Callum Shakespeare

Tracking quicksilver: Estimation of mercury waste from consumer products and subsequent verification by analysis of soil, water, sediment, and plant sample from the Cebu City, Philippines, landfill

Publisher: Springer Science and Business Media LLC

Date: 25-02-2015

DOI: 10.1007/S10661-015-4321-0

Abstract: Source attribution of mercury (Hg) is critical for policy development to minimize the impact of Hg in wastes. Mercury content of consumer products and its subsequent release into the waste stream of Cebu City, Philippines, is estimated through surveys that employed validated, enumerator-administered questionnaires. Initially, a citywide survey (n = 1636) indicates that each household annually generates 1.07 ppm Hg (i.e., mg Hg/kg waste) and that linear and compact fluorescent l s (17.2 %) and thermometers (52.1 %) are the major sources of Hg. A subsequent survey (n = 372) in the vicinity of the city's municipal solid waste landfill shows that residents in the area annually generate 0.38 ppm Hg per household, which is less than the citywide mean surprisingly though, less affluent respondents living closer to the landfill site reported more Hg from thermometers and sphygmomanometers. Analysis of collected soil (0.238 ppm), leachate water (6.5 ppb), sediment (0.109 ppm), and three plants (0.393 to 0.695 ppm) shows no significant variation throughout five stations in and around the landfill site, although the period of collection is significant for soil (P = 0.001) and Cenchrus echinatus (P = 0.016). Detected Hg in the landfill is considerably less than the annual estimated release, indicating that there is minimal accumulation of Hg in the soil or in plants. As a result of this project, a policy brief has been provided to the Cebu City council in aid of hazardous waste legislation.

A generalized mathematical model of geostrophic adjustment and frontogenesis: uniform potential vorticity

Publisher: Cambridge University Press (CUP)

Date: 06-11-2013

DOI: 10.1017/JFM.2013.526

Abstract: Fronts, or regions with strong horizontal density gradients, are ubiquitous and dynamically important features in the ocean and atmosphere. In the atmosphere, fronts are associated with some of the most severe weather events, while in the ocean, fronts are associated with enhanced turbulence, water mass transformation and biological activity. Here, we examine the dynamics involved in the formation of fronts, or frontogenesis, in detail using a generalized mathematical framework. This extends previous work which has generally revolved around two limiting cases: fronts generated through forcing due to a convergent large-scale flow, and fronts generated spontaneously during the geostrophic adjustment of an initially unbalanced flow. Here, we introduce a new generalized momentum coordinate to simultaneously describe forced and spontaneous frontogenesis. The nonlinear, inviscid, Boussinesq, hydrostatic governing equations for uniform PV flow are solved for arbitrary Rossby and Froude number. The solution is then examined in three distinct cases. Firstly, for a zero potential vorticity (PV) flow bounded by rigid lids, a general solution is derived for the transient response of the fluid to an arbitrary initial mass imbalance and deformation field. The deformation frontogenesis solution of Hoskins & Bretherton ( J. Atmos. Sci. , vol. 29, 1972, pp. 11–37) and the mass imbalance solution of Blumen ( J. Phys. Oceanogr. , vol. 30, 2000, pp. 31–39) emerge as two limits of this general solution. Secondly, the problem of geostrophic adjustment of an initial mass imbalance (no deformation field) is considered for uniform PV flow bounded by rigid lids. The general solution is derived, composed of an adjusted state and a transient component describing the propagation of inertia–gravity waves. The criteria for the occurrence of a frontal discontinuity is determined in terms of the Rossby and Froude numbers. The uniform PV solution reduces identically to the zero PV solution of Blumen in the limit of vanishing background stratification. Thirdly, we examine the more general case of uniform PV flow with a deformation field and either balanced or unbalanced initial conditions. In this case the solution is composed of a time-varying mean state – matching the Hoskins & Bretherton solution in the limit of small strain – and an inertia gravity wave field, the dynamics of which are examined in detail. Our analysis provides a unifying framework capable of describing frontal formation and geostrophic adjustment in a wide variety of settings.

Ten-Year Treatment Outcomes of Neovascular Age-Related Macular Degeneration from Two Regions.

