ORCID Profile
0000-0001-7692-2798
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Physical Oceanography | Glaciology | Climate Change Processes | Oceanography
Effects of Climate Change and Variability on Antarctic and Sub-Antarctic Environments (excl. Social Impacts) | Expanding Knowledge in the Environmental Sciences | Expanding Knowledge in the Earth Sciences |
Publisher: Wiley
Date: 02-10-2021
Publisher: American Geophysical Union (AGU)
Date: 02-11-2021
DOI: 10.1029/2021GL094871
Abstract: The expansion of Antarctic sea ice since 1979 in the presence of increasing greenhouse gases remains one of the most puzzling features of current climate change. Some studies have proposed that the formation of the ozone hole, via the Southern Annular Mode, might explain that expansion, and a recent paper highlighted a robust causal link between summertime Southern Annular Mode (SAM) anomalies and sea ice anomalies in the subsequent autumn. Here we show that many models are able to capture this relationship between the SAM and sea ice, but also emphasize that the SAM only explains a small fraction of the year‐to‐year variability. Finally, examining multidecadal trends, in models and in observations, we confirm the findings of several previous studies and conclude that the SAM–and thus the ozone hole–are not the primary drivers of the sea ice expansion around Antarctica in recent decades.
Publisher: Wiley
Date: 25-03-2021
Publisher: Copernicus GmbH
Date: 26-04-2019
Abstract: Abstract. Transport from the Northern Hemisphere (NH) midlatitudes to the Arctic plays a crucial role in determining the abundance of trace gases and aerosols that are important to Arctic climate via impacts on radiation and chemistry. Here we examine this transport using an idealized tracer with a fixed lifetime and predominantly midlatitude land-based sources in models participating in the Chemistry Climate Model Initiative (CCMI). We show that there is a 25 %–45 % difference in the Arctic concentrations of this tracer among the models. This spread is correlated with the spread in the location of the Pacific jet, as well as the spread in the location of the Hadley Cell (HC) edge, which varies consistently with jet latitude. Our results suggest that it is likely that the HC-related zonal-mean meridional transport rather than the jet-related eddy mixing is the major contributor to the inter-model spread in the transport of land-based tracers into the Arctic. Specifically, in models with a more northern jet, the HC generally extends further north and the tracer source region is mostly covered by surface southward flow associated with the lower branch of the HC, resulting in less efficient transport poleward to the Arctic. During boreal summer, there are poleward biases in jet location in free-running models, and these models likely underestimate the rate of transport into the Arctic. Models using specified dynamics do not have biases in the jet location, but do have biases in the surface meridional flow, which may result in differences in transport into the Arctic. In addition to the land-based tracer, the midlatitude-to-Arctic transport is further examined by another idealized tracer with zonally uniform sources. With equal sources from both land and ocean, the inter-model spread of this zonally uniform tracer is more related to variations in parameterized convection over oceans rather than variations in HC extent, particularly during boreal winter. This suggests that transport of land-based and oceanic tracers or aerosols towards the Arctic differs in pathways and therefore their corresponding inter-model variabilities result from different physical processes.
Publisher: Wiley
Date: 29-04-2021
Publisher: American Geophysical Union (AGU)
Date: 13-08-2019
DOI: 10.1029/2018JD030195
Publisher: Wiley
Date: 31-08-2020
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-12282
Abstract: & & The Southern Ocean is one of today's largest sink of carbon, having absorbed about 10\\% of the anthropogenic carbon emissions. Southern Ocean's dynamics are principally modulated by the strength of the Southern Hemispheric westerlies, & which are projected to increase over the coming century. Here, using a high-resolution ocean-sea-ice-carbon cycle model, we explore the impact of idealized changes in Southern Hemispheric westerlies on the ocean carbon storage . We find that a 20\\% strengthening of the Southern Hemispheric westerlies leads to a $\\sim$25 Gt loss of natural carbon, while an additional 13 Gt of anthropogenic carbon is absorbed compared to the control run, thus resulting in a net loss of $\\sim$12 GtC from the ocean over a period of 42 years. This tendency is enhanced if the westerlies are also shifted polewards, with a total natural carbon loss of almost 37 GtC, and an additional anthropogenic carbon uptake of 18 GtC. While both experiments display a large natural carbon loss south of 10$^\\circ$S, the litude is three times greater in the poleward strengthening case, which is & not fully compensated by the increase in anthropogenic carbon content. However, the poleward wind shift leads to significant differences in the pattern of DIC change due to a weakening of the upper overturning cell, & which leads to an increase in natural and total carbon north of 35$^\\circ$S in the upper 2000 m.& &
Publisher: Copernicus GmbH
Date: 07-03-2023
DOI: 10.5194/EGUSPHERE-2023-390
