Surface Roughness Imaging of Currents Shows Divergence and Strain in the Wind Direction

Images of sea surface roughness-for example, obtained by synthetic aperture radars (SAR) or by radiometers viewing areas in and around the sun glitter-at times provide clear observations of meso- and sub-mesoscale oceanic features. Interacting with the surface wind waves, particular deformation properties of surface currents are responsible for those manifestations. Ignoring other sources of surface roughness variations, the authors limit their discussion to the mean square slope (mss) variability. This study confirms that vortical currents and currents with shear in the wind direction shall not be expressed in surface roughness images. Only divergent currents or currents with no divergence but strained in the wind direction can exhibit surface roughness signatures. More specifically, nondivergent currents might be traced with a 45 degrees sensitivity to the wind direction. A simple method is proposed in order to interpret high-resolution roughness images, where roughness variations are proportional to partial derivative u/partial derivative x + alpha partial derivative v/partial derivative y, a linear combination of the along-wind and crosswind current gradients. The polarization parameter a depends upon the sensor look direction and the directional properties of the surface waves selected by the sensor. The use of multiple look directions or possible acquisitions with different wind directions shall thus help to retrieve surface currents from surface roughness observations.