Search

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.

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.

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.