Wang, Yang, and Sonya Legg, October 2023: Enhanced dissipation of internal tides in a mesoscale baroclinic eddy. Journal of Physical Oceanography, 53(10), DOI:10.1175/JPO-D-23-0045.12293-2316. Abstract
The dissipation of low-mode internal tides as they propagate through mesoscale baroclinic eddies is examined using a series of numerical simulations, complemented by three-dimensional ray tracing calculations. The incident mode-1 internal tide is refracted into convergent energy beams, resulting in a zone of reduced energy flux in the lee of the eddy. The dissipation of internal tides is significantly enhanced in the upper water column within strongly baroclinic (anticyclonic) eddies, exhibiting a spatially asymmetric pattern, due to trapped high-mode internal tides. Where the eddy velocity opposes the internal tide propagation velocity, high-mode waves can be trapped within the eddy, whereas high modes can freely propagate away from regions where eddy and internal wave velocities are in the same direction. The trapped high modes with large vertical shear are then dissipated, with the asymmetric distribution of trapping leading to the asymmetric distribution of dissipation. Three-dimensional ray tracing solutions further illustrate the importance of the baroclinic current for wave trapping. Similar enhancement of dissipation is also found for a baroclinic cyclonic eddy. However, a barotropic eddy is incapable of facilitating robust high modes and thus cannot generate significant dissipation of internal tides, despite its strong velocities. Both energy transfer from low to high modes in the baroclinic eddy structure and trapping of those high modes by the eddy velocity field are therefore necessary to produce internal wave dissipation, a conclusion confirmed by examining the sensitivity of the internal tide dissipation to eddy radius, vorticity, and vertical scale.
Oceanic lee waves are generated in the deep stratified ocean by the flow of ocean currents over sea floor topography, and when they break, they can lead to mixing in the stably stratified ocean interior. While the theory of linear lee waves is well established, the nonlinear mechanisms leading to mixing are still under investigation. Tidally driven lee waves have long been observed in the ocean, along with associated mixing, but observations of lee waves forced by geostrophic eddies are relatively sparse and largely indirect. Parameterizations of the mixing due to ocean lee waves are now being developed and implemented in ocean climate models. This review summarizes current theory and observations of lee wave generation and mixing driven by lee wave breaking, distinguishing between steady and tidally oscillating forcing. The existing parameterizations of lee wave–driven mixing informed by theory and observations are outlined, and the impacts of the parameterized lee wave–driven mixing on simulations of large-scale ocean circulation are summarized.
Nazarian, Robert, Christian M Burns, Sonya Legg, Maarten C Buijsman, Harpreet Kaur, and Brian K Arbic, November 2021: On the magnitude of canyon-induced mixing. Journal of Geophysical Research: Oceans, 126(11), DOI:10.1029/2021JC017671. Abstract
The location of mixing due to internal tides is important for both the ocean circulation as well as local biogeochemical processes. Numerous observations and modeling studies have shown that submarine canyons may be regions of enhanced internal tide-driven mixing, but there has not yet been a systematic study of all submarine canyons resolved in bathymetric datasets. Here, we parameterize the internal tide-driven dissipation from a suite of simulations and pair this with a global high-resolution, internal tide-resolving model and bathymetric dataset to estimate the internal-tide-driven dissipation that occurs in all documented submarine canyons. We find that submarine canyons dissipate a significant fraction of the incoming internal tide's energy, which is consistent with observations. When globally integrated, submarine canyons are responsible for dissipating 30.8–75.3 GW, or 3.2%–7.8% of the energy input into the M2-frequency internal tides. This percentage of the internal tide energy that is dissipated in submarine canyons is comparable to or larger than previous calculations using extrapolations from observations of single canyons.
Spingys, Carl P., Alberto C Naveira Garabato, and Sonya Legg, et al., April 2021: Mixing and transformation in a deep western boundary current: A case study. Journal of Physical Oceanography, 51(4), DOI:10.1175/JPO-D-20-0132.11205-1222. Abstract
Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focusing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unraveled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to restratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling.
We develop a parameterization for representing the effects of submesoscale symmetric instability (SI) in the ocean interior. SI may contribute to water mass modification and mesoscale energy dissipation in flow systems throughout the World Ocean. Dense gravity currents forced by surface buoyancy loss over shallow shelves are a particularly compelling test case, as they are characterized by density fronts and shears susceptible to a wide range of submesoscale instabilities. We present idealized experiments of Arctic shelf overflows employing the GFDL-MOM6 in z* and isopycnal coordinates. At the highest resolutions, the dense flow undergoes geostrophic adjustment and forms bottom- and surface-intensified jets. The density front along the topography combined with geostrophic shear initiates SI, leading to onset of secondary shear instability, dissipation of geostrophic energy, and turbulent mixing. We explore the impact of vertical coordinate, resolution, and parameterization of shear-driven mixing on the representation of water mass transformation. We find that in isopycnal and low-resolution z* simulations, limited vertical resolution leads to inadequate representation of diapycnal mixing. This motivates our development of a parameterization for SI-driven turbulence. The parameterization is based on identifying unstable regions through a balanced Richardson number criterion and slumping isopycnals toward a balanced state. The potential energy extracted from the large-scale flow is assumed to correspond to the kinetic energy of SI which is dissipated through shear mixing. Parameterizing submesoscale instabilities by combining isopycnal slumping with diapycnal mixing becomes crucial as ocean models move toward resolving mesoscale eddies and fronts but not the submesoscale phenomena they host.
We document the configuration and emergent simulation features from the Geophysical Fluid Dynamics Laboratory (GFDL) OM4.0 ocean/sea‐ice model. OM4 serves as the ocean/sea‐ice component for the GFDL climate and Earth system models. It is also used for climate science research and is contributing to the Coupled Model Intercomparison Project version 6 Ocean Model Intercomparison Project (CMIP6/OMIP). The ocean component of OM4 uses version 6 of the Modular Ocean Model (MOM6) and the sea‐ice component uses version 2 of the Sea Ice Simulator (SIS2), which have identical horizontal grid layouts (Arakawa C‐grid). We follow the Coordinated Ocean‐sea ice Reference Experiments (CORE) protocol to assess simulation quality across a broad suite of climate relevant features. We present results from two versions differing by horizontal grid spacing and physical parameterizations: OM4p5 has nominal 0.5° spacing and includes mesoscale eddy parameterizations and OM4p25 has nominal 0.25° spacing with no mesoscale eddy parameterization.
