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

• Main Authors: Alpers, Werner, Brandt, Peter, Lazar, Alban, Dagorne, Dominique, Sow, Bamol, Faye, Saliou, Hansen, Morten W., Rubino, Angelo, Poulain, Pierre-Marie, Brehmer, Patrice
• Other Authors: Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Variabilité climatique tropicale et globale (VARCLIM), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Instrumentation, Moyens analytiques, Observatoires en Géophysique et Océanographie (IMAGO), EGU
• Format: Conference Object
• Language: English
• Published: HAL CCSD 2013
• Subjects: [PHYS.PHYS.PHYS-GEO-PH]Physics [physics]/Physics [physics]/Geophysics [physics.geo-ph]; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]; North Atlantic
• Online Access: https://hal.science/hal-00873732

International audience High-resolution satellite images and oceanographic field measurements have revealed that oceanic eddies with diameters ranging from 1 to several hundred km are ubiquitous phenomena in the World's ocean. While eddies with horizontal scales above 100 km have been studied extensively using altimeter data, only few papers exist dealing with observations of eddies with horizontal scales below 50 km. These small-scale eddies cannot be resolved by conventional altimeters, but they can be observed from space by high-resolution optical/infrared sensors and by synthetic aperture radars (SARs). In this paper we report about a single small-scale cyclonic (cold) eddy which was generated at the headland of Cap-Vert off the coast of Senegal following a sudden freshening of the trade winds. Due to favorable cloud conditions, we were able to track the time evolution of the eddy for 31 days by satellite images acquired in the visible/ infrared band. Furthermore, the eddy was also imaged during this period by a space-borne SAR. Cold eddies become visible on SAR images via the change in the small-scale sea surface roughness caused by the damping of short surface waves by biogenic surface films or/and by the change of the stability of the air-sea interface. Biogenic surface films consist of surface-active material secreted by biota in the cold eddy. The satellite data we are using are from the MODIS sensor onboard the American Aqua satellite, the AVHHR sensor onboard the European MetOp satellite, and the Advanced SAR (ASAR) onboard the European Envisat satellite. The sea surface temperature (SST) and chlorophyll-a (CHL) maps derived from MODIS data show that the eddy propagated from its birth place at Cap-Vert in the Senegal upwelling region westward into the open North Atlantic. During the 31 days of satellite observations, the eddy moved 200 km westward thereby carrying nutrients from the upwelling region into the oligotrophic North Atlantic, where it caused enhanced CHL concentration. Maximum CHL ...