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The Lagrangian and Eulerian surface current signatures of a low-mode internal tide propagating through a turbulent balanced flow are compared in idealized numerical simulations. Lagrangian and Eulerian total (i.e., coherent plus incoherent) tidal amplitudes are found to be similar. Compared to Eulerian diagnostics, the Lagrangian tidal signal is more incoherent with comparable or smaller incoherence time scales and larger incoherent amplitudes. The larger level of incoherence in Lagrangian data is proposed to result from the deformation of an Eulerian internal tide signal induced by drifter displacements. Based on the latter hypothesis, a theoretical model successfully predicts Lagrangian autocovariances by relating Lagrangian and Eulerian autocovariances and the properties of the internal tides and jet. These results have implications for the separation of balanced flow and internal tides signals in the sea level data collected by the future Surface Water and Ocean Topography (SWOT) satellite mission.

The surface kinetic energy of a 1/48° global ocean simulation and its distribution as a function of frequency and location are compared with the one estimated from 15,329 globally distributed surface drifter observations at hourly resolution. These distributions follow similar patterns with a dominant low‐frequency component and well‐defined tidal and near‐inertial peaks globally. Quantitative differences are identified with deficits of low‐frequency energy near the equator (factor 2) and at near‐inertial frequencies (factor 3) and an excess of energy at semidiurnal frequencies (factor 4) for the model. Owing to its hourly resolution and its near‐global spatial coverage, the array of surface drifters is an invaluable tool to evaluate the realism of tide‐resolving high‐resolution ocean simulations used in observing system simulation experiments. Sources of bias between model and drifter data are discussed, and associated leads for future work highlighted. Plain Language Summary Ocean currents predominately control the transport and distribution of physical properties (such as heat and momentum) and biochemical tracers (such as carbon, oxygen, and nutrients) in the global oceans. These currents are generally most energetic at the ocean surface and distributed across a broad range of temporal and spatial scales. Global ocean models simulate ocean variability from large‐scale circulations to tidal motions and are used to provide guidance for new global oceanographic observations, especially for future high‐resolution satellite missions. Therefore, it is crucial to assess the realism of global ocean models. In this study, we use drifter observations of global surface currents to compare with the output from a 1/48° global ocean simulation. Our results show that the model exhibits a broad qualitative consistency with the surface drifter data but also displays some key differences at high‐frequency dynamics. The model shows a 3 times deficit of near‐inertial variance and a 4 times excess of semidiurnal variance when compared with surface drifter data. Our findings demonstrate that the global drifter data set with hourly temporal resolution provides a resource for assessment of the surface circulation predicted by state‐of‐the‐art global ocean models.

International audience; Several methods that reconstruct the three-dimensional ocean dynamics from sea level are presented and evaluated in the Gulf Stream region with a 1/60° realistic numerical simulation. The use of sea level is motivated by its better correlation with interior pressure or quasi-geostrophic potential vorticity (PV) compared to sea surface temperature and sea surface salinity, and, by its observability via satellite altimetry. The simplest method of reconstruction relies on a linear estimation of pressure at depth from sea level. Another method consists in linearly estimating PV from sea level first and then performing a PV inversion. The last method considered, labeled SQG for surface quasi-geostrophy, relies on a PV inversion but assumes no PV anomalies. The first two methods show comparable skill at levels above -800 m. They moderately outperform SQG which emphasizes the difficulty of estimating interior PV from surface variables. Over the 250-1,000 m depth range, the three methods skillfully reconstruct pressure at wavelengths between 500 and 200 km whereas they exhibit a rapid loss of skill between 200 and 100 km wavelengths. Applicability to a real case scenario and leads for improvements are discussed.

Understanding how internal tides distort and lose coherence as they propagate through the ocean has been identified as a key issue for interpreting data from the upcoming wide-swath altimeter mission SWOT. This study addresses that issue through the analysis of numerical experiments where a low mode internal tide propagates through a quasi-geostrophic turbulent jet. We present a set of reduced equations based on vertical mode projections that describe the dynamics of the low-mode internal tides as they propagate through the slower turbulent field. Diagnostics of the terms responsible for the interaction between the wave and the slow circulation are computed from the numerical outputs. The large scale change of stratification, on top of eddies and jet meanders, contributes significantly to these interaction terms which is shown to be consistent with an independent scaling analysis. The sensitivity of interaction terms to a degradation of the slow field spatial and horizontal resolution indicates that current day observing systems (Argo network, altimetry) may lack spatial resolution in order to correctly predict internal tide evolution. The upcoming SWOT satellite mission may improve upon this situation. The number of vertical modes required to properly estimate interaction terms is discussed. These results advocate development of a simplified model based on solving a modest number of the reduced equations subject to a prescribed mesoscale field and internal tide sources. Such a model would constitute an inexpensive way to calculate the distortion and loss of coherence of low-mode internal tides as they propagate through mesoscale circulation.