Abstract: Abstract. While the Southern Ocean (SO) provides the largest oceanic sink of carbon, some observational studies have suggested that the total SO CO2 uptake exhibited large (~0.3 GtC/yr) decadal-scale variability over the last 30 years, with a similar SO CO2 uptake in 2016 than in the early 1990s. Here, using an eddy-rich ocean, sea-ice, carbon cycle model, with a nominal resolution of 1/10th degree, we explore the changes in total, natural and anthropogenic CO2 fluxes in the Southern Ocean over the period 1970–2021 and the processes leading to the CO2 flux variability. Over that period, the simulated total CO2 uptake increases by 0.5 GtC/yr, half of which occurs between 1970 and 1982. The simulated total CO2 flux exhibits decadal-scale variability with an litude of ~0.1 GtC/yr in phase with observations and with variability in the Southern Annular Mode (SAM). Notably, a stagnation of the total CO2 uptake is simulated between 1982 and 2000, while a re-invigoration is simulated between 2000 and 2012. This decadal-scale variability results from enhanced outgassing of natural CO2 south of the sub-Antarctic front due to the strengthening and poleward shift of the southern hemispheric (SH) westerlies. These wind changes also lead to enhanced anthropogenic CO2 uptake south of the polar front, even though the correlation is low and the litude 75 % smaller than for natural CO2 changes. The total SO CO2 uptake capability thus reduced since 1970 in response to a shift towards positive phases of the SAM. Both the multi-decadal and annual changes in SO fluxes can be mostly explained by variations in surface dissolved inorganic carbon (DIC) brought about by a combination of Ekman-driven vertical advection and DIC diffusion at the base of the mixed layer, thus indicating that even in an eddy-rich ocean model a strengthening and/or poleward shift of the southern hemispheric westerlies enhance CO2 outgassing. The projected poleward strengthening of the SH westerlies over the coming century will thus reduce the capability of the SO to mitigate the increase in atmospheric CO2.
Publisher: Wiley
Date: 28-06-2020
Publisher: Wiley
Date: 29-06-2020
Publisher: Copernicus GmbH
Date: 30-08-2018
DOI: 10.5194/ACP-2018-841
Abstract: Abstract. Transport from the Northern Hemisphere (NH) midlatitudes to the Arctic plays a crucial role in determining the abundance of trace gases and aerosols that are important to Arctic climate via impacts on radiation and chemistry. Here we examine this transport using an idealized tracer with fixed lifetime and predominantly midlatitude land-based sources in models participating in the Chemistry Climate Model Initiative (CCMI). We show that there is a 20 %–40 % difference in the Arctic concentrations of this tracer among the models. This spread is found to be generally related to the spread in location of the Pacific jet, with lower Arctic tracer concentrations occurring in models with a more northern jet, during both winter and summer. However, the underlying mechanism for this relationship does not involve the jet directly, but instead involves differences in the surface meridional flow over the tracer source region, that vary with jet latitude. Specifically, in models with a more northern jet, the Hadley Cell (HC) generally extends further north and the tracer source region is mostly covered by surface southward flow associated with the lower branch of the HC, resulting in less efficient transport poleward to the Arctic. During boreal summer, there are poleward biases in jet location in free-running models, and these models likely underestimate the rate of transport into the Arctic. Models using specified dynamics do not have biases in the jet location, but do have biases in the surface meridional flow, which results in differences in the transport into the Arctic. In addition to the land-based tracer, the midlatitude-to-Arctic transport is further examined by another idealized tracer with zonally uniform sources. With equal sources from lands and oceans, the intermodel spread of this zonally uniform tracer is more related to variations of parameterized convection over oceans than variations of HC extent particularly during boreal summer. This suggests that transport of land-based and oceanic tracers or aerosols towards the Arctic differ in pathways and therefore their corresponding intermodel variabilities result from different physical processes.
Publisher: American Meteorological Society
Date: 07-2012
Abstract: The relationship between two commonly used diagnostics of stirring in ocean and atmospheric flows, the finite-time Lyapunov exponents λ and relative dispersion R2, is examined for a simple uniform strain flow and ocean flow inferred from altimetry. Although both diagnostics are based on the separation of initially close particles, the two diagnostics measure different aspects of the flow and, in general, there is not a one-to-one relationship between the diagnostics. For a two-dimensional flow with time-independent uniform strain, there is a single time-independent λ, but there is a wide range of values of R2 for in idual particle pairs. However, it is shown that the upper and lower limits of R2 for in idual pairs, the mean value over a large ensemble of pairs, and the probability distribution function (PDF) of R2 have simple relationships with λ. Furthermore, these analytical expressions provide a reasonable approximation for the R2–λ relationship in the surface ocean flow based on geostrophic velocities derived from satellite altimeter measurements. In particular, the bimodal distribution, upper and lower bounds, and mean values from the ocean flow are similar to the analytical expressions for a uniform strain flow. How well, as well as over what integration time scale, this holds depends on the spatial and temporal variations within the ocean region being considered.
