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Ramsey Harcourt

Principal Oceanographer





Research Interests

Large Eddy Simulation (LES), Computational Fluid Dynamics, Deep Convection, Wave and Ice Boundary Layers, Response of Drifters to Convection

Department Affiliation

Ocean Physics


2000-present and while at APL-UW

Northern Arabian Sea Circulation-Autonomous Research (NASCar): A research initiative based on autonomous sensors

Centurioni, L.R., and 33 others, including R.R. Harcourt, C.M. Lee, L. Rainville, and A.Y. Shcherbina, "Northern Arabian Sea Circulation-Autonomous Research (NASCar): A research initiative based on autonomous sensors," Oceanography, 30, 74-87, doi:10.5670/oceanog.2017.224, 2017.

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1 Jun 2017

The Arabian Sea circulation is forced by strong monsoonal winds and is characterized by vigorous seasonally reversing currents, extreme differences in sea surface salinity, localized substantial upwelling, and widespread submesoscale thermohaline structures. Its complicated sea surface temperature patterns are important for the onset and evolution of the Asian monsoon. This article describes a program that aims to elucidate the role of upper-ocean processes and atmospheric feedbacks in setting the sea surface temperature properties of the region. The wide range of spatial and temporal scales and the difficulty of accessing much of the region with ships due to piracy motivated a novel approach based on state-of-the-art autonomous ocean sensors and platforms. The extensive data set that is being collected, combined with numerical models and remote sensing data, confirms the role of planetary waves in the reversal of the Somali Current system. These data also document the fast response of the upper equatorial ocean to monsoon winds through changes in temperature and salinity and the connectivity of the surface currents across the northern Indian Ocean. New observations of thermohaline interleaving structures and mixing in setting the surface temperature properties of the northern Arabian Sea are also discussed.

On the role of sea-state in bubble-mediated air-sea gas flux during a winter storm

Liang, J.-H., S.R. Emerson, E.A. D'Asaro, C.L. McNeil, R.R. Harcourt, P.P. Sullivan, B. Yang, and M.F. Cronin, "On the role of sea-state in bubble-mediated air-sea gas flux during a winter storm," J. Geophys. Res., 122, 2671-2685, doi:10.1002/2016JC012408, 2017.

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1 Apr 2017

Oceanic bubbles play an important role in the air-sea exchange of weakly soluble gases at moderate to high wind speeds. A Lagrangian bubble model embedded in a large eddy simulation model is developed to study bubbles and their influence on dissolved gases in the upper ocean. The transient evolution of mixed-layer dissolved oxygen and nitrogen gases at Ocean Station Papa (50°N, 145°W) during a winter storm is reproduced with the model. Among different physical processes, gas bubbles are the most important in elevating dissolved gas concentrations during the storm, while atmospheric pressure governs the variability of gas saturation anomaly (the relative departure of dissolved gas concentration from the saturation concentration). For the same wind speed, bubble-mediated gas fluxes are larger during rising wind with smaller wave age than during falling wind with larger wave age. Wave conditions are the primary cause for the bubble gas flux difference: when wind strengthens, waves are less-developed with respect to wind, resulting in more frequent large breaking waves. Bubble generation in large breaking waves is favorable for a large bubble-mediated gas flux. The wave-age dependence is not included in any existing bubble-mediated gas flux parameterizations.

Submesoscale streamers exchange water on the north wall of the Gulf Stream

Klymak, J.M., R.K. Shearman, J. Gula, C.M. Lee, E.A. D'Asaro, L.N. Thomas, R.R. Harcourt, A.Y. Shcherbina, M.A. Sundermeyer, J. Molemaker, and J.C. McWilliams, "Submesoscale streamers exchange water on the north wall of the Gulf Stream," Geophys. Res. Lett., 43, 1226-1233, doi:10.1002/2015GL067152, 2016.