Publisher: Elsevier BV

Date: 02-2019

DOI: 10.1016/J.AJO.2019.10.007

Abstract: To report and compare 10-year treatment outcomes of vascular endothelial growth factor (VEGF) inhibitors for neovascular age-related macular degeneration (nAMD) from Australia and New Zealand (ANZ) and Switzerland. Retrospective, comparative, interventional case series. We analyzed 712 treatment-naive eyes (ANZ, n = 474 Switzerland, n = 321) starting anti-VEGF for nAMD in routine clinical practice between January 1, 2006, and December 31, 2008, tracked in the prospectively designed observational database, the Fight Retinal Blindness! registry. The primary outcome was mean change in visual acuity (VA [in logMAR letters]) in eyes that completed 10 years of treatment. The mean VA in 132 eyes (28%) from ANZ patients who completed 10 years of treatment dropped by 0.9 letters from baseline (95% confidence interval [CI], -4.9 to 3.1 P = 0.7) with 42% achieving ≥20/40, whereas the 37 eyes (12%) from Swiss subjects lost 14.9 letters (95% CI, -24 to -5.7 P < 0.001) with 35% achieving ≥20/40. Eyes from ANZ patients received more injections than eyes from Swiss subjects over 10 years (a median of 53 vs 42, respectively) from fewer visits with better disease control (proportion of visits with active disease: 38% vs 69%, respectively), suggesting a treat-and-extend regimen versus a pro re nata regimen (treatment given only when the lesion is active). Macular atrophy and subretinal fibrosis were the main reasons for 10 letter loss in the subset of eyes analyzed retrospectively. The mean VA of eyes from both regions that discontinued treatment within 10 years had fallen below the baseline at their final visit. Eyes with nAMD may achieve satisfactory long-term visual outcomes if they receive adequate treatment. Central macular atrophy does not develop universally in eyes receiving long-term treatment with VEGF inhibitors as previously feared. Visual outcomes were better in eyes from ANZ patients, likely because they received more injections.

Asymmetric internal tide generation in the presence of a steady flow

Publisher: Wiley

Date: 28-06-2020

DOI: 10.1002/ESSOAR.10503467.1

The spontaneous generation of inertia–gravity waves during frontogenesis forced by large strain: numerical solutions

Publisher: Cambridge University Press (CUP)

Date: 07-05-2015

DOI: 10.1017/JFM.2015.197

Abstract: A fully nonlinear numerical model is used to investigate spontaneous wave generation during two-dimensional frontogenesis forced by a horizontal strain field. The model uses the idealised configuration of an infinitely long straight front and uniform potential vorticity, with a uniform imposed convergent strain across the front. Shakespeare & Taylor ( J. Fluid Mech. , vol. 757, 2014, pp. 817–853) formulated a generalised analytical model (ST14) for this system that extends the classical Hoskins & Bretherton ( J. Atmos. Sci. , vol. 29, 1972, pp. 11–37) model (HB) to large strain rates ( ${\\it\\alpha}\\sim f$ ). Here, we use a numerical model to simulate the fully nonlinear problem and compare the results with the predictions of the analytical model for a variety of strain rates. Even for weak strains ( ${\\it\\alpha}=0.2f$ ), the confinement of the secondary circulation and the spontaneous generation of waves, predicted by ST14, are shown to be important corrections to the HB solution. These inviscid predictions are also robust for an equilibrated front where strain-forced frontogenesis is balanced by diffusion. For strong strains the wavefield becomes of leading-order importance to the solution. In this case the frontal circulation is tightly confined, and the vertical velocity is an order of magnitude larger than in the HB model. The addition of a strain field that weakens with time allows the release and propagation of the spontaneously generated waves. We also consider fronts with both large vorticity and strain rate, beyond the validity of the ST14 model.

