Sea states across current fronts: a case study using numerical wave models and CFOSAT-SWIM data across Agulhas current

Gwendal Marechal, Ifremer, CNRS, IRD, University of Western Brittany, LOPS, Plouzané, France, Fabrice Ardhuin, University of California, San Diego, Scripps Institution of Oceanography, La Jolla, United States, Alexis Mouche, Ifremer, LOPS, Plouzané, France, Danièle Hauser, CNRS & University Paris-Saclay, LATMOS, Guyancourt, France, Fabrice Collard, OceanDataLab, Plouzané, France and Cédric Tourain, CNES, Toulouse, France
Abstract:
Surface currents are a major source of wind-wave variability offshore and at the coasts, with, among other aspects, expected large impacts on air-sea fluxes, navigation safety and coastal hazards. Unfortunately, only limited data was available so far with relatively poor spatial resolution and a limited range of current or sea state parameters. Recent advances in the modelling of surface currents have revealed strong mesoscale and submesoscale structures (filaments, eddies, fronts) that modify wind waves properties.

Here we investigate the response of wave height and directions to the offshore current front of the Agulhas using a combination of numerical model, and satellite data including altimeters, SAR Doppler centroid from Envisat and Sentinel 1, sun glitter data from Sentinel 2, and the new SWIM wave spectrometer which is aboard the China-France Ocean SATellite (CFOSAT). CFOSAT was launched in October 2018, and its SWIM instrument is a new type of space-borne radar that provides unprecedented measurements of wave spectra, down to wavelengths of 40 meters in most conditions.

We particularly question the necessary current magnitude and vorticity that is required to explain the observed sharp gradients in wave height and mean directions observed in altimeter and SWIM and data. This is done by performing realistic simulations of waves using the WAVEWATCH III model, forced by currents from two model set-ups for the ocean circulation, based on the Coastal and Regional Ocean Community (CROCO) and the MITgcm model. When currents are smoothed to resolutions up to 30 km, the wave model generally reproduces the sharp height and direction gradient at the edge of the Agulhas current, but the gradients fade for coarser surface current fields. According to the model, these large gradients are also the location of enhanced wave breaking and upper ocean mixing, which may have an impact on the dynamics of currents at smaller scales.

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