MOM6 makes use of a vertical Lagrangian‐remap algorithm that enables general vertical coordinates. We show that use of a hybrid depth‐isopycnal coordinate reduces the mid‐depth ocean warming drift commonly found in pure z* vertical coordinate ocean models. To test the need for the mesoscale eddy parameterization used in OM4p5, we examine the results from a simulation that removes the eddy parameterization. The water mass structure and model drift are physically degraded relative to OM4p5, thus supporting the key role for a mesoscale closure at this resolution.
We revisit the challenges and prospects for ocean circulation models following Griffies et al. (2010). Over the past decade, ocean circulation models evolved through improved understanding, numerics, spatial discretization, grid configurations, parameterizations, data assimilation, environmental monitoring, and process-level observations and modeling. Important large scale applications over the last decade are simulations of the Southern Ocean, the Meridional Overturning Circulation and its variability, and regional sea level change. Submesoscale variability is now routinely resolved in process models and permitted in a few global models, and submesoscale effects are parameterized in most global models. The scales where nonhydrostatic effects become important are beginning to be resolved in regional and process models. Coupling to sea ice, ice shelves, and high-resolution atmospheric models has stimulated new ideas and driven improvements in numerics. Observations have provided insight into turbulence and mixing around the globe and its consequences are assessed through perturbed physics models. Relatedly, parameterizations of the mixing and overturning processes in boundary layers and the ocean interior have improved. New diagnostics being used for evaluating models alongside present and novel observations are briefly referenced. The overall goal is summarizing new developments in ocean modeling, including: how new and existing observations can be used, what modeling challenges remain, and how simulations can be used to support observations.
Naveira Garabato, Alberto C., Eleanor E Frajka-Williams, Carl P Spingys, Sonya Legg, Kurt L Polzin, A Forryan, E Povl Abrahamsen, Christian E Buckingham, and Stephen M Griffies, July 2019: Rapid mixing and exchange of deep-ocean waters in an abyssal boundary current. Proceedings of the National Academy of Sciences, 116(27), DOI:10.1073/pnas.1904087116. Abstract
The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Here, we use innovative observations of a major branch of the overturning circulation—an abyssal boundary current in the Southern Ocean—to identify a previously undocumented mixing mechanism, by which deep-ocean waters are efficiently laundered through intensified near-boundary turbulence and boundary–interior exchange. The linchpin of the mechanism is the generation of submesoscale dynamical instabilities by the flow of deep-ocean waters along a steep topographic boundary. As the conditions conducive to this mode of mixing are common to many abyssal boundary currents, our findings highlight an imperative for its representation in models of oceanic overturning.
In this study we revisit the problem of rotating dense overflow dynamics by performing nonhydrostatic numerical simulations, resolving submesoscale variability. Thermohaline stratification and buoyancy forcing are based on data from the Eurasian Basin of the Arctic Ocean, where overflows are particularly crucial to exchange of dense water between shelves and deep basins, yet relatively little studied. A nonlinear equation of state is used, allowing proper representation of thermohaline structure and mixing. We examine three increasingly complex scenarios: nonrotating 2D, rotating 2D, and rotating 3D. The nonrotating 2D case behaves according to known theory – the gravity current descends alongslope until reaching a relatively shallow neutral buoyancy level. However, in the rotating cases we have identified novel dynamics: in both 2D and 3D the submesoscale range is dominated by symmetric instability (SI). Rotation leads to geostrophic adjustment, causing dense water to be confined within the forcing region longer and attain a greater density anomaly. In the 2D case, Ekman drainage leads to descent of the geostrophic jet, forming a highly dense alongslope front. Beams of negative Ertel potential vorticity develop parallel to the slope, initiating SI and vigorous mixing in the overflow. In 3D, baroclinic eddies are responsible for cross-isobath dense water transport but SI again develops along the slope and at eddy edges. Remarkably, through two different dynamics the 2D SI-dominated case and 3D eddy-dominated case attain roughly the same final water mass distribution, highlighting the potential role of SI in driving mixing within certain regimes of dense overflows.
Mouw, Colleen, S Clem, Sonya Legg, and J Stockard, December 2018: Meeting Mentoring Needs in Physical Oceanography: An Evaluation of the Impact of MPOWIR. Oceanography, 31(4), DOI:10.5670/oceanog.2018.405. Abstract
After a decade of program offerings, the Mentoring Physical Oceanography Women to Increase Retention (MPOWIR) program initiated a community-wide survey to (1) assess the impact MPOWIR has had on retention of women in the field of physical oceanography, and (2) gauge where needs are being met and where gaps still exist. To investigate the impact of MPOWIR, we compare MPOWIR participants with male and female cohorts that did not participate in MPOWIR but were at a similar career stage. The survey results indicate MPOWIR has had a substantial impact by aiding individuals in finding and developing mentoring relationships. MPOWIR women are far more likely to have a mentor, and they report having mentors in addition to their advisors, indicating proactive seeking of mentoring relationships. Survey results identify many unmet mentoring needs for both men and women, but MPOWIR participants appear to be receiving more from their mentoring relationships than their non-MPOWIR cohorts. The majority of survey respondents reported there were challenges to achieving career goals, but MPOWIR participants were significantly more likely to have attained their career goals, even though they had received their PhDs more recently. Eighty-eight percent of survey respondents with PhDs were employed in oceanography, irrespective of participation in MPOWIR. MPOWIR women indicate the program has had a large impact on their lives, with the greatest effect being expansion of professional networks and exposure to professional development skills. Senior participants in the program (who serve as mentors to junior scientists) also reported significant professional and personal growth from being involved. Data obtained independently of the survey show that, of the 173 women who have participated in MPOWIR, the recent PhDs are predominantly in postdoctoral positions as expected, but for participants receiving their PhDs prior to 2012, an impressive 80% are in faculty or university/government/nonprofit research positions. Thus, MPOWIR appears to have had an important impact on retention and career satisfaction of its participants.
Wang, He, Sonya Legg, and Robert Hallberg, July 2018: The Effect of Arctic Freshwater Pathways on North Atlantic Convection and the Atlantic Meridional Overturning Circulation. Journal of Climate, 31(13), DOI:10.1175/JCLI-D-17-0629.1. Abstract
This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic Meridional Overturning Circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea and the Nordic Seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean state climate.
MacKinnon, J A., M H Alford, Joseph K Ansong, Brian K Arbic, A Barna, B P Briegleb, F O Bryan, Maarten C Buijsman, Eric P Chassignet, Gokhan Danabasoglu, S Diggs, Stephen M Griffies, Robert Hallberg, S R Jayne, M Jochum, J Klymak, E Kunze, William G Large, Sonya Legg, B Mater, and Angelique Melet, et al., November 2017: Climate Process Team on Internal-Wave Driven Ocean Mixing. Bulletin of the American Meteorological Society, 98(11), DOI:10.1175/BAMS-D-16-0030.1. Abstract
Recent advances in our understanding of internal-wave driven turbulent mixing in the ocean interior are summarized. New parameterizations for global climate ocean models, and their climate impacts, are introduced.
Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.
Nazarian, Robert, and Sonya Legg, October 2017: Internal Wave Scattering in Continental Slope Canyons, Part 1: Theory and Development of a Ray Tracing Algorithm. Ocean Modelling, 118, DOI:10.1016/j.ocemod.2017.07.002. Abstract
When internal waves interact with topography, such as continental slopes, they can transfer wave energy to local dissipation and diapycnal mixing. Submarine canyons comprise approximately ten percent of global continental slopes, and can enhance the local dissipation of internal wave energy, yet parameterizations of canyon mixing processes are currently missing from large-scale ocean models. As a first step in the development of such parameterizations, we conduct a parameter space study of M2 tidal-frequency, low-mode internal waves interacting with idealized V-shaped canyon topographies. Specifically, we examine the effects of varying the canyon mouth width, shape and slope of the thalweg (line of lowest elevation). This effort is divided into two parts. In the first part, presented here, we extend the theory of 3-dimensional internal wave reflection to a rotated coordinate system aligned with our idealized V-shaped canyons. Based on the updated linear internal wave reflection solution that we derive, we construct a ray tracing algorithm which traces a large number of rays (the discrete analog of a continuous wave) into the canyon region where they can scatter off topography. Although a ray tracing approach has been employed in other studies, we have, for the first time, used ray tracing to calculate changes in wavenumber and ray density which, in turn, can be used to calculate the Froude number (a measure of the likelihood of instability). We show that for canyons of intermediate aspect ratio, large spatial envelopes of instability can form in the presence of supercritical sidewalls. Additionally, the canyon height and length can modulate the Froude number. The second part of this study, a diagnosis of internal wave scattering in continental slope canyons using both numerical simulations and this ray tracing algorithm, and test of robustness of the ray tracing, is presented in the companion article.
Nazarian, Robert, and Sonya Legg, October 2017: Internal Wave Scattering in Continental Slope Canyons, Part 2: A Comparison of Ray Tracing and Numerical Simulations. Ocean Modelling, 118, DOI:10.1016/j.ocemod.2017.07.005. Abstract
When internal waves interact with topography, such as continental slopes, they can transfer wave energy to local dissipation and diapycnal mixing. Submarine canyons comprise approximately ten percent of global continental slopes, and can enhance the local dissipation of internal wave energy, yet parameterizations of canyon mixing processes are currently missing from large-scale ocean models. As a first step in the development of such parameterizations, we conduct a parameter space study of M2 tidal-frequency, low-mode internal waves interacting with idealized V-shaped canyon topographies. Specifically, we examine the effect of varying the canyon mouth width, shape and slope of the thalweg (line of lowest elevation) (i.e. flat bottom or near-critical slope). In Part 1 of this study (Nazarian and Legg, 2017a), we developed a ray tracing algorithm and used it to estimate how canyons can increase the wave Froude number, by increasing energy density and increasing vertical wavenumber. Here in Part 2 we examine the internal wave scattering in continental slope canyons using numerical simulations, and compare the results with the linear ray tracing predictions. We find that at intermediate canyon widths, a large fraction of incoming wave energy can be dissipated, which can be explained as a consequence of the increase in ray density and, for near-critical slope canyons, increase in vertical wave number, which leads to lower Richardson number followed by instability. Relative to a steep continental slope without a canyon, we find that V-shaped flat bottom canyons always dissipate more energy and are an effective geometry for wave trapping and subsequent energy loss. When both flat bottom canyons and near-critical slope canyons are made narrower, less wave energy enters the canyon, but a larger fraction of that energy is lost to dissipation due to subsequent reflections and wave trapping. There is agreement between the diagnostics calculated from the numerical model and the linear ray tracing, lending support for the use of linear theory to understand the fundamental dynamics of internal wave scattering in canyons.
Ummenhofer, C, A Subramanian, and Sonya Legg, November 2017: Maintaining Momentum in Climate Model Development. EOS, 98, DOI:10.1029/2017EO086501.
Yi, Y R., Sonya Legg, and Robert Nazarian, October 2017: The Impact of Topographic Steepness on Tidal Dissipation at Bumpy Topography. Fluids, 2(4), 55, DOI:10.3390/fluids2040055. Abstract
Breaking internal waves are an important contributor to mixing in the stratified ocean interior. We use two-dimensional, nonhydrostatic numerical simulations to examine the breaking of internal waves generated by tidal flow over sinusoidal bottom topography. We explore the sensitivity of the internal wave breaking to the topographic steepness and Coriolis frequency, focusing on the vertical structure of kinetic energy dissipation and the ratio of local dissipation to the barotropic-to-baroclinic energy conversion. When the tidal frequency is twice the local Coriolis frequency, wave breaking above the topography is driven by wave–wave interactions which transfer wave energy from the tidal forcing frequency to the inertial frequency. The greater shear associated with the inertial frequency waves leads to enhanced dissipation in a thick layer above the topography. The topographic steepness strongly modulates this dependence of dissipation on Coriolis frequency; for some steep sinusoidal topographies, most wave energy propagates downward into the topographic troughs, eliminating the possibility for significant breaking above the topographic peaks. Current parameterizations of tidal dissipation in use in global ocean models need to be adapted to include the dependence of the local dissipation on both the Coriolis frequency and the topographic steepness.
Turbulent mixing driven by breaking internal tides plays a primary role in the meridional overturning and oceanic heat budget. Most current climate models explicitly parameterize only the local dissipation of internal tides at the generation sites, representing the remote dissipation of low-mode internal tides which propagate away through a uniform background diffusivity. In this study, a simple energetically-consistent parameterization of the low-mode internal tide dissipation is derived and implemented in Geophysical Fluid Dynamics Laboratory’s ESM2G Earth System Model. The impact of remote and local internal tide dissipation on the ocean state is examined using a series of simulations with the same total amount of energy input for mixing, but with different scalings of the vertical profile of dissipation with the stratification and with different idealized scenarios for the distribution of the low-mode internal tide energy dissipation: uniformly over ocean basins, continental slopes, or continental shelves. In these idealized scenarios, the ocean state, including the meridional overturning circulation, ocean ventilation, main thermocline thickness and ocean heat uptake, is particularly sensitive to the vertical distribution of mixing by breaking low-mode internal tides. Less sensitivity is found to the horizontal distribution of mixing, provided that distribution is in the open ocean. Mixing on coastal shelves only impacts the large-scale circulation and water mass properties where it modifies water masses originating on shelves. More complete descriptions of the distribution of the remote part of internal-tide driven mixing, particularly in the vertical and relative to water mass formation regions, are therefore required to fully parameterize ocean turbulent mixing.