Special Issue Ten Years of Remote Sensing at Barcelona Expert Center.-- 14 pages, 5 figures, 3 tables, 2 appendices, The expected amplitude of fixed-point sea surface temperature (SST) fluctuations induced by barotropic and baroclinic tidal flows is estimated from tidal current atlases and SST observations. The fluctuations considered are the result of the advection of pre-existing SST fronts by tidal currents. They are thus confined to front locations and exhibit fine-scale spatial structures. The amplitude of these tidally induced SST fluctuations is proportional to the scalar product of SST frontal gradients and tidal currents. Regional and global estimations of these expected amplitudes are presented. We predict barotropic tidal motions produce SST fluctuations that may reach amplitudes of 0.3 K. Baroclinic (internal) tides produce SST fluctuations that may reach values that are weaker than 0.1 K. The amplitudes and the detectability of tidally induced fluctuations of SST are discussed in the light of expected SST fluctuations due to other geophysical processes and instrumental (pixel) noise. We conclude that actual observations of tidally induced SST fluctuations are a challenge with present-day observing systems, Aurélien Ponte benefited from funding via the ANR project EQUINOx (ANR-17-CE01-0006-01) and CNES TOSCA project “New dynamical tools for submesoscale characterization in SWOT data”. Cristina González-Haro benefited from funding by CNES

At times, high-resolution images of sea surface roughness can provide stunning details of submesoscale upper-ocean dynamics. As interpreted, transformations of short-scale wind waves by horizontal current gradients are responsible for those spectacular observations. Those observations could prove particularly useful to validate numerical ocean models that reach increasingly high resolutions. Focusing on surface roughness at optical wavelengths, two steps have recently been performed in that direction. First, it was shown in a previous paper by Rascle et al. that surface roughness variations not only trace surface current divergence but also other characteristics of the current gradient tensor, mainly the strain in the wind direction. The wind direction with respect to the current gradient thus stands out as an important interpretative parameter. The second step is the purpose of the present paper, where the effect of the viewing direction is investigated. To this end, the authors discuss pairs of quasi-simultaneous sun-glitter images, taken from different satellite positions, to provide different viewing configurations, namely, quasi-orthogonal azimuth angles at similar zenith angles. As evidenced, upwind and crosswind viewing observations can be markedly different. As further confirmed with idealized numerical simulations, this anisotropy well traces anisotropic surface current areas, while more isotropic contrasts likely trace areas dominated by surface divergence conditions. These findings suggest the potential to directly separate divergence from other deformations by using high-resolution roughness observations at multiple azimuth view angles.

International audience; Recent realistic high resolution modeling studies show a net increase of submesoscale activity in fall and winter when the mixed layer depth is at its maximum. This submesoscale activity increase is associated with a reduced deepening of the mixed layer. Both phenomena can be related to the development of mixed layer instabilities, which convert available potential energy into submesoscale eddy kinetic energy and contribute to a fast restratification by slumping the horizontal density gradient in the mixed layer. In the present work, the mixed layer formation and restratification was studied by uniformly cooling a fully turbulent zonal jet in a periodic channel at different resolutions, from eddy resolving (10 km) to submesoscale permitting (2 km). The effect of the submesoscale activity, highlighted by these different horizontal resolutions, was quantified in terms of mixed layer depth, restratification rate and buoyancy fluxes. Contrary to many idealized studies focusing on the restratification phase only, this study addresses a continuous event of mixed layer formation followed by its complete restratification. The robustness of the present results was established by ensemble simulations. The results show that, at higher resolution, when submesoscale starts to be resolved, the mixed layer formed during the surface cooling is significantly shallower and the total restratification almost three times faster. Such differences between coarse and fine resolution models are consistent with the submesoscale upward buoyancy flux, which balances the convection during the formation phase and accelerates the restratification once the surface cooling is stopped. This submesoscale buoyancy flux is active even below the mixed layer. Our simulations show that mesoscale dynamics also cause restratification, but on longer time scales. Finally, the spatial distribution of the mixed layer depth is highly heterogeneous in the presence of submesoscale activity, prompting the question of whether it is possible to parameterize submesoscale effects and their effects on the marine biology as a function of a spatially-averaged mixed layer depth.

International audience; Time series of current velocity profiles and thermistor chains were obtained throughout a cross-bay transect for ~90 days for the purpose of comparing observed wind-driven stratified flows to theory. This study concentrates on the synoptic scale wind and its influence on the bay’s circulation. The maximum water column stratification was 3 – 4 °C/m throughout the deployment and influenced wind-driven flows. Low-pass filtered flows showed more complicated structures than those expected from theory: a depth-dependent recirculating structure with the along-bay flow over one half of the transect moving in opposite direction to the other half. Analysis of complex empirical orthogonal functions indicated that the first six modes explained 80 % of the flow variability. Therefore, there was no predominantly energetic mode of variability. All modesexhibited a rich spatial structure with vertical and lateral variations. For all modes there was vertically sheared bidirectional flow, as expected from theory, with the largest eigenvector (mode value) asymmetrically influenced by Earth’s rotation and advection of momentum.

