The onset of the Spring Bloom in the MEDOC area: mesoscale spatial variability

In the northwestern Mediterranean Sea, Coastal Zone Color Scanner images su ggest that the eddies that participate in the restratification following de ep convection interact with the spring phytoplankton bloom. The mechanisms for this interaction are studied using a biogeochemical model embedded in a n eddy-resolving primitive equation ocean model. The model is initialized w ith a patch of dense water surrounded by a stratified ocean, which is chara cteristic of the winter situation. The atmospheric forcing is artificially held constant, in order to focus solely on the mesoscale variability. After a few days, meanders develop at the periphery of the patch, inducing its s inking and spreading. Mesoscale upward motions are responsible for the shoa ling of the mixing layer in the trough of the meanders. As sunlight is the main factor regulating primary production at this time of year, this shoali ng increases the mean exposure time of the phytoplankton cells and thus enh ances productivity. Consequently, the majority of phytoplankton production is obtained at the edge of the patch, in agreement with in situ data. Throu gh advection, phytoplankton is then subducted from these sources towards th e crest of the meanders. Our results suggest that this mesoscale transport is responsible for a decorrelation between phytoplankton biomass and primar y production. (C) 1999 Elsevier Science Ltd. All rights reserved.