoyancy fluxes as well as wind stress.
Convection Enhances Mixing in the Southern Ocean
Publisher: American Geophysical Union (AGU)
Date: 11-05-2018
DOI: 10.1029/2018GL077711
Sequential changes in ocean circulation and biological export productivity during the last glacial–interglacial cycle: a model–data study
Publisher: Copernicus GmbH
Date: 15-01-2021
Abstract: Abstract. We conduct a model–data analysis of the marine carbon cycle to understand and quantify the drivers of atmospheric CO2 concentration during the last glacial–interglacial cycle. We use a carbon cycle box model, “SCP-M”, combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and Southern Ocean biological export productivity across marine isotope stages spanning 130 000 years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea-ice cover and shallow-water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimized in each marine isotope stage against proxy data for atmospheric CO2, δ13C and Δ14C and deep-ocean δ13C, Δ14C and CO32-. Our model–data results suggest that global overturning circulation weakened during Marine Isotope Stage 5d, coincident with a ∼ 25 ppm fall in atmospheric CO2 from the last interglacial period. There was a transient slowdown in Atlantic meridional overturning circulation during Marine Isotope Stage 5b, followed by a more pronounced slowdown and enhanced Southern Ocean biological export productivity during Marine Isotope Stage 4 (∼ −30 ppm). In this model, the Last Glacial Maximum was characterized by relatively weak global ocean and Atlantic meridional overturning circulation and increased Southern Ocean biological export productivity (∼ −20 ppm during MIS 3 and MIS 2). Ocean circulation and Southern Ocean biological export productivity returned to modern values by the Holocene period. The terrestrial biosphere decreased by 385 Pg C in the lead-up to the Last Glacial Maximum, followed by a period of intense regrowth during the last glacial termination and the Holocene (∼ 600 Pg C). Slowing ocean circulation, a colder ocean and to a lesser extent shallow carbonate dissolution contributed ∼ −70 ppm to atmospheric CO2 in the ∼ 100 000-year lead-up to the Last Glacial Maximum, with a further ∼ −15 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological export productivity was one of the ingredients required to achieve the Last Glacial Maximum atmospheric CO2 level. We find that the incorporation of glacial–interglacial proxy data into a simple quantitative ocean transport model provides useful insights into the timing of past changes in ocean processes, enhancing our understanding of the carbon cycle during the last glacial–interglacial period.
Topographic influence on submesoscale dynamics in the Southern Ocean
Publisher: American Geophysical Union (AGU)
Date: 24-02-2015
DOI: 10.1002/2014GL062720
The geography of numerical mixing in a suite of global ocean models
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-9319
Abstract: & & Numerical mixing, the physically spurious diffusion of tracers due to the numerical discretization of advection, is known to contribute to biases in ocean circulation models. However, quantifying numerical mixing is non-trivial, with most studies utilizing specifically targeted experiments in idealized settings. Here, we present a precise method based on water-mass transformation for quantifying numerical mixing, including its spatial structure, that can be applied to any conserved variable in global general circulation ocean models. The method is applied to a suite of global MOM5 ocean-sea ice model simulations with differing grid spacings and sub-grid scale parameterizations. In all configurations numerical mixing drives across-isotherm heat transport of comparable magnitude to that associated with explicitly-parameterized mixing. Numerical mixing is prominent at warm temperatures in the tropical thermocline, where it is sensitive to the vertical diffusivity and resolution. At colder temperatures, numerical mixing is sensitive to the presence of explicit neutral diffusion, suggesting that much of the numerical mixing in these regions acts as a proxy for neutral diffusion when it is explicitly absent. Comparison of equivalent (with respect to vertical resolution and explicit mixing parameters) 1/4-degree and 1/10-degree horizontal resolution configurations shows only a modest enhancement in numerical mixing at the eddy-permitting 1/4-degree resolution. Our results provide a detailed view of numerical mixing in ocean models and pave the way for future improvements in numerical methods.& &
The wineglass effect shapes particle export to the deep ocean in mesoscale eddies
Publisher: American Geophysical Union (AGU)
Date: 19-09-2016
DOI: 10.1002/2015GL066463
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End Date: 12-2018
Amount: $3,000,000.00
Funder: Australian Research Council
View Funded ActivityLinkage Infrastructure, Equipment And Facilities - Grant ID: LE150100089
Start Date: 2015
End Date: 12-2015
Amount: $490,000.00
Funder: Australian Research Council
View Funded Activity