The Wavelength Dependence of the Propagation of Near-Inertial Internal Waves

Publisher: American Meteorological Society

Date: 10-2022

DOI: 10.1175/JPO-D-21-0266.1

Abstract: Wind-generated near-inertial internal waves (NIWs) are triggered in the mixed layer and propagate down into the ocean interior. Observational and numerical studies have shown the effects of background vorticity and high shear on propagating NIWs. However, the impacts of the background mean flow on NIWs as a function of the waves’ horizontal wavelength have yet to be fully investigated. Here, two distinct cases are analyzed, namely, the propagation of wind-generated, large-scale NIWs in negative vorticity and the behavior of small-scale NIWs in high shear. The propagation and energetics of the respective NIWs are investigated using a realistic eddy-resolving numerical simulation of the Kuroshio region. The large-scale NIWs display a rapid vertical propagation to depth in negative vorticity areas, while the small-scale NIWs are confined to shallower depths in high-shear regions. Furthermore, the dominant energy sources and sinks of near-inertial energy are estimated as the respective NIWs propagate into the ocean’s interior. The qualitative analysis of NIW energetics reveals that the wind triggers the generation of both the large-scale and small-scale NIWs, but the waves experience further lification as they draw energy from the background mean flow upon propagation in negative vorticity and high-shear regions, respectively. In addition, the study demonstrates that small-scale NIWs can be induced independently by wind fluctuations and do not necessarily rely on straining nor refraction of large-scale NIWs by mesoscale motions.

A simple technique for developing and visualising stratified fluid dynamics: the hot double-bucket

Publisher: Springer Science and Business Media LLC

Date: 22-04-2021

DOI: 10.1007/S00348-021-03190-Y

Dissipating and reflecting internal waves

Publisher: American Meteorological Society

Date: 21-05-2021

DOI: 10.1175/JPO-D-20-0261.1

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.

Spontaneous Wave Generation at Strongly Strained Density Fronts

Publisher: American Meteorological Society

Date: 07-2016

DOI: 10.1175/JPO-D-15-0043.1

Abstract: A simple analytical model is presented describing the spontaneous generation of inertia–gravity waves at density fronts subjected to strong horizontal strain rates. The model considers fronts of arbitrary horizontal and vertical structure in a semi-infinite domain, with a single boundary at the ocean surface. Waves are generated because of the acceleration of the steady uniform strain flow around the density front, analogous to the generation of lee waves via flow over a topographic ridge. Significant wave generation only occurs for sufficiently strong strain rates α 0.2 f and sharp fronts H / L 0.5 f / N , where f is the Coriolis parameter, N is the stratification, and H and L are the height and width scales of the front, respectively. The frequencies of the generated waves are entirely determined by the strain rate. The lowest-frequency wave predicted to be generated via this mechanism has a Lagrangian frequency ω = 1.93 f as measured in a reference frame moving with the background strain flow. The model is intended as a first-order description of wave generation at submesoscale (1 to 10 km wide) fronts where large strain rates are commonplace. The analytical model compares well with fully nonlinear numerical simulations of the submesoscale regime.

A New Mechanism for Mode Water Formation Involving Cabbeling and Frontogenetic Strain at Thermohaline Fronts. Part II: Numerical Simulations

Publisher: American Meteorological Society

Date: 07-2017

DOI: 10.1175/JPO-D-17-0001.1

Abstract: Submesoscale-resolving numerical simulations are used to investigate a mechanism for sustained mode water formation via cabbeling at thermohaline fronts subject to a confluent strain flow. The simulations serve to further elucidate the mechanism and refine the predictions of the analytical model of Thomas and Shakespeare. Unlike other proposed mechanisms involving air–sea fluxes, the cabbeling mechanism, in addition to driving significant mode water formation, uniquely determines the thermohaline properties of the mode water given knowledge of the source water masses on either side of the front. The process of mode water formation in the simulations is as follows: Confluent flow associated with idealized mesoscale eddies forces water horizontally toward the front. The frontogenetic circulation draws this water near adiabatically from the full depth of the thermohaline front up to the surface 25 m, where resolved submesoscale instabilities drive intense mixing across the thermohaline front, creating the mode water. The mode water is denser than the surrounding stratified fluid and sinks to fill its neutral buoyancy layer at depth. This layer gradually expands up to the surface, and eddies composed entirely of this mode water detach from the front and accumulate in the diffluent regions of the domain. The process continues until the source water masses are exhausted. The temperature–salinity ( T – S ) relation of the resulting mode water is biased to the properties of the source water that has the larger isopycnal T – S anomaly. This mechanism has the potential to drive O (1) Sv (1 Sv ≡ 10 6 m 3 s −1 ) mode water formation and may be important in determining the properties of mode water in the global oceans.