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16 Feb 2016

The Gulf Stream is a major conduit of warm surface water from the tropics to the subpolar North Atlantic. Here we observe and simulate a submesoscale (<20 km) mechanism by which the Gulf Stream exchanges water with subpolar water to the north. Along isopycnals, the front has a sharp compensated temperature-salinity contrast, with distinct mixed water between the two water masses 2 and 4 km wide. This mixed water does not increase downstream despite substantial energy available for mixing. A series of streamers detrain this water at the crest of meanders. Subpolar water replaces the mixed water and resharpens the front. The water mass exchange accounts for a northward flux of salt of 0.5–2.5 psu m2 s-1, (large-scale diffusivity O (100 m2 s-1)). This is similar to bulk-scale flux estimates of 1.2 psu m2 s-1 and supplies fresher water to the Gulf Stream required for the production of 18° subtropical mode water.

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The LatMix summer campaign: Submesoscale stirring in the upper ocean

Shcherbina, A.Y., and 37 others including E. D'Asaro, R.R. Harcourt, C.M. Lee, R.-C. Lien, and T.B. Sanford, "The LatMix summer campaign: Submesoscale stirring in the upper ocean," Bull. Am. Meteor. Soc., 96, 1257-1279, doi:10.1175/BAMS-D-14-00015.1, 2015.

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1 Aug 2015

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s-1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

An improved second-moment closure model of Langmuir turbulence

Harcourt, R.R., "An improved second-moment closure model of Langmuir turbulence," J. Phys. Oceanogr., 45, 84-103, doi:10.1175/JPO-D-14-0046.1, 2015.

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1 Jan 2015

A prior second-moment closure (SMC) model of Langmuir turbulence in the upper ocean is modified by introduction of inhomogeneous pressure–strain rate and pressure–scalar gradient closures that are similar to the high Reynolds number, near-wall treatments for solid wall boundaries. This repairs several near-surface defects in the algebraic Reynolds stress model (ARSM) of the prior SMC by redirecting Craik–Leibovich (CL) vortex force production of turbulent kinetic energy out of the surface-normal vertical component and into a horizontal one, with an associated reduction in near-surface CL production of vertical momentum flux. A surface-proximity function introduces a new closure parameter that is tuned to previous results from large-eddy simulations (LES), and a numerical SMC model based on stability functions from the new ARSM produces improved comparisons with mean profiles of momentum and TKE components from steady-state LES results forced by aligned wind and waves. An examination of higher-order quasi-homogeneous closures and a numerical simulation of Langmuir turbulence away from the boundaries both show the near-surface inhomogeneous closure to be both necessary for consistency and preferable for simplicity.

Quantifying upper ocean turbulence driven by surface waves

D'Asaro, E.A., J. Thomson, A.Y. Shcherbina, R.R. Harcourt, M.F. Cronin, M.A. Hemer, and B. Fox-Kemper, "Quantifying upper ocean turbulence driven by surface waves," Geophys. Res. Lett, 41, 102-107, doi:10.1002/1013GL058193, 2014.

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1 Jan 2014

Nearly all operational ocean models use air-sea fluxes and the ocean shear and stratification to estimate upper ocean boundary layer mixing rates. This approach implicitly parameterizes surface wave effects in terms of these inputs. Here, we test this assumption using parallel experiments in a lake with small waves and in the open ocean with much bigger waves. Under the same wind stress and adjusting for buoyancy flux, we find the mixed layer average turbulent vertical kinetic energy in the open ocean typically twice that in the lake. The increase is consistent with models of Langmuir turbulence, in which the wave Stokes drift, and not wave breaking, is the dominant mechanism by which waves energize turbulence in the mixed layer. Applying these same theories globally, we find enhanced mixing and deeper mixed layers resulting from the inclusion of Langmuir turbulence in the boundary layer parameterization, especially in the Southern Ocean.

Waves and the equilibrium range at Ocean Weather Station P

Thomson, J., E.A. D'Asaro, M.F. Cronin, W.E. Rogers, R.R. Harcourt, and A. Shcherbina, "Waves and the equilibrium range at Ocean Weather Station P," J. Geophys. Res., 118, 5951-5962, doi:10.1002/2013JC008837, 2013.