Publisher: Copernicus GmbH
Date: 25-05-2018
Abstract: Abstract. Understanding and modeling the large-scale transport of trace gases and aerosols is important for interpreting past (and projecting future) changes in atmospheric composition. Here we show that there are large differences in the global-scale atmospheric transport properties among the models participating in the IGAC SPARC Chemistry–Climate Model Initiative (CCMI). Specifically, we find up to 40 % differences in the transport timescales connecting the Northern Hemisphere (NH) midlatitude surface to the Arctic and to Southern Hemisphere high latitudes, where the mean age ranges between 1.7 and 2.6 years. We show that these differences are related to large differences in vertical transport among the simulations, in particular to differences in parameterized convection over the oceans. While stronger convection over NH midlatitudes is associated with slower transport to the Arctic, stronger convection in the tropics and subtropics is associated with faster interhemispheric transport. We also show that the differences among simulations constrained with fields derived from the same reanalysis products are as large as (and in some cases larger than) the differences among free-running simulations, most likely due to larger differences in parameterized convection. Our results indicate that care must be taken when using simulations constrained with analyzed winds to interpret the influence of meteorology on tropospheric composition.
Publisher: Copernicus GmbH
Date: 30-08-2018
Publisher: American Geophysical Union (AGU)
Date: 05-06-0330
DOI: 10.1029/2021GL093215
Abstract: Southern Hemisphere (SH) stratospheric sudden warmings (SSWs) result in smaller Antarctic ozone holes and are linked to extreme midlatitude weather on subseasonal to seasonal timescales. Therefore, it is of interest how often such events occur and whether we should expect more events in the future. Here, we use a pair of novel multimillennial simulations with a stratosphere‐resolving coupled ocean‐atmosphere climate model to show that the frequency of SSWs, such as observed 2002 and 2019, is about one in 22 years for 1990 conditions. In addition, we show that we should expect the frequency of SSWs, and that of more moderate vortex weakening events, to strongly decrease by the end of this century.
Publisher: Copernicus GmbH
Date: 30-11-2017
Publisher: Wiley
Date: 28-10-2020
Publisher: Copernicus GmbH
Date: 30-11-2017
Abstract: Abstract. Understanding and modeling the large-scale transport of trace gases and aerosols is important for interpreting past (and projecting future) changes in atmospheric composition. Here we show that there are large differences in the global-scale atmospheric transport properties among models participating in the IGAC SPARC Chemistry-Climate Model Initiative (CCMI). Specifically, we find up to 40 % differences in the transport timescales connecting the Northern Hemisphere (NH) midlatitude surface to the Arctic and to Southern Hemisphere high latitudes, where the mean age ranges between 1.7 years and 2.6 years. We show that these differences are related to large differences in vertical transport among the simulations and, in particular, to differences in parameterized convection over the oceans. While stronger convection over NH midlatitudes is associated with slower transport to the Arctic, stronger convection in the tropics and subtropics is associated with faster interhemispheric transport. We also show that the differences among simulations constrained with fields derived from the same reanalysis products are as large as (and, in some cases, larger than) the differences among free-running simulations, due to larger differences in parameterized convection. Our results indicate that care must be taken when using simulations constrained with analyzed winds to interpret the influence of meteorology on tropospheric composition.
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.
Publisher: Copernicus GmbH
Date: 07-03-2023
Publisher: Wiley
Date: 22-03-2021
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.
Publisher: Cambridge University Press (CUP)
Date: 11-1988
DOI: 10.1017/S0022112088002757
Abstract: Viscoelastic theory is used to describe the response of a floating ice sheet to a moving vehicle. We adopt a two-parameter memory function to describe the behaviour of the ice, subjected to a steadily moving line or point load. The viscoelastic dissipation produces an asymmetric quasi-static response at subcritical speed, renders a finite response at the critical speed, and d s the shorter leading waves rather more severely than the longer trailing waves at supercritical speed. We extend earlier asymptotic theory to consider the anisotropic d ing of the flexural waves. There is enhanced agreement between theory and experiment.
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.
No related organisations have been discovered for Darryn Waugh.
Start Date: 08-2021
End Date: 12-2027
Amount: $20,000,000.00
Funder: Australian Research Council
View Funded Activity