Diagnosing Instantaneous Forcing and Feedbacks of Downwelling Longwave Radiation at the Surface: A Simple Methodology and Its Application to CMIP5 Models

Publisher: American Meteorological Society

Date: 15-06-2022

DOI: 10.1175/JCLI-D-21-0865.1

Abstract: Climate models predict large increases in downwelling longwave radiation (DLR) at Earth’s surface as atmospheric CO 2 concentrations increase. Here we introduce a novel methodology that allows these increases to be decomposed into direct radiative forcing due to enhanced CO 2 and feedbacks due to subsequent changes in atmospheric properties. For the first time, we develop explicit analytic expressions for the radiative forcing and feedbacks, which are calculable from time-mean fields of near-surface air temperature, specific humidity, pressure, total column water vapor, and total cloud fraction. Our methodology captures 90%–98% of the variance in changes in clear-sky and all-sky DLR in five CMIP5 models, with a typical error of less than 10%. The longwave feedbacks are decomposed into contributions from changes in temperature, specific humidity, water vapor height scale, and cloud fraction. We show that changes in specific humidity and height scale are closely linked to changes in near-surface air temperature and therefore, in the global average, that 90% of the increase in all-sky DLR may be attributed to a feedback from increasing near-surface air temperature. Mean-state clouds play a major role in changes in DLR by masking the clear-sky longwave and enhancing the temperature feedback via increased blackbody radiation. The impact of changes in cloud cover (the cloud feedback) on the DLR is small (∼2%) in the global average, but significant in particular geographical regions.

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.

Nonhydrostatic Wave Generation at Strained Fronts

Publisher: American Meteorological Society

Date: 07-2016

DOI: 10.1175/JAS-D-14-0294.1

Abstract: A simple analytical model is developed to describe wave generation during frontogenesis forced by a horizontal strain field. In contrast to previous models, neither geostrophic nor hydrostatic balance is assumed. The generated waves are trapped in the strain field and form steady bands of enhanced vertical flow on either side of the surface front on scales from 1 to 100 km. The predictions of the analytical model are confirmed by comparison with fully nonlinear numerical simulations.

Attribution of horizontal and vertical contributions to spurious mixing in an Arbitrary Lagrangian – Eulerian ocean model

Publisher: Elsevier BV

Date: 11-2017

DOI: 10.1016/J.OCEMOD.2017.09.008

Interdependence of Internal Tide and Lee Wave Generation at Abyssal Hills: Global Calculations

Publisher: American Meteorological Society

Date: 03-2020

DOI: 10.1175/JPO-D-19-0179.1

Abstract: The generation of internal waves at abyssal hills has been proposed as an important source of bottom-intensified mixing and a sink of geostrophic momentum. Using the theory of Bell, previous authors have calculated either the generation of lee waves by geostrophic flow or the generation of the internal tide by the barotropic tide, but never both together. However, the Bell theory shows that the two are interdependent: that is, the presence of a barotropic tide modifies the generation of lee waves, and the presence of a geostrophic (time mean) flow modifies the generation of the internal tide. Here we extend the theory of Bell to incorporate multiple tidal constituents. Using this extended theory, we recalculate global wave fluxes of energy and momentum using the abyssal-hill spectra, model-derived abyssal ocean stratification and geostrophic flow estimates, and the TPX08 tidal velocities for the eight major constituents. The energy flux into lee waves is suppressed by 13%–19% as a result of the inclusion of tides. The generated wave flux is dominated by the principal lunar semidiurnal tide (M2), and its harmonics and combinations, with the strongest fluxes occurring along midocean ridges. The internal tide generation is strongly asymmetric because of Doppler shifting by the geostrophic abyssal flow, with 55%–63% of the wave energy flux (and stress) directed upstream, against the geostrophic flow. As a consequence, there is a net wave stress associated with generation of the internal tide that reaches magnitudes of 0.01–0.1 N m −2 in the vicinity of midocean ridges.

The life cycle of Spontaneously generated internal waves

Publisher: American Meteorological Society

Date: 02-2018

DOI: 10.1175/JPO-D-17-0153.1

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.

Spontaneous generation of internal waves

Publisher: AIP Publishing

Date: 06-2019

DOI: 10.1063/PT.3.4225

Abstract: The waves responsible for transporting energy in the ocean and atmosphere are not solely a product of the combined forces of winds, tides, and mountains.

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

DOI: 10.1175/JPO-D-11-0198.1

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.

