A small-scale oceanic eddy off the coast of West Africa studied by multi-sensor satellite and surface drifter data

摘要

A small-scale oceanic eddy, which was generated in autumn 2011 at the headland of Cap-Vert off the coast of Senegal, West Africa, and then propagated westward into the open North Atlantic Ocean, is studied by multi-sensor satellite and surface drifter data. The eddy was generated after a sudden increase of the trade winds causing an enhanced southward flow and upwelling at the coast of Senegal. After this wind burst event, an extremely nonlinear cyclonic eddy with a radius of about 10 to 20. km evolved downstream of Cap-Vert with Rossby number larger than one. Our analysis suggests that the eddy was generated by flow separation at the headland of Cap-Vert. The eddy was tracked on its way into the open North Atlantic Ocean from satellites over 31. days via its sea surface temperature and chlorophyll-a (CHL) signature and by a satellite-tracked surface drifter. The satellite images show that this small-scale eddy transported nutrients from the upwelling region westward into the oligotrophic North Atlantic thus giving rise to enhanced CHL concentration there. Maximum CHL concentration was encountered few days after vortex generation, which is consistent with a delayed plankton growth following nutrient supply into the euphotic zone within the eddy. Furthermore, the eddy was imaged by the synthetic aperture radar (SAR) onboard the Envisat satellite. It is shown that the radar signatures of cold eddies result from damping of short surface waves by biogenic surface films which arise from surface-active material secreted by the biota in the cold eddy as well as by the change of the stability of the air-sea interface.