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1 Nov 2013

Wave and wind measurements at Ocean Weather Station P (OWS-P, 50°N 145°W) are used to evaluate the equilibrium range of surface wave energy spectra. Observations are consistent with a local balance between wind input and breaking dissipation, as described by Philips (1985). The measurements include direct covariance wind stress estimates and wave breaking dissipation rate estimates during a 3 week research cruise to OWS-P. The analysis is extended to a wider range of conditions using observations of wave energy spectra and wind speed during a 2 year mooring deployment at OWS-P. At moderate wind speeds (5–15 m/s), mooring wave spectra are in agreement, within 5% uncertainty, with the forcing implied by standard drag laws and mooring wind measurements. At high wind speeds (>15 m/s), mooring wave spectra are biased low, by 13%, relative to the forcing implied by standard drag laws and mooring wind measurements. Deviations from equilibrium are associated with directionality and variations at the swell frequencies. A spectral wave hindcast accurately reproduces the mooring observations, and is used to examine the wind input.

A second moment closure model of Langmuir turbulence

Harcourt, R.R., "A second moment closure model of Langmuir turbulence," J. Phys. Oceanogr., 43, 673-697, doi:10.1175/JPO-D-12-0105, 2013.

1 Apr 2013

Determining vertical water velocities from Seaglider

Frajka-Williams, E., C.C. Eriksen, P.B. Rhines, and R.R. Harcourt, "Determining vertical water velocities from Seaglider," J. Atmos. Ocean. Technol., 28, 1641-1656, doi:10.1175/2011JTECHO830.1, 2011.

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1 Dec 2011

Vertical velocities in the world's oceans are typically small, less than 1 cm s-1, posing a significant challenge for observational techniques. Seaglider, an autonomous profiling instrument, can be used to estimate vertical water velocity in the ocean. Using a Seaglider's flight model and pressure observations, vertical water velocities are estimated along glider trajectories in the Labrador Sea before, during, and after deep convection. Results indicate that vertical velocities in the stratified ocean agree with the theoretical Wentzel–Kramers–Brillouin (WKB) scaling of w; and in the turbulent mixed layer, scale with buoyancy, and wind forcing. It is estimated that accuracy is to within 0.5 cm s-1. Because of uncertainties in the flight model, velocities are poor near the surface and deep apogees, and during extended roll maneuvers. Some of this may be improved by using a dynamic flight model permitting acceleration and by better constraining flight parameters through pilot choices during the mission.

Enhanced turbulence and energy dissipation at ocean fronts

D'Asaro, E., C. Lee, L. Rainville, L. Thomas, and R. Harcourt, "Enhanced turbulence and energy dissipation at ocean fronts," Science, 332, 318-322, doi:0.1126/science.1201515, 2011.

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15 Apr 2011

The ocean surface boundary layer mediates air-sea exchange. In the classical paradigm and in current climate models, its turbulence is driven by atmospheric forcing. Observations at a 1-km-wide front within the Kuroshio found the rate of energy dissipation within the boundary layer to be enhanced by 10 to 20 times, suggesting that the front not the atmospheric forcing supplied the energy for the turbulence. The data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence. The resulting boundary layer is stratified, in contrast to the classically well-mixed layer. These effects will be strongest at the intense fronts found in the Kuroshio, Gulf Stream, and Antarctic Circumpolar Current, key players in the climate system.

Measurement of vertical kinetic energy and vertical velocity skewness in oceanic boundary layers by imperfectly Lagrangian floats

Harcourt, R.R., and E.A. D'Asaro, "Measurement of vertical kinetic energy and vertical velocity skewness in oceanic boundary layers by imperfectly Lagrangian floats," J. Atmos. Ocean. Technol., 27, 1918-1935, doi:10.1175/2010JTECHO731.1, 2010.

