ORCID Profile
0000-0002-6817-925X
Current Organisation
UNSW Sydney
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Physical oceanography | Computational Fluid Dynamics | Oceanography | Oceanography | Physical Oceanography | Photogrammetry and remote sensing | Biological oceanography | Chemical oceanography | Maritime Engineering not elsewhere classified | Geophysical Fluid Dynamics
Marine Oceanic Processes (excl. climate related) | Natural Hazards in Marine Environments | Expanding Knowledge in the Earth Sciences |
Publisher: Wiley
Date: 24-12-2021
Publisher: American Geophysical Union (AGU)
Date: 10-2015
DOI: 10.1002/2015JC010972
Publisher: American Geophysical Union (AGU)
Date: 06-2017
DOI: 10.1002/2016JC011968
Publisher: American Geophysical Union (AGU)
Date: 02-2015
DOI: 10.1002/2014JC010357
Publisher: Cambridge University Press (CUP)
Date: 23-03-2023
DOI: 10.1017/JFM.2023.134
Abstract: A dynamical understanding of the physical process of surface gravity wave breaking remains an unresolved problem in fluid dynamics. Conceptually, breaking can be described by inception and onset, where breaking inception is the initiation of unknown irreversible processes within a wave crest that precede the visible manifestation of breaking onset. In the search for an energetic indicator of breaking inception, we use an ensemble of non-breaking and breaking crests evolving within unsteady wave packets simulated in a numerical wave tank to investigate the evolution of each term in the kinetic energy balance equation. We observe that breaking onset is preceded by around one quarter of a wave period by a rapid increase in the rate of convergence of kinetic energy that triggers an irreversible acceleration of the kinetic energy growth rate. This energetic signature, which is present only for crests that subsequently break, arises when the kinetic energy growth rate exceeds a critical threshold. At this point the additional kinetic energy convergence cannot be offset by converting excess kinetic energy to potential energy or by dissipation through friction. Our results suggest that the ratio of the leading terms of the kinetic energy balance equation at the time of this energetic signature is proportional to the strength of the breaking crest. Hence this energetic inception point both predicts the occurrence of breaking onset and indicates the strength of the breaking event.
Publisher: Informa UK Limited
Date: 04-2011
Publisher: American Physical Society (APS)
Date: 29-11-2007
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.
Publisher: American Meteorological Society
Date: 08-2011
Abstract: Several recent studies diagnose lateral stirring and mixing in the upper ocean using altimetry-derived velocity fields to advect “virtual” particles and fields offline. However, the limited spatiotemporal resolution of altimetric maps leads to errors in the inferred diagnostics, because unresolved scales are necessarily imperfectly modeled. The authors examine a range of tracer diagnostics in two models of baroclinic turbulence: the standard Phillips model, in which dispersion is controlled by large-scale eddies, and the Eady model, where dispersion is determined by local scales of motion. These models serve as a useful best- and worst-case comparison and a valuable test of the resolution sensitivity of tracer diagnostics. The effect of unresolved scales is studied by advecting tracers using model velocity fields subs led in space and time and comparing the derived tracer diagnostics with their “true” value obtained from the fully resolved flow. The authors find that eddy diffusivity and absolute dispersion, which are governed by large-scale dynamics, are insensitive to spatial s ling error in either flow. Measures that depend strongly on small scales, such as relative dispersion and finite-time Lyapunov exponents, are highly sensitive to spatial s ling in the Eady model. Temporal s ling error is found to have a more complicated behavior because of the onset of particle overshoot leading to scrambling of Lagrangian diagnostics. This leads to a potential restriction on the utility of raw altimetry maps for studying mixing in the upper ocean. The authors conclude that offline diagnostics of mixing in ocean flows with an energized submesoscale should be viewed with some caution.
Publisher: Springer Science and Business Media LLC
Date: 25-08-2009
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.
