Multi-sensor investigation of upwelling including Sentinel-1 SAR data

In studies of upwelling usually data from infrared and optical sensors are used which provide information on the sea surface temperature (SST) and the chlorophyll-a (Chl-a) concentration. In this paper, we show that also synthetic aperture radars (SAR) images can also give valuable contribution to such studies. Upwelling regions become detectable by SAR because they are associated with a reduction of the radar backscatter due to 1) a change of the stability of the air-sea interface or/and 2) the presence of biogenic slicks. While the change of the stability of the air-sea interface due to the presence of cold surface water in the upwelling region causes only a small reduction of the radar backscatter, biogenic cause a very strong reduction, usually of more than 10 dB. In areas of strong upwelling, the biological productivity is high due to increased nutrient supply from lower water levels. The biota living in this area secrete surface active material that ascends to the sea surface and forms there biogenic slicks, which often cover large areas in the form as monomolecular layers. The biogenic slicks damp the short-scale surface waves, which are responsible for the radar backscattering, as strongly as mineral oil films, and thus areas covered with the surface films become visible on SAR images as areas of strongly reduced radar backscatter (dark areas). Biogenic slicks often are entrained in the surface current field of eddies and thus render the structure of eddies visible on SAR images. Furthermore, boundaries of upwelling regions are often associated with strong convergent surface flow causing enhancement of the radar backscatter.
This paper focuses on upwelling events off the coast of Namibia (caused by northward directed coastal winds), off the coast of South Africa (caused by intensification of the meandering Agulhas Return Current), off the southern coast of Sicily (caused by strong southeastward directed coastal winds, in particular by the Mistral). These upwelling events are studied using Sentinel-1 SAR images, Modis SST and Chl-a maps, and model data of geostrophic surface currents. We show that this synergism yields new insights into upwelling mechanism. In particular, we show that upwelling events are often associated with the generation of filaments, internal waves, and small-scale eddies, which are detectable by SAR.