1 Nov 2010

Three-dimensional structure and temporal evolution of submesoscale thermohaline intrusions in the North Pacific subtropical frontal zone

Shcherbina, A.Y., M.C. Gregg, M.H. Alford, M.H., and R.R. Harcourt, "Three-dimensional structure and temporal evolution of submesoscale thermohaline intrusions in the North Pacific subtropical frontal zone," J. Phys. Oceanogr., 40, 1669-1689, doi:10.1175/2010JPO4373.1, 2010.

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1 Aug 2010

Four instances of persistent intrusive deformation of the North Pacific Subtropical Front were tagged individually by a Lagrangian float and tracked for several days. Each feature was mapped in three dimensions using repeat towed observations referenced to the float. Isohaline surface deformations in the frontal zone included sheetlike folds elongated in the alongfront direction and narrow tongues extending across the front. All deformations appeared as protrusions of relatively cold, and fresh, water across the front. No corresponding features of the opposite sign or isolated lenslike structures were observed. The sheets were O(10 m) thick, protruded about 10 km into the warm saline side of the front, and were coherent for 10–30 km along the front. Having about the same thickness and cross-frontal extent as the sheets, tongues extended less than 5 km along the front.

All of the intrusions persisted as long as they were followed, several days to one week. Their structures evolved on both inertial (23 h) and subinertial (10 days) time scales in response to differential lateral advection. The water mass surrounding the intrusions participated in gradual anticyclonic rotation as a part of a mesoscale meander of the subtropical front. The intrusions may be interpreted as a manifestation of three-dimensional submesoscale turbulence of the frontal zone, driven by the mesoscale. Absence of large features of the opposite sign may be indicative of the asymmetry of the underlying dynamics.

Characterizing thermohaline intrusions in the North Pacific subtropical frontal zone

Shcherbina, A.Y., M.C. Gregg, M.H. Alford, and R.R. Harcourt, "Characterizing thermohaline intrusions in the North Pacific subtropical frontal zone," J. Phys. Oceanogr., 39, 2735-2756, 2009.

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1 Nov 2009

A monthlong field survey in July 2007, focused on the North Pacific subtropical frontal zone (STFZ) near 30°N, 158°W, combined towed depth-cycling conductivity-temperature-depth (CTD) profiling with shipboard current observations. Measurements were used to investigate the distribution and structure of thermohaline intrusions. The study revealed that local extrema of vertical salinity profiles, often used as intrusion indicators, were only a subset of a wider class of distortions in thermohaline fields due to interleaving processes. A new method to investigate interleaving based on diapycnal spiciness curvature was used to describe an expanded class of laterally coherent intrusions. STFZ intrusions were characterized by their overall statistics and by a number of case studies. Thermohaline interleaving was particularly intense within 5 km of two partially compensated fronts, where intrusions with both positive and negative salinity anomalies were widespread. The vertical and cross-frontal scales of the intrusions were on the order of 10 m and 5 km, respectively. Though highly variable, the slopes of these features were typically intermediate between those of isopycnals and isohalines. Although the influence of double-diffusive processes sometime during the evolution of intrusions could not be excluded, the broad spectrum of the observed features suggests that any role of double diffusion was secondary.

Large-eddy simulation of Langmuir turbulence in pure wind seas

Harcourt, R.R., and E.A. D'Asaro, "Large-eddy simulation of Langmuir turbulence in pure wind seas," J. Phys. Oceanogr., 38, 1542-1562, 2008.

1 Jul 2008

Thermobaric cabbeling over Maud Rise: Theory and large eddy simulation

Harcourt, R.R., "Thermobaric cabbeling over Maud Rise: Theory and large eddy simulation," Prog. Oceanogr., 67, 186-244, DOI: 10.1016/j.pocean.2004.12.001, 2005

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1 Oct 2005

A Large Eddy Simulation (LES) of the wintertime upper ocean below seasonal Antarctic ice cover over Maud Rise was carried out using observed time-dependent surface forcing from 1994 Antarctic Zone Flux Experiment (ANZFLUX) observations. Surface ice formation increases the density of the cold, fresher Surface Mixed Layer (SML), that overlies warmer, saltier Weddell Deep Water (WDW). This reduces the stability of the thermocline until it reaches a critical point for instabilities arising from the nonlinear equation of state (NES) for seawater density ρ. This simulation was intended to model the thermobaric detrainment of SML fluid, a NES instability predicted to result from the dependence of seawater density on the product ΘP of temperature and pressure.