Publisher: American Geophysical Union (AGU)
Date: 03-2017
DOI: 10.1002/2016JC012241
Publisher: American Meteorological Society
Date: 05-2020
Abstract: Submesoscale lenses of water with anomalous hydrographic properties have previously been observed in the East Australian Current (EAC) system, embedded within the thermocline of mesoscale anticyclonic eddies. The waters within these lenses have high oxygen content and temperature–salinity properties that signify a surface origin. However, it is not known how these lenses form. This study presents field observations that provide insight into a possible generation mechanism via subduction at upper-ocean fronts. High-resolution hydrographic and velocity measurements of submesoscale activity were taken across a front between a mesoscale eddy dipole downstream of the EAC separation point. The front had O (1) Rossby number, strong vertical shear, and flow conducive to symmetric instability. Frontogenesis was measured in conjunction with subduction of an anticyclonic water parcel, indicative of intrathermocline eddy formation. Twenty-five years of satellite imagery reveals the existence of strong mesoscale strain coupled with strong temperature fronts in this region and indicates the conditions that led to frontal subduction observed here are a persistent feature. These processes impact the vertical export of tracers from the surface and dissipation of mesoscale kinetic energy, implicating their importance for understanding regional ocean circulation and biological productivity.
Publisher: American Geophysical Union (AGU)
Date: 11-2017
DOI: 10.1002/2017JC013097
Publisher: Springer Science and Business Media LLC
Date: 05-2002
Publisher: Wiley
Date: 12-05-2021
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: American Geophysical Union (AGU)
Date: 04-2019
DOI: 10.1029/2018JC014482
Publisher: Cambridge University Press (CUP)
Date: 08-01-2008
DOI: 10.1017/S002211200700941X
Abstract: The theory of turbulent resistivity in ‘wavy’ magnetohydrodynamic turbulence in two dimensions is presented. The goal is to explore the theory of quenching of turbulent resistivity in a regime for which the mean field theory can be rigorously constructed at large magnetic Reynolds number Rm . This is achieved by extending the simple two-dimensional problem to include body forces, such as buoyancy or the Coriolis force, which convert large-scale eddies into weakly interacting dispersive waves. The turbulence-driven spatial flux of magnetic potential is calculated to fourth order in wave slope – the same order to which one usually works in wave kinetics. However, spatial transport, rather than spectral transfer, is the object here. Remarkably, adding an additional restoring force to the already tightly constrained system of high Rm magnetohydrodynamic turbulence in two dimensions can actually increase the turbulent resistivity, by admitting a spatial flux of magnetic potential which is not quenched at large Rm , although it is restricted by the conditions of applicability of weak turbulence theory. The absence of Rm -dependent quenching in this wave-interaction-driven flux is a consequence of the presence of irreversibility due to resonant nonlinear three-wave interactions, which are independent of collisional resistivity. The broader implications of this result for the theory of mean field electrodynamics are discussed.
Publisher: American Meteorological Society
Date: 11-2019
Abstract: Horizontal fluxes of heat and other scalar quantities in the ocean are due to correlations between the horizontal velocity and tracer fields. However, the limited spatial resolution of ocean models means that these correlations are not fully resolved using the velocity and temperature evaluated on the model grid, due to the limited spatial resolution and the boxcar-averaged nature of the velocity and the scalar field. In this article, a method of estimating the horizontal flux due to unresolved spatial correlations is proposed, based on the depth-integrated horizontal transport from the seafloor to the density surface whose spatially averaged height is the height of the calculation. This depth-integrated horizontal transport takes into account the subgrid velocity and density variations to compensate the standard estimate of horizontal transport based on staircase-like velocity and density. It is not a parameterization of unresolved eddies, since it utilizes data available in ocean models without relying on any presumed parameter such as diffusivity. The method is termed the horizontal residual mean (HRM). The method is capable of estimating the spatial-correlation-induced water transport in a 1/4° global ocean model, using model data smoothed to 3/4°. The HRM extra overturning has a peak in the Southern Ocean of about 1.5 Sv (1 Sv ≡ 10 6 m 3 s −1 ). This indicates an extra heat transport of 0.015 PW on average in the same area. It is expected that implementing the scheme in a coarse-resolution ocean model will improve its representation of lateral heat fluxes.
Publisher: American Physical Society (APS)
Date: 29-03-2022
Publisher: Wiley
Date: 15-06-2016
DOI: 10.1002/QJ.2833
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.