The propagation of remotely generated superinertial internal tides constitutes a difficulty for the modelling of regional ocean tidal variability which we illustrate in several ways. First, the M2 tidal solution inside a control region located along the Southern California Bight coastline is monitored while the extent of the numerical domain is increased (up to 512 x 512 km). While the amplitude and phase of sea level averaged over the region is quasi-insensitive to domain size, a steady increase of kinetic energy, predominantly baroclinic, is observed with increasing domain size. The increasing flux of energy into the control region suggests that this trend is explained by the growing contribution from remote generation sites of internal tide which can propagate up to the control region. Increasing viscosities confirms this interpretation by lowering baroclinic energy levels and limiting their rate of increase with domain size. Doubling the grid spacing allows consideration of numerical domains 2 times larger. While the coarse grid has lower energy levels than the finer grid, the rate of energy increase with domain size appears to be slowing for the largest domain of the coarse grid simulations. Forcing the smallest domain with depth-varying tidal boundary conditions from the simulation in the largest domain produces energy levels inside the control region comparable to those in the control region for the largest domain, thereby confirming the feasibility of a nested approach. In contrast, simulations forced with a subinertial tidal constituent (K1) show that when the propagation of internal tide is limited, the control region kinetic energy is mostly barotropic and the magnitudes of variations of the kinetic energy with domain size are reduced. (C) 2012 Elsevier Ltd. All rights reserved.

An idealized model is developed for the three-dimensional response of a coastal basin (e.g., lagoon, bay, or estuary) to time-periodic wind stress. This model handles basins that are deeper and/or shallower than an Ekman depth with wind forcing frequencies ranging from subinertial to superinertial. Here the model is used to describe how the response (current and sea level) of a basin deeper than one Ekman depth depends on the wind forcing frequency. At low subinertial frequencies, the response is similar to the steady wind case and is hence called “quasi steady.” There is a near-surface Ekman transport to the right of the wind balanced by a return flow at depth. Lateral bathymetric variations introduce an along-basin circulation that decays with increasing frequency and sets the extent of the quasi-steady response in the frequency domain. At the inertial frequency, the wind forces a damped resonant response with large vertical shear and weak depth-integrated flow. This result is potentially important for coastal basins located near ±30° latitude and forced by a diurnal breeze. At superinertial frequencies, the response becomes irrotational and is amplified near seiche frequencies. The response to a sudden onset of wind is computed in the time domain and confirms that the slow growth of the along-basin circulation controls the spinup process. The response of basins shallower than an Ekman depth, and the validity of the model inside semienclosed basins, are also discussed.

The role of wind-driven upwelling in stratifying a semiarid bay in the Gulf of California is demonstrated with observations in Bahia Concepcion, Baja California Sur, Mexico. The stratification in Bahia Concepcion is related to the seasonal heat transfer from the atmosphere as well as to cold water intrusions forced by wind-driven upwelling. During winter, the water column is relatively well-mixed by atmospheric cooling and by northwesterly, downwelling-favorable, winds that typically exceed 10 m/s. During summer, the water column is gradually heated and becomes stratified because of the heat flux from the atmosphere. The wind field shifts from downwelling-favorable to upwelling-favorable at the beginning of summer, i.e., the winds become predominantly southeasterly. The reversal of wind direction triggers a major cold water intrusion at the beginning of the summer season that drops the temperature of the entire water column by 3–5 °C. The persistent upwelling-favorable winds during the summer provide a continuous cold water supply that helps maintain the stratification of the bay.

Internal gravity waves of tidal frequency are generated as the ocean tides push water upward onto the continental shelf. Such waves also arrive at the continental slope from deep water and are heavily modified by the change in water depth. The wave generation and wave shoaling effects have an additional level of complexity where a canyon is sliced into the continental slope. Recently, steps have been taken to simulate internal tides in canyons, to understand the physical processes of internal tides in canyons, and also to compute the ramifications on sound propagation in and near the canyons. Internal tides generated in canyons can exhibit directionality, with the directionality being consistent with an interesting multiple-scattering effect. The directionality imparts a pattern to the sound-speed anomaly field affecting propagation. The directionality also means that short nonlinear internal waves, which have specific strong effects on sound, can have interesting patterns near the canyons. In addition to the directionality of internal tides radiated from canyons, the internal tide energy within the canyons can be patchy and may unevenly affect sound.

Studies using idealized bathymetry have shown that steep slopes can have important effects on the geometric structure of ocean sound fields. Other studies have provided insight into the temporal variability of sound fields in the moving ocean. There is a general understanding that both effects occur together in the ocean, but their relative importance is not fully known, nor is the way they interact. The processes are often linked because slopes can generate strong internal tides and waves. Here, the coupled spatial and temporal variability is investigated using time-stepped three-dimensional (3.5-D) sound propagation modeling. The needed ocean sound-speed information is generated using modern regional ocean modeling. Solutions of the 3D parabolic acoustic wave equation are generated for a few hundred hertz. The spatial patterns of sound reveal that temporal intensity variation (scintillation) caused by internal tides can be enhanced at areas where sound encounters steep slopes. The magnitude of the effec...