Instead, model results demonstrate a different instability arising from the combination of thermobaricity with cabbeling, the NES effect due primarily to the dependence of ρ on Θ2. This combined thermobaric cabbeling instability drives turbulent convection in a deep interior mixed layer (IML) that may grow hundreds of meters thick below the thermocline, largely decoupled from SML dynamics. In the LES, thermobaric cabbeling and IML convection shoals the SML through entrainment from below until ice motion increases in the observationally-based model forcing. Increased upper ocean model heat flux due to higher ice speed melts surface ice, increasing thermocline stratification and eventually bringing the simulated instability to a halt.

In an auxiliary simulation the lull preceding strong ice motion in field observations is artificially extended by temporarily holding model surface forcing constant until the SML shoals entirely, bringing the modified WDW of the IML, 2°C above freezing, directly to the surface. Subsequently, reverting to the observed surface forcing and its attendant strong ice motion melts the ice cover entirely, demonstrating a possible mechanism for open ocean Antarctic polynya formation. The same process, as halted prematurely in the LES using the forcing observed, may also be responsible for thick, deep internal layers and localized "chimney" structures observed in the Weddell Sea.

Fully Lagrangian floats in Labrador Sea deep convection: Comparison of numerical and experimental results

Harcourt, R.R., E.L. Steffen, R.W. Garwood, and E.A. D'Asaro, "Fully Lagrangian floats in Labrador Sea deep convection: Comparison of numerical and experimental results," J. Phys. Oceanogr., 32, 493-510, doi: 10.1175/1520-0485(2002)032, 2002.

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1 Feb 2002

Measurements of deep convection from fully Lagrangian floats deployed in the Labrador Sea during February and March 1997 are compared with results from model drifters embedded in a large eddy simulation (LES) of the rapidly deepening mixed layer. The deep Lagrangian floats (DLFs) have a large vertical drag, and are designed to nearly match the density and compressibility of seawater. The high-resolution numerical simulation of deep convective turbulence uses initial conditions and surface forcing obtained from in situ oceanic and atmospheric observations made by the R/V Knorr. The response of model floats to the resolved large eddy fields of buoyancy and velocity is simulated for floats that are 5 g too buoyant, as well as for floats that are correctly ballasted. Mean profiles of potential temperature, Lagrangian rates of heating and acceleration, vertical turbulent kinetic energy (TKE), vertical heat flux, potential temperature variance, and float probability distribution functions (PDFs) are compared for actual and model floats.

Horizontally homogeneous convection, as represented by the LES model, accounts for most of the first and second order statistics from float observations, except that observed temperature variance is several times larger than model variance. There are no correspondingly large differences in vertical TKE, heat flux, or mixed layer depth. The augmented temperature variance may be due to mixing across large-scale temperature and salinity gradients that are largely compensated in buoyancy. The rest of the DLF statistics agree well with the response of correctly ballasted model floats in the lowest 75% of the mixed layer, and are less consistent with results from buoyantly ballasted model floats.

Other differences between observation and simulation in the mean profiles of heat flux, vertical TKE, and Lagrangian heating and vertical acceleration rates are confined to the upper quarter of the mixed layer. These differences are small contributions to layer-averaged quantities, but represent statistically significant profile features. Larger observed values of heat flux and vertical TKE in the upper quarter of the mixed layer are more consistent with model floats ballasted light. Float buoyancy, however, cannot fully account for the observed PDFs, temperature profiles, and Lagrangian rates of heating and acceleration. A test of Lagrangian self-consistency comparing vertical TKE and Lagrangian acceleration also shows that DLF measurements are not significantly affected by excess float buoyancy. These upper mixed layer features may instead be due to the interaction of wind-driven currents and baroclinicity.

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