Subramanian, A, C Ummenhofer, A Giannini, Marika M Holland, Sonya Legg, A Mahadevan, D Perovich, J Small, João Teixeira, and L Thompson, August 2016: Translating process understanding to improve climate models, A US CLIVAR white paper, Report 2016-3, DOI:10.5065/D63X851Q 48pp. Abstract
Turnewitsch, R, Matthew Dumont, K Kiriakoulakis, and Sonya Legg, et al., December 2016: Tidal influence on particulate organic carbon export fluxes around a tall seamount. Progress in Oceanography, 149, DOI:10.1016/j.pocean.2016.10.009. Abstract
As tall seamounts may be ‘stepping stones’ for dispersion and migration of deep open ocean fauna, an improved understanding of the productivity at and food supply to such systems needs to be formed. Here, the 234Th/238U approach for tracing settling particulate matter was applied to Senghor Seamount – a tall sub-marine mountain near the tropical Cape Verde archipelago – in order to elucidate the effects of topographically-influenced physical flow regimes on the export flux of particulate organic carbon (POC) from the near-surface (topmost ⩽ 100 m) into deeper waters. The comparison of a suitable reference site and the seamount sites revealed that POC export at the seamount sites was ∼2–4 times higher than at the reference site. For three out of five seamount sites, the calculated POC export fluxes are likely to be underestimates. If this is taken into account, it can be concluded that POC export fluxes increase while the passing waters are advected around and over the seamount, with the highest export fluxes occurring on the downstream side of the seamount. This supports the view that biogeochemical and biological effects of tall seamounts in surface-ocean waters might be strongest at some downstream distance from, rather than centred around, the seamount summit. Based on measured (vessel-mounted ADCP) and modelled (regional flow field: AVISO; internal tides at Senghor: MITgcm) flow dynamics, it is proposed that tidally generated internal waves result in a ‘screen’ of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased POC export above the seamount: (1) sudden increased upward transport of nutrients into the euphotic zone, driving brief pulses of primary production of new particulate matter, followed by the particles’ export into deeper waters; and (2) pulses of increased shear-driven aggregation of smaller, slower-settling into larger, faster-settling particles. This study shows that, under certain conditions, there can be an effect of a tall seamount on aspects of surface-ocean biogeochemistry, with tidal dynamics playing a prominent role. It is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic-pelagic communities at and near tall seamounts.
Alford, M H., S L Peacock, J A MacKinnon, J D Nash, Maarten C Buijsman, L R Centuroni, Shenn-Yu Chao, Ming-Huei Chang, D M Farmer, O B Fringer, Ke-Hsien Fu, P C Gallacher, H C Graber, K R Helfrich, S M Jachec, C R Jackson, J Klymak, D-S Ko, T M Shaun Johnston, and Sonya Legg, et al., May 2015: The formation and fate of internal waves in the South China Sea. Nature, 521(7550), DOI:10.1038/nature14399. Abstract
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6, 7. For over a decade, studies8, 9, 10, 11 have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
Internal lee waves generated by geostrophic flows over rough topography are thought to be a significant energy sink for eddies and energy source for deep ocean mixing. The sensitivity of the energy flux into lee waves from pre-industrial, present and possible future climate conditions is explored in this study using linear theory. The bottom stratification and geostrophic velocity fields needed for the calculation of the energy flux into lee waves are provided by Geophysical Fluid Dynamics Laboratory’s global coupled carbon-climate Earth System Model, ESM2G. The unresolved mesoscale eddy energy is parameterized as a function of the large-scale available potential energy. Simulations using historical and Representative Concentration Pathway (RCP) scenarios were performed over the 1861-2200 period. Our diagnostics suggest a decrease of the global energy flux into lee waves of order 20% from pre-industrial to future climate conditions under the RCP8.5 scenario. In the Southern Ocean, the energy flux into lee waves exhibits a clear annual cycle with maximum values in austral winter. The long-term decrease of the global energy flux into lee waves and the annual cycle of the energy flux in the Southern Ocean are mostly due to changes in bottom velocity.
The sensitivity of large scale ocean circulation and climate to overflow representation is studied using coupled climate models, motivated by the differences between two models differing only in their ocean components: CM2G (which uses an isopycnal–coordinate ocean model) and CM2M (which uses a z-coordinate ocean model). Analysis of the control simulations of the two models shows that the Atlantic Meridional Overturning Circulation (AMOC) and the North Atlantic climate have some differences, which may be related to the representation of overflow processes. Firstly, in CM2G, as in the real world, overflows have two branches flowing out of the Nordic Seas, to the east and west of Iceland, respectively, while only the western branch is present in CM2M. This difference in overflow location results in different horizontal circulation in the North Atlantic. Secondly, the diapycnal mixing in the overflow downstream region is much larger in CM2M than in CM2G, which affects the entrainment and product water properties. Two sensitivity experiments are conducted in CM2G to isolate the effect of these two model differences: in the first experiment, the outlet of the eastern branch of the overflow is blocked, and the North Atlantic horizontal circulation is modified due to the absence of the eastern branch of the overflow, although the AMOC has little change; in the second experiment, the diapycnal mixing downstream of the overflow is enhanced, resulting in changes in the structure and magnitude of the AMOC.
Buijsman, Maarten C., J Klymak, and Sonya Legg, et al., March 2014: Three Dimensional Double Ridge Internal Tide Resonance in Luzon Strait. Journal of Physical Oceanography, 44(3), DOI:10.1175/JPO-D-13-024.1. Abstract
The three-dimensional (3D) double ridge internal tide interference in Luzon Strait in the South China Sea is examined by comparing 3D and 2D (two-dimensional) realistic simulations. Both the 3D simulations and observations indicate the presence of 3D first-mode (semi)diurnal standing waves in the 3.6 km deep trench in the Strait. As in an earlier 2D study, barotropic to baroclinic energy conversion, flux divergence, and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides. The resonance in the 3D simulation is several times stronger than the 2D simulations for the central Strait. Idealized experiments indicate that, in addition to ridge height, the resonance is only a function of separation distance, and not of the along ridge length, i.e. the enhanced resonance in 3D is not caused by 3D standing waves or basin modes. Instead, the difference in resonance between the 2D and 3D simulations is attributed to the topographic blocking of the barotropic flow by the 3D ridges, affecting wave generation, and a more constructive phasing between the remotely generated internal waves, arriving under oblique angles, and the barotropic tide. Most of the resonance occurs for the first mode. The contribution of the higher modes is reduced because of 3D radiation, multiple generation sites, scattering, and a rapid decay in amplitude away from the ridge.
