Surface Roughness Changes by Finescale Current Gradients: Properties at Multiple Azimuth View Angles

摘要

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 which reach increasingly high resolutions.Focusing on surface roughness at optical wavelengths, two steps have recently been performed in that direction. First, it was shown by Rascle et al. (2014, Journal of Physical Oceanography) 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 we investigate the effect of the viewing direction. To this end, we 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.