Publisher: American Geophysical Union (AGU)
Date: 25-06-2018
DOI: 10.1029/2018GL078429
Publisher: AIP Publishing
Date: 07-2010
DOI: 10.1063/1.3456726
Abstract: The efficiency with which an incompressible flow mixes a passive scalar field that is continuously replenished by a steady source-sink distribution has been quantified using the suppression of the mean scalar variance below the value it would attain in the absence of the stirring. We examine the relationship this mixing measure has to the effective diffusivity obtained from homogenization theory, particularly establishing precise connections in the case of a stirring velocity field that is periodic in space and time and varies on scales much smaller than that of the source. We explore theoretically and numerically via the Childress–Soward family of flows how the mixing measures lose their linkage to the homogenized diffusivity when the velocity and source field do not enjoy scale separation. Some implications for homogenization-based parametrizations of mixing by flows with finite scale separation are discussed.
Publisher: American Meteorological Society
Date: 05-2012
Abstract: Attempts to monitor ocean eddy heat transport are strongly limited by the sparseness of available observations and the fact that heat transport is a quadratic, sign-indefinite quantity that is particularly sensitive to unresolved scales. In this article, a suite of stochastic filtering strategies for estimating eddy heat transport are tested in idealized two-layer simulations of mesoscale oceanic turbulence at high and low latitudes under a range of observation scenarios. A novel feature of these filtering strategies is the use of computationally inexpensive stochastic models to forecast the underlying nonlinear dynamics. The stochastic model parameters can be estimated by regression fitting to climatological energy spectra and correlation times or by adaptively learning these parameters “on-the-fly” from the observations themselves. The authors show that, by extracting high-wavenumber information that has been aliased into the low wavenumber band, “stochastically super-resolved” velocity fields with a nominal resolution increase of a factor of 2 or more can be derived. Observations of the upper-layer streamfunction are projected onto an empirical orthogonal function basis for the vertical structure to produce filtered estimates for both upper- and lower-layer streamfunctions and hence net heat transport. The resulting time-mean poleward eddy heat transport is significantly closer to the true value when compared with standard estimates based upon optimal interpolation. By contrast, the temporal variability of the heat transport is underestimated because of poor temporal resolution. Implications for estimating poleward eddy heat transport using current and next-generation altimeters are discussed.
Publisher: American Astronomical Society
Date: 11-04-2008
DOI: 10.1086/588654
Publisher: American Geophysical Union (AGU)
Date: 12-2020
DOI: 10.1029/2020JC016580
Abstract: Accurate forecasting of ocean currents in dynamic regions remains a critical challenge due to the sparsity of observations in global ocean observing networks and the limited resolution of present‐day regional ocean models. Lately, traditional observing platforms have been complemented by newly available data streams capable of s ling at higher spatial and/or temporal resolutions in dynamically significant regions in near‐real time. However, the relative merits and trade‐offs of incorporating these “nontraditional” observations into ocean state estimates have not been thoroughly investigated. Here, we perform a detailed statistical and dynamical comparison of two high‐resolution reanalysis products assimilating different combinations of traditional and nontraditional observations in the East Australian Current (EAC) system, a vigorous western boundary current. We show that sea‐surface height and temperature are well‐constrained by satellite measurements however, below the surface, a reanalysis incorporating fully available observations better represents the ocean state. The core of the EAC jet is effectively constrained by subsurface observations from deep water moorings upstream of jet separation, while radar‐derived nearshore surface velocities in the separation zone are found to resolve the submesoscale cyclonic band inshore of the EAC. Cost function sensitivity analysis of both products reveals excessive model adjustment at depth causing the reanalyzes to overestimate alongshore transport relative to a 22‐year freely evolving simulation. Overall, the assimilation of nontraditional observations delivers marked improvement in representing dynamical features of the EAC. However, this improvement is not as pronounced in the model forecast due to the introduction of nonphysical dynamics or forcing, suggesting that other improvements such as increased model resolution are required.
Location: United States of America
Start Date: 2018
End Date: 2020
Funder: Australian Research Council
View Funded ActivityStart Date: 07-2023
End Date: 06-2026
Amount: $693,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2021
End Date: 12-2023
Amount: $772,031.00
Funder: Australian Research Council
View Funded ActivityStart Date: 09-2018
End Date: 04-2022
Amount: $440,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2023
End Date: 12-2025
Amount: $389,674.00
Funder: Australian Research Council
View Funded Activity