Clem, S, Sonya Legg, S Lozier, and Colleen Mouw, December 2014: The Impact of MPOWIR: A Decade of Investing in Mentoring Women in Physical Oceanography. Oceanography, 27(4), DOI:10.5670/oceanog.2014.113. Abstract
MPOWIR (Mentoring Physical Oceano-graphy Women to Increase Retention) is a US community-initiated and community-led mentoring program aimed at improving the retention of women physical oceanographers in academic and/or research positions. This article describes the MPOWIR program elements designed by the US physical oceanography community, quantifies the participation in these programs, describes MPOWIR's impact to date, and outlines future directions. An examination of surveys to date indicates that MPOWIR, several years after its implementation, is having a positive impact on the retention of junior women in physical oceanography, primarily by giving them a broad professional network and focused mentoring.
We propose a new framework for parameterization of ocean convection processes. The new framework is termed “patchy convection” since our aim is to represent the heterogeneity of mixing processes that take place within the horizontal scope of a grid cell. We focus on applying this new scheme to represent the effect of pre-conditioning for deep convection by subgrid scale eddy variability. The nw parameterization separates the grid-cell into two regions of different stratification, applies convective mixing separately to each region, and then recombines the density profile to produce the grid-cell mean density profile. The scheme depends on two parameters: the areal fraction of the vertically-mixed region within the horizontal grid cell, and the density difference between the mean and the unstratified profiles at the surface. We parameterize this density difference in terms of an unresolved eddy kinetic energy. We illustrate the patchy parameterization using a 1D idealized convection case before evaluating the scheme in two different global ocean-ice simulations with prescribed atmospheric forcing; i) diagnosed eddy velocity field applied only in the Labrador Sea ii) diagnosed global eddy velocity field. The global simulation results indicate that the patchy convection scheme improves the warm biases in the deep Atlantic Ocean and Southern Ocean. This proof-of-concept study is a first step in developing the patchy parameterization scheme, which will be extended in future to use a prognostic eddy field as well as to parameterize convection due to under-ice brine rejection.
Legg, Sonya, January 2014: Scattering of low-mode internal waves at finite isolated topography. Journal of Physical Oceanography, 44(1), DOI:10.1175/JPO-D-12-0241.1. Abstract
A series of two-dimensional numerical simulations examine the breaking of first mode internal waves at isolated ridges, independently varying the relative height of the topography compared to the depth of the ocean, h0/H0; the relative steepness of the topographic slope, compared to the slope of the internal wave group velocity, γ; and the Froude number of the incoming internal wave, Fr0. The fraction of the incoming wave energy which is reflected back toward deep water, transmitted beyond the ridge, and lost to dissipation and mixing, are diagnosed from the simulations. For critical slopes, with γ = 1, the fraction of incoming energy lost at the slope scales approximately like h0/H0, independent of incoming wave Froude number. For subcritical slopes, with γ < 1, waves break and lose a substantial proportion of their energy if the maximum Froude number, estimated as Frmax = Fr0/(1 − h0/H0)2, exceeds a critical value, found empirically to be about 0.3. The dissipation at subcritical slopes therefore increases as both incoming wave Froude number and topographic height increase. At critical slopes, the dissipation is enhanced along the slope facing the incoming wave. In contrast, at subcritical slopes, dissipation is small until the wave amplitude is sufficiently enhanced by the shoaling topography to exceed the critical Froude number; then large dissipation extends all the way to the surface. The results are shown to generalize to variable stratification and different topographies, including axisymmetric seamounts. The regimes for low-mode internal wave-breaking at isolated critical and subcritical topography identified by these simulations provide guidance for the parameterization of the mixing due to radiated internal tides.
Diapycnal mixing plays a key role in maintaining the ocean stratification and meridional overturning circulation (MOC). In the ocean interior, it is mainly sustained by breaking internal waves. Two important classes of internal waves are internal tides and lee waves, respectively generated by barotropic tides and geostrophic flows interacting with rough topography. Currently, regarding internal-wave driven mixing, most climate models only explicitly parameterize the local dissipation of internal tides. In this study, we explore the combined effects of internal tide and lee wave driven mixing on the ocean state. We perform a series of sensitivity experiments using the CM2G ocean-ice-atmosphere coupled model, including a parameterization of lee wave driven mixing using a recent estimate for the global map of energy conversion into lee waves, in addition to the tidal mixing parameterization. We show that although the global energy input in the deep ocean into lee waves (0.2 TW) is small compared to that into internal tides (1.4 TW), lee wave driven mixing makes a significant impact on the ocean state, notably on the ocean thermal structure and stratification, as well as on the MOC. The vertically-integrated circulation is also impacted in the Southern Ocean, which accounts for half the lee wave energy flux. Finally, we show that the different spatial distribution of the internal tide and lee wave energy input impacts the sensitivity described in this study. These results suggest that lee wave driven mixing should be parameterized in climate models, preferably using more physically-based parameterizations that allow the internal lee wave driven mixing to evolve in a changing ocean.
Klymak, J, Maarten C Buijsman, Sonya Legg, and R Pinkel, July 2013: Parameterizing surface and internal tide scattering and breaking on supercritical topography: the one- and two-ridge cases. Journal of Physical Oceanography, 43(7), DOI:10.1175/JPO-D-12-061.1. Abstract
A parameterization is presented for turbulence dissipation due to internal tides generated at and impinging upon topography steep enough to be “supercritical†with respect to the tide. The parameterization requires knowledge of the topography, stratification, and the remote forcing, either barotropic or baroclinic. Internal modes that are arrested at the crest of the topography are assumed to dissipate, and faster modes assumed to propagate away. The energy flux into each mode is predicted using a knife-edge topography that allows linear numerical solutions. The parameterization is tested using high-resolution two-dimensional numerical models of barotropic and internal tides impinging on an isolated ridge, and for the generation problem on a two-ridge system. The recipe is seen to work well compared to numerical simulations of isolated ridges, so long as the ridge has a slope steeper than twice the critical steepness. For less steeply sloped ridges, near-critical generation becomes more dominant. For the two-ridge case, the recipe works well when compared to numerical model runs with very thin ridges. However, as the ridges are widened, even by a small amount, the recipe does poorly in an unspecified manner, because the linear response at high modes becomes compromised as it interacts with the slopes.