Curved Density Fronts: Cyclogeostrophic Adjustment and Frontogenesis

Publisher: American Meteorological Society

Date: 10-2016

DOI: 10.1175/JPO-D-16-0137.1

Abstract: Curvature can play a significant role in the dynamics of density fronts at small scales and in low-latitude regions of the ocean. Fronts can be displaced from balance by rapid forcing and undergo an adjustment toward a more stable state or be strained and sharpened by surrounding flow in a process known as frontogenesis. This study investigates the role of curvature in adjustment and frontogenesis using the idealized configuration of an axisymmetric eddy and associated circular front. As a result of the curvature, the balanced state of this system is not geostrophic balance, where pressure and Coriolis forces exactly balance, but cyclogeostrophic balance, where pressure and Coriolis forces combine to supply a net inwards centripetal force on fluid parcels. The parameter range for which cyclogeostrophically balanced states exist for a given unbalanced initial condition is determined. This parameter range is smaller for anticyclonic fronts (i.e., fronts curved around a warm core), which have larger angular velocities than comparable straight fronts, implying they are more likely to break down during adjustment. The reverse is true for cyclonic fronts. A model for the sharpening of a curved front in a background strain flow, analogous to the Hoskins and Bretherton (1972) model for a straight front, is developed. Relative to a straight front subject to the same strain rate, vertical velocities are weaker for an anticyclonic front and stronger for a cyclonic front. Anticyclonic fronts collapse to a near discontinuity during frontogenesis far more rapidly than cyclonic fronts for the same strain rate.

Lagrangian Filtering: A novel method for separating internal waves from non-wave flows in high-resolution simulations

Publisher: Wiley

Date: 12-05-2021

DOI: 10.1002/ESSOAR.10507016.1

Magnetization pinning in conducting films demonstrated using broadband ferromagnetic resonance

Publisher: AIP Publishing

Date: 15-11-2010

DOI: 10.1063/1.3493242

Abstract: The broadband microstrip ferromagnetic resonance (FMR), cavity FMR, and Brillouin light scattering spectroscopy techniques have been applied for detection and characterization of a magnetic inhomogeneity in a film s le. In the case of a 100 nm thick permalloy film, an additional magnetically depleted top sublayer has been detected due to pinning effect it produces on the magnetization in the bulk of the film. The pinning results in appearance of an exchange standing spin wave mode in the broadband FMR absorption spectrum, whose litudes are different depending on whether the film or the film substrate faces the microstrip transducer. Comparison of the experimental litudes for this mode with results of our theory for both film placements revealed that the depleted layer is located at the film surface facing away from the film substrate. Subsequent broadband FMR characterization of a large number of other presumably single-layer films with thicknesses in the range 30–100 nm showed the same result.

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.

Changes in real‐world treatment patterns for diabetic macular oedema from 2009 to 2019 and 5‐year outcomes: Data from the Fight Retinal Blindness! Registry

Publisher: Wiley

Date: 11-06-2020

DOI: 10.1111/CEO.13781

Observed Eddy–Internal Wave Interactions in the Southern Ocean

Publisher: American Meteorological Society

Date: 10-2020

DOI: 10.1175/JPO-D-20-0001.1

Abstract: The physical mechanisms that remove energy from the Southern Ocean’s vigorous mesoscale eddy field are not well understood. One proposed mechanism is direct energy transfer to the internal wave field in the ocean interior, via eddy-induced straining and shearing of preexisting internal waves. The magnitude, vertical structure, and temporal variability of the rate of energy transfer between eddies and internal waves is quantified from a 14-month deployment of a mooring cluster in the Scotia Sea. Velocity and buoyancy observations are decomposed into wave and eddy components, and the energy transfer is estimated using the Reynolds-averaged energy equation. We find that eddies gain energy from the internal wave field at a rate of −2.2 ± 0.6 mW m −2 , integrated from the bottom to 566 m below the surface. This result can be decomposed into a positive (eddy to wave) component, equal to 0.2 ± 0.1 mW m −2 , driven by horizontal straining of internal waves, and a negative (wave to eddy) component, equal to −2.5 ± 0.6 mW m −2 , driven by vertical shearing of the wave spectrum. Temporal variability of the transfer rate is much greater than the mean value. Close to topography, large energy transfers are associated with low-frequency buoyancy fluxes, the underpinning physics of which do not conform to linear wave dynamics and are thereby in need of further research. Our work suggests that eddy–internal wave interactions may play a significant role in the energy balance of the Southern Ocean mesoscale eddy and internal wave fields.

