Llido, Jérôme, Eric Machu, Isabelle Dadou and Véronique Garçon

1-Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, 18, Avenue Edouard Belin, 31401 Toulouse Cedex 4, France, Tel: +33.5.61.33.27.85, Fax: + 33.5.61.25.32.05, E-mail: llido@notos.cst.cnes.fr

Seasonal and interannual variability of phytoplankton in the Agulhas Retroflection Current System from a coupled physical and biological model

The Agulhas system, south of South Africa, plays an important role as a key active region in the oceanic carbon cycle. The frontal system formed by the Agulhas Return Current (ARC) and the Subtropical Convergence (STC) is a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a. The variability of this frontal system is analyzed in a three dimensional eddy permitting (1/3º) coupled physical (AGAPE)/ biological (NPZD) model using two different atmospheric forcings: an ECMWF monthly climatology and monthly means of the NCEP reanalysis over the 1995-2001 period. Modeled chlorophyll fields compare rather well with the SeaWiFS chlorophyll data. However in years 1998 and 1999, a northward inclination of the Agulhas Front from 30ºE eastwards is simulated which is not present on the SeaWiFS observations. This is linked to the NCEP forcing used. Deeper stratification and enhanced cross-frontal transport explains the higher concentrations of phytoplankton found in winter in the South West Indian Ocean subtropical gyre. In the frontal system, deep winter mixing conditions removes phytoplankton from the euphotic zone and thus inhibits growth. Simulation of primary production is in good agreement with estimates derived from satellite observations.