Melet, Angelique, Robert Hallberg, Sonya Legg, and Kurt L Polzin, March 2013: Sensitivity of the Ocean State to the Vertical Distribution of Internal-Tide Driven Mixing. Journal of Physical Oceanography, 43(3), DOI:10.1175/JPO-D-12-055.1. Abstract
The ocean interior stratification and meridional overturning circulation are largely sustained by diapycnal mixing. The breaking of internal tides is a major source of diapycnal mixing. Many recent climate models parameterize internal-tide breaking using the scheme of St Laurent et al. (2002). While this parameterization dynamically accounts for internal-tide generation, the vertical distribution of the resultant mixing is ad hoc, prescribing energy dissipation to decay exponentially above the ocean bottom with a fixed length scale. Recently, Polzin (2009) formulated a dynamically based parameterization, in which the vertical profile of dissipation decays algebraically with a varying decay scale, accounting for variable stratification using WKB stretching. We compare two simulations using the St Laurent and Polzin formulations in the CM2G ocean-ice-atmosphere coupled model, with the same formulation for internal-tide energy input. Focusing mainly on the Pacific Ocean, where the deep low-frequency variability is relatively small, we show that the ocean state shows modest but robust and significant sensitivity to the vertical profile of internal-tide driven mixing. Therefore, not only the energy input to the internal tides matters, but also where in the vertical it is dissipated.
Buijsman, Maarten C., Sonya Legg, and J Klymak, August 2012: Double Ridge Internal Tide Interference and its Effect on Dissipation in Luzon Strait. Journal of Physical Oceanography, 42(8), DOI:10.1175/JPO-D-11-0210.1. Abstract
Luzon Strait between Taiwan and the Phillipines features two parallel north-south oriented ridges. The barotropic tides that propagate over these ridges cause strong internal waves and dissipation. The energy dissipation mechanisms and the role of the baroclinic wave fields in this dissipation are investigated using numerical simulations with the MITgcm. The model is integrated over two-dimensional configurations along a zonal transect at 20.6°N for a maximum duration of a spring-neap cycle. Nearly all dissipation occurs at the steep ridge crests due to high-mode turbulent lee waves with horizontal scales of several kilometers and vertical scales of hundreds of meters. The spatial structure and timing of the predicted velocities and dissipation agree with observations, and confirm the existence of these lee waves. The lee wave strength is greatly affected by the internal waves generated at the other ridge. When semidiurnal barotropic tides are dominant, the internal wave beams from both ridges nearly superpose after one surface reflection. The remotely generated internal waves from both ridges are therefore in phase with each other and the barotropic tides at the ridges. The barotropic to baroclinic energy conversion, energy flux divergence, ridge top velocities, and dissipation are stronger compared to the sum of the single east and west ridge cases. When diurnal tides are dominant, the wave fields are more out of phase, and the conversion, divergence, and the dissipation are less than or equal to the single ridge cases combined.
Klymak, J, Sonya Legg, M H Alford, Maarten C Buijsman, R Pinkel, and J D Nash, June 2012: The direct breaking of internal waves at steep topography. Oceanography, 25(2), DOI:10.5670/oceanog.2012.50. Abstract
Internal waves are often observed to break close to the seafloor topography that generates them, or from which they scatter. This breaking is often spectacular, with turbulent structures observed hundreds of meters above the seafloor, and driving turbulence dissipations and mixing up to 10,000 times open-ocean levels. This article provides an overview of efforts to observe and understand this turbulence, and to parameterize it near steep "supercritical" topography (i.e., topography that is steeper than internal wave energy characteristics). Using numerical models, we demonstrate that arrested lee waves are an important turbulence-producing phenomenon. Analogous to hydraulic jumps in water flowing over an obstacle in a stream, these waves are formed and then break during each tidal cycle. Similar lee waves are also observed in the atmosphere and in shallow fjords, but in those cases, their wavelengths are of similar scale to the topography, whereas in the ocean, they are small compared to the water depth and obstacle size. The simulations indicate that these nonlinear lee waves propagate against the generating flow (usually the tide) and are arrested because they have the same phase speed as the oncoming flow. This characteristic allows estimation of their size a priori and, using a linear model of internal tide generation, computation of how much energy they trap and turn into turbulence. This approach yields an accurate parameterization of mixing in numerical models, and these models are being used to guide a new generation of observations.
Coles, V, L Gerber, Sonya Legg, and S Lozier, June 2011: Mentoring Groups: A non-exit strategy for women in physical oceanography. Oceanography, 24(2), DOI:10.5670/oceanog.2011.43.
In this study, we investigate the dynamics of a dense gravity currents over different sizes of ridges and canyons. We employ a high resolution idealized isopycnal model and perform a large number of experiments changing the aspect ratio of a ridge/canyon, the Coriolis parameter, the reduced gravity, the background slope and initial overflow thickness. The control run (smooth topography) is in an eddy-regime and the frequencies of the eddies coincide with those of the Filchner overflow Darelius et al., 2009. Our idealized corrugation experiments show that corrugations steer the plume downslope, and that ridges are more effective than canyons in transporting the overflow to the deep ocean. We find that a corrugation Burger number (Buc) can be used as a parameter to describe the flow over topography. Buc is a combination of a Froude number and the aspect ratio. The maximum downslope transport of a corrugation can be increased when the height of the corrugation increases (Buc increases) or when the width of the corrugation decreases (Buc increases).
In addition, we propose a new parameterization of mixing as a function of Buc that can be used to account for unresolved shear in coarse resolution models. The new parameterization captures the increased local shear, thus increasing the turbulent kinetic energy and decreasing the gradient Richardson number. We find reasonable agreement in the overflow thickness and transport between the models with this parameterization and the high resolution models. We conclude that mixing effects of corrugations can be implemented as unresolved shear in an eddy diffusivity formulation and this parameterization can be used in coarse resolution models.
Nikurashin, M, and Sonya Legg, February 2011: A mechanism for local dissipation of internal tides generated at rough topography. Journal of Physical Oceanography, 41(2), DOI:10.1175/2010JPO4522.1. Abstract
Fine- and micro-structure observations indicate that turbulent mixing is enhanced within O(1) km above
rough topography. Enhanced mixing is associated with internal wave breaking and, in many regions of the
ocean, has been linked to the breaking and dissipation of internal tides. The generation and dissipation of
internal tides are explored in this study using a high-resolution two-dimensional nonhydrostatic numerical
model, which explicitly resolves the instabilities leading to wave breaking, configured in an idealized domain
with a realistic multiscale topography and flow characteristics. The control simulation, chosen to represent the
Brazil Basin region, produces a vertical profile of energy dissipation and temporal characteristics of finescale
motions that are consistent with observations. Results suggest that a significant fraction of mixing in the
bottom O(1) km of the ocean is sustained by the transfer of energy from the large-scale internal tides to
smaller-scale internal waves by nonlinear wave–wave interactions. The time scale of the energy transfer to
the smaller scales is estimated to be on the order of a few days. A suite of sensitivity experiments is carried
out to examine the dependence of the energy transfer time scale and energy dissipation on topographic
roughness, tidal amplitude, and Coriolis frequency parameters. Implications for tidal mixing parameterizations
are discussed.