Particle‐Based Lagrangian Filtering for Locating Wave‐Generated Thermal Refugia for Coral Reefs

Publisher: American Geophysical Union (AGU)

Date: 07-2020

DOI: 10.1029/2020JC016106

Asymmetric Internal Tide Generation in the Presence of a Steady Flow

Publisher: American Geophysical Union (AGU)

Date: 10-2020

DOI: 10.1029/2020JC016503

A New Mechanism for Mode Water Formation involving Cabbeling and Frontogenetic Strain at Thermohaline Fronts

Publisher: American Meteorological Society

Date: 09-2015

DOI: 10.1175/JPO-D-15-0007.1

Abstract: A simple analytical model is used to elucidate a potential mechanism for steady-state mode water formation at a thermohaline front that involves frontogenesis, submesoscale lateral mixing, and cabbeling. This mechanism is motivated in part by recent observations of an extremely sharp, density-compensated front at the North Wall of the Gulf Stream. Here, the intergyre, along-isopycnal, salinity–temperature difference is compressed into a span of a few kilometers, making the flow susceptible to cabbeling. The sharpness of the front is caused by frontogenetic strain, which is presumably balanced by submesoscale lateral mixing processes. The balance is studied with the simple model, and a scaling is derived for the amount of water mass transformation resulting from the ensuing cabbeling. The transformation scales with the strain rate, equilibrated width of the front, and the square of the isopycnal temperature contrast across the front. At the major ocean fronts where mode waters are found, this isopycnal temperature contrast decreases with increasing density near the isopycnal layers where mode waters reside. This implies that cabbeling should result in a convergent diapycnal mass flux into mode water density classes. The scaling for the transformation suggests that at these fronts the process could generate 0.01–1 Sverdrups (Sv 1 Sv ≡ 10 6 m 3 s −1 ) of mode water. These formation rates, while smaller than mode water formation by air–sea fluxes, should be independent of season and thus could fill select isopycnal layers continuously and play an important role in the dynamics of mode waters on interannual time scales.

The spontaneous generation of inertia–gravity waves during frontogenesis forced by large strain: theory

Publisher: Cambridge University Press (CUP)

Date: 26-09-2014

DOI: 10.1017/JFM.2014.514

Abstract: Density fronts are common features of ocean and atmosphere boundary layers. Field observations and numerical simulations have shown that the sharpening of frontal gradients, or frontogenesis, can spontaneously generate inertia–gravity waves (IGWs). Although significant progress has been made in describing frontogenesis using approximations such as quasi-geostrophy (Stone, J. Atmos. Sci. , vol. 23, 1966, pp. 455–565, Williams & Plotkin J. Atmos. Sci. , vol. 25, 1968, pp. 201–206) semi-geostrophy (Hoskins, Annu. Rev. Fluid Mech. , vol. 14, 1982, pp. 131–151), these models omit waves. Here, we further develop the analytical model of Shakespeare & Taylor ( J. Fluid Mech. , vol. 736, 2013, pp. 366–413) to describe the spontaneous emission of IGWs from an initially geostrophically balanced front subjected to a time-varying horizontal strain. The model uses the idealised configuration of an infinitely long, straight front and uniform potential vorticity (PV) fluid, with a uniform imposed convergent strain across the front, similar to Hoskins & Bretherton ( J. Atmos. Sci. , vol. 29, 1972, pp. 11–37). Inertia–gravity waves are generated via two distinct mechanisms: acceleration of the large-scale flow and frontal collapse. Wave emission via frontal collapse is predicted to be exponentially small for small values of strain but significant for larger strains. Time-varying strain can also generate finite- litude waves by accelerating the cross-front flow and disrupting geostrophic balance. In both cases waves are trapped by the oncoming strain flow and can only propagate away from the frontal zone when the strain field weakens sufficiently, leading to wave emission that is strongly localised in both time and space.

