Statistical analyses of eddies in the Western Mediterranean Sea based on Synthetic Aperture Radar imagery

Abstract

We analysed 7188 Sentinel-1A Synthetic Aperture Radar (SAR) images acquired in a 3-year period, between October 2014 and September 2017, in the Western Mediterranean Sea and found imprints of about 14,300 oceanic eddies with diameters ranging from 0.4 km to 160.1 km. Those eddies manifest on the SAR imagery either through the accumulation of surfactants or through wave-current interaction, and considering the favourable wind-speed ranges for both mechanisms we performed statistical analyses to gain insight into the eddies' spatio-temporal distribution, their typical sizes and shapes, and into the predominant mechanisms that are responsible for their formation. The vast majority of the detected eddies is elliptical, with a tendency for only large mesoscale eddies towards a circular shape. We found no general influence of the wind on the eddies' orientation, although mesoscale eddies tend to orient into the main current direction in each subregion. Submesoscale eddies do not show such tendency. Submesoscale eddies in the Western Mediterranean Sea are predominantly cyclonic (>90%), and with increasing eddy diameter there is a transition from cyclonic to anticyclonic sense of rotation. In general, we found highest densities of submesoscale eddies in autumn, when the biological activity is still high and when mean regional winds are already strong, but variable. In addition, instabilities of thermal fronts during summer and autumn and instabilities of the main and secondary currents influence the formation of submesoscale eddies. Some of our results confirm those previously obtained for other regions.