We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to
enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements
of observational measures rendering information to help evaluate simulations and to guide development priorities.
http://www.oceanobs09.net/blog/?p=88
Breaking internal waves in the vicinity of topography can reach heights of over 100 m and are thought to enhance basin-wide energy dissipation and mixing in the ocean. The scales at which these waves are modelled often include the breaking of large waves (10 s of meters), but not the turbulence dissipation scales (centimeters). Previous approaches to parameterize the turbulence have been to use a universally large viscosity, or to use mixing schemes that rely on Richardson-number criteria.
A simple alternative is presented that enhances mixing and viscosity in the presence of breaking waves by assuming that dissipation is governed by the equivalence of the density overturning scales to the Ozmidov scale (, where LT is the size of the density overturns, and N the stratification). Eddy diffusivities and viscosities are related to the dissipation by the Osborn relation (Kz=ΓεN-2) to yield a simple parameterization , where Γ≈0.2 is the flux coefficient. This method is compared to previous schemes for flow over topography to show that, when eddy diffusivity and viscosity are assumed to be proportional, it dissipates the correct amount of energy, and that the dissipation reported by the mixing scheme is consistent with energy losses in the model. A significant advantage of this scheme is that it has no tunable parameters, apart from the turbulent Prandtl number and flux coefficient. A disadvantage is that the overturning scales of the turbulence must be relatively well-resolved.
Klymak, J, Sonya Legg, and R Pinkel, September 2010: A simple parameterization of turbulent tidal mixing near supercritical topography. Journal of Physical Oceanography, 40(9), DOI:10.1175/2010JPO4396.1. Abstract
A simple parameterization for tidal dissipation near supercritical topography, designed to be applied at deep mid-ocean ridges, is presented. In this parameterization, radiation of internal tides is quantified using a linear knife-edge model. Vertical internal wave modes that have non-rotating phase speeds slower than the tidal advection speed are assumed to dissipate locally, primarily due to hydraulic effects near the ridge crest. Evidence for high modes being dissipated is given in idealized numerical models of tidal flow over a Gaussian ridge. These idealized models also give guidance for where in the water column the predicted dissipation should be placed. The dissipation recipe holds if the Coriolis frequency f is varied so long as hN/W >> f, where N is the stratification, h the topographic height, and W a width scale. This parameterization is not applicable to shallower topography, which has significantly more dissipation as near-critical process dominate the observed turbulence. The parameterization compares well against simulations of tidal dissipation at the Kauai ridge, but predicts less dissipation than estimated from observations of the full Hawaiian ridge, perhaps due to unparameterized wave-wave interactions.
Klymak, J, Sonya Legg, and R Pinkel, February 2010: High-mode stationary waves in stratified flow over large obstacles. Journal of Fluid Mechanics, 644, DOI:10.1017/S0022112009992503. Abstract
Simulations of steady two-dimensional stratified flow over an isolated obstacle are presented where the obstacle is tall enough so that the topographic Froude number, Nhm/Uo ≫ 1. N is the buoyancy frequency, hm the height of the topography from the channel floor and Uo the flow speed infinitely far from the obstacle. As for moderate Nhm/Uo (~1), a columnar response propagates far up- and downstream, and an arrested lee wave forms at the topography. Upstream, most of the water beneath the crest is blocked, while the moving layer above the crest has a mean velocity Um = UoH/(H−hm). The vertical wavelength implied by this velocity scale, λo = 2πUm/N, predicts dominant vertical scales in the flow. Upstream of the crest there is an accelerated region of fluid approximately λo thick, above which there is a weakly oscillatory flow. Downstream the accelerated region is thicker and has less intense velocities. Similarly, the upstream lift of isopycnals is greatest in the first wavelength near the crest, and weaker above and below. Form drag on the obstacle is dominated by the blocked response, and not on the details of the lee wave, unlike flows with moderate Nhm/Uo.
Directly downstream, the lee wave that forms has a vertical wavelength given by λo, except for the deepest lobe which tends to be thicker. This wavelength is small relative to the fluid depth and topographic height, and has a horizontal phase speed cpx = −Um, corresponding to an arrested lee wave. When considering the spin-up to steady state, the speed of vertical propagation scales with the vertical component of group velocity cgz = αUm, where α is the aspect ratio of the topography. This implies a time scale = tNα/2π for the growth of the lee waves, and that steady state is attained more rapidly with steep topography than shallow, in contrast with linear theory, which does not depend on the aspect ratio.
Griffies, Stephen M., Alistair Adcroft, V Balaji, Robert Hallberg, Sonya Legg, Torge Martin, and Anna Pirani, et al., February 2009: Sampling Physical Ocean Field in WCRP CMIP5 Simulations: CLIVAR Working Group on Ocean Model Development (WGOMD) Committee on CMIP5 Ocean Model Output, International CLIVAR Project Office, CLIVAR Publication Series No. 137, 56pp. PDF
Legg, Sonya, Tal Ezer, Stephen M Griffies, Robert Hallberg, and L Jackson, et al., May 2009: Improving oceanic overflow representation in climate models: The gravity current entrainment climate process team. Bulletin of the American Meteorological Society, 90(5), DOI:10.1175/2008BAMS2667.1. Abstract
Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model developers with those conducting observational, numerical, and laboratory process studies of overflows. Here, the organization of the Climate Process Team is described, and a few of the successes and lessons learned during this collaboration are highlighted, with some emphasis on the well-observed Mediterranean overflow. The Climate Process Team has developed several different overflow parameterizations, which are examined in a hierarchy of ocean models, from comparatively well-resolved regional models to the largest-scale global climate models.
Green, J A M., J H Simpson, Sonya Legg, and M R Palmer, 2008: Internal waves, baroclinic energy fluxes and mixing at the European shelf edge. Continental Shelf Research, 28(7), DOI:10.1016/j.csr.2008.01.014. Abstract
The energy flux in internal waves generated at
the Celtic Sea shelf break was estimated by (i) applying perturbation theory
to a week-long dataset from a mooring at 200 m depth, and (ii) using a 2D
non-hydrostatic circulation model over the shelf break. The dataset
consisted of high resolution time-series of currents and vertical
stratification together with two 25-h sets of vertical profiles of the
dissipation of turbulent kinetic energy. The observations indicated an
average energy flux of 139 W m−1, travelling along the shelf
break towards the northwest. The average energy flux across the shelf break
at the mooring was only 8 W m−1. However, the waves propagating
onshelf transported up to 200 W m−1, but they were only present
51% of the time. A comparison between the divergence of the baroclinic
energy flux and observed dissipation within the seasonal thermocline at the
mooring showed that the dissipation was at least one order of magnitude
larger. Results from a 2D model along a transect perpendicular to the shelf
break showed a time-averaged onshelf energy flux of 153–425 W m−1,
depending on the magnitude of the barotropic forcing. A divergence zone of
the energy flux was found a few kilometre offshore of the location of the
observations in the model results, and fluxes on the order of several kW m−1
were present in the deep waters further offshelf from the divergence zone.