The clear‐sky downwelling long‐wave radiation at the surface in current and future climates

Publisher: Wiley

Date: 10-2021

DOI: 10.1002/QJ.4176

Abstract: Downwelling long‐wave radiation is a crucial component of the energy balance of the land and ocean surface. Here we develop a semi‐analytic model for the downwelling long‐wave dependent on five governing parameters: the near‐surface air temperature, the near‐surface specific humidity, the surface air pressure, the e‐folding height‐scale of water vapour, and the CO concentration. The model predicts the hourly clear‐sky long‐wave in the ERA5 reanalysis product with a global mean error of 8.2 Wm, and on average captures 97% of the temporal variation at in idual locations. We show that the model may be used to calculate clear‐sky downwelling long‐wave from only surface observations of temperature and humidity by using the time‐mean water vapour height‐scale from the ERA5, interpolated to the location of the observation. Using this method replicates sub‐hourly observations from in idual sites having a range of climates with errors of 12–25 Wm. Furthermore, the inclusion of CO allows the model to be used to study changes in downwelling long‐wave at the surface as CO concentrations vary. We validate the model's representation of CO by comparison with five CMIP5 climate models. Our model thus provides a simple yet accurate framework to understand the key parameters controlling downwelling long‐wave and its variability in the current and future climates.

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.

On the Momentum Flux of Internal Tides

Publisher: American Meteorological Society

Date: 04-2019

DOI: 10.1175/JPO-D-18-0165.1

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.

The viscous lee wave problem and its implications for ocean modelling

Publisher: Elsevier BV

Date: 05-2017

DOI: 10.1016/J.OCEMOD.2017.03.006

Outcomes of cataract surgery in eyes with diabetic macular oedema: Data from the Fight Retinal Blindness! Registry

Publisher: Wiley

Date: 14-01-2020

DOI: 10.1111/CEO.13707

Abstract: There are limited data on real-world outcomes of cataract surgery in eyes receiving intravitreal treatments for diabetic macular oedema (DMO). Cataract surgery may exacerbate oedema in some eyes with DMO resulting in inferior outcomes. Matched, case-controlled retrospective study of observational data in routine clinical practice. Eyes receiving intravitreal treatments for DMO tracked in the Fight Retinal Blindness! Registry. Eyes that underwent cataract surgery were identified and matched 1:1 with phakic controls also receiving intravitreal injections for DMO. We also assessed potential factors that were associated with better visual acuity (VA) outcomes. Change in VA 6 months after cataract surgery. Cataract surgery was identified in 208 eyes of 156 patients of which 147 eyes had 6 months of observations before and after surgery. The mean VA 6 months after surgery improved by 10.6 letters and was similar to their matched phakic controls (68.8 vs 69.2 letters P = 0.8). Mean CST both 6 months before (341 μm) and after (360 μm) surgery were similar (P = 0.08). However, these eyes had thicker maculae and they received more injections than their matched phakic controls both before and after surgery. Eyes with worse VA before surgery and those that had received intravitreal treatment in the 4 weeks preceding surgery were more likely to gain vision. Visual outcomes of cataract surgery in eyes receiving intravitreal therapy for DMO were reasonably better. Their maculae were thicker and required more injections in the 6 months before and after surgery than their phakic controls.

The Drag on the Barotropic Tide due to the Generation of Baroclinic Motion

Publisher: American Meteorological Society

Date: 12-2020

DOI: 10.1175/JPO-D-19-0167.1

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.

Spontaneous surface generation and interior amplification of internal waves in a regional-scale ocean model

Publisher: American Meteorological Society

Date: 04-2017

DOI: 10.1175/JPO-D-16-0188.1

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.

University Of Toronto

Location: Canada

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Australian National University

Location: Australia

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University Of Cambridge

Location: United Kingdom of Great Britain and Northern Ireland

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Discovery Projects - Grant ID: DP230101836

Start Date: 12-2023

End Date: 12-2026

Amount: $340,000.00

Funder: Australian Research Council

Discovery Early Career Researcher Award - Grant ID: DE180100087

Start Date: 06-2018

End Date: 06-2021

Amount: $328,075.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP190100791

Start Date: 02-2019

End Date: 10-2022

Amount: $270,000.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE200100040

Start Date: 04-2020

End Date: 07-2021

Amount: $580,000.00

Funder: Australian Research Council

Linkage Projects - Grant ID: LP200100406

Start Date: 06-2021

End Date: 06-2025

Amount: $1,161,512.00

Funder: Australian Research Council

ARC Centres Of Excellence - Grant ID: CE170100023

Start Date: 08-2017

End Date: 12-2024

Amount: $30,050,000.00

Funder: Australian Research Council