The modelled fluxes exhibited qualitative agreements with the phase and
hourly onshelf magnitudes of the observed energy fluxes. Both the
observations and the model results show an intermittent onshelf energy flux
of 100–200 W m−1, but these waves could only propagate
20–30 km
onshore before dissipating. This conclusion was supported by a 25-h dataset
sampled some 180 km onto the shelf, where a weak wave energy flux was found
going towards the shelf break. We therefore conclude that shelf break
generated internal waves are unlikely to be the main source of energy for
mixing on the inner part of the shelf.
This paper presents a new parameterization for shear-driven, stratified, turbulent mixing that is pertinent to climate models, in particular the shear-driven mixing in overflows and the Equatorial Undercurrent. This parameterization satisfies a critical requirement for climate applications by being simple enough to be implemented implicitly and thereby allowing the parameterization to be used with time steps that are long compared to both the time scale on which the turbulence evolves and the time scale with which it alters the large-scale ocean state.
The mixing is expressed in terms of a turbulent diffusivity that is dependent on the shear forcing and a length scale that is the minimum of the width of the low Richardson number region (Ri = N2/|uz|2, where N is the buoyancy frequency and |uz| is the vertical shear) and the buoyancy length scale over which the turbulence decays [Lb = Q1/2/N, where Q is the turbulent kinetic energy (TKE)]. This also allows a decay of turbulence vertically away from the low Richardson number region over the buoyancy scale, a process that the results show is important for mixing across a jet. The diffusivity is determined by solving a vertically nonlocal steady-state TKE equation and a vertically elliptic equilibrium equation for the diffusivity itself.
High-resolution nonhydrostatic simulations of shear-driven stratified mixing are conducted in both a shear layer and a jet. The results of these simulations support the theory presented and are used, together with discussions of various limits and reviews of previous work, to constrain parameters.
Legg, Sonya, L Jackson, and Robert Hallberg, 2008: Eddy-resolving modeling of overflows In Ocean Modeling in an Eddying Regime, Geophysical Monograph 177, M. W. Hecht, and H. Hasumi, eds., Washington, DC, American Geophysical Union, 63-82.
Legg, Sonya, and J Klymak, September 2008: Internal hydraulic jumps and overturning generated by tidal flow over a tall steep ridge. Journal of Physical Oceanography, 38(9), DOI:10.1175/2008JPO3777.1. Abstract
Recent observations from the Hawaiian Ridge indicate episodes of overturning and strong dissipation coupled with the tidal cycle near the top of the ridge. Simulations with realistic topography and stratification suggest that this overturning has its origins in transient internal hydraulic jumps that occur below the shelf break at maximum ebb tide, and then propagate up the slope as internal bores when the flow reverses. A series of numerical simulations explores the parameter space of topographic slope, barotropic velocity, stratification, and forcing frequency to identify the parameter regime in which these internal jumps are possible. Theoretical analysis predicts that the tidally driven jumps may occur when the vertical tidal excursion is large, which is shown to imply steep topographic slopes, such that dh/dxN/ω > 1. The vertical length scale of the jumps is predicted to depend on the flow speed such that the jump Froude number is of order unity. The numerical results agree with the theoretical predictions, with finite-amplitude internal hydraulic jumps and overturning forming during strong offslope tidal flow over steep slopes. These results suggest that internal hydraulic jumps may be an important mechanism for local tidally generated mixing at tall steep topography.
The work presented in this paper is part of an effort to understand and improve the representation of overflows in large scale, coarse resolution ocean climate models. To this end we developed a regional model of the Faroe Bank Channel overflow using the MITgcm (Massachusetts Institute of Technology General Circulation Model), a typical global ocean model using discrete levels as the vertical co-ordinate. In order to isolate the numerical diffusion resulting from the advection of tracers, the model is run without any turbulence closure schemes, without convective adjustment or any other physically based parameterization of mixing. Comparison between the model results and recent observations of the Faroe Bank Channel plume allows assessment of the model performance, including its ability to correctly represent the mixing and the downslope transport in the plume. It is found that at the highest resolution used in this paper (2.5 km – horizontal and 25 m – vertical) the structure of the modeled plume and the magnitude of the entrainment is comparable to the observed plume.
The dependence of the mixing on various model parameters, such as vertical and horizontal resolution, vertical viscosity, drag coefficient and inflow forcing, is tested extensively. The numerical mixing in the model is found to be most sensitive to changes in the horizontal resolution, and to a lesser extent on vertical resolution and vertical viscosity. The inflow forcing and drag coefficient show only a very minor effect on the mixing.
The results presented in the paper identify the shortcomings of the model at coarser resolutions which need to be addressed when attempting to represent such overflows realistically in large scale climate and ocean models.
A series of idealised numerical simulations of dense water flowing down a broad uniform slope are presented, employing both a z-coordinate model (the MIT general circulation model) and an isopycnal coordinate model (the Hallberg Isopycnal Model). Calculations are carried out at several different horizontal and vertical resolutions, and for a range of physical parameters. A subset of calculations are carried out at very high resolution using the non-hydrostatic variant of the MITgcm. In all calculations dense water descends the slope while entraining and mixing with ambient fluid. The dependence of entrainment, mixing and down-slope descent on resolution and vertical coordinate are assessed. At very coarse resolutions the z-coordinate model generates excessive spurious mixing, and dense water has difficulty descending the slope. However, at intermediate resolutions the mixing in the z-coordinate model is less than found in the high-resolution non-hydrostatic simulations, and dense water descends further down the slope. Isopycnal calculations show less resolution dependence, although entrainment and mixing are both reduced slightly at coarser resolution. At intermediate resolutions the z-coordinate and isopycnal models produce similar levels of mixing and entrainment. These results provide a benchmark against which future developments in overflow entrainment parameterizations in both z-coordinate and isopycnal models may be compared.
Legg, Sonya, and K M H Huijts, 2006: Preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography. Deep-Sea Research, Part II, 53(1-2), DOI:10.1016/j.dsr2.2005.09.014. Abstract
Much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean. Here the process of internal-tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using two-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters. While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency. Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography.
