THE ONSET OF A BLOOM AFTER DEEP WINTER CONVECTION IN THE NORTHWESTERNMEDITERRANEAN SEA - MESOSCALE PROCESS STUDY WITH A PRIMITIVE EQUATIONMODEL

The importance of mesoscale processes for primary production predictio ns is examined in a process study concerning the onset of the spring b loom after deep winter convection in the northwestern Mediterranean se a. Winter deep convection brings nutrient to the enlightened surface l ayer, but inhibits photosynthesis; phytoplankton biomasses are very lo w. As soon as restratification occurs, vertical mixing is blocked and a strong bloom onsets. Coastal Zone Color Scanner images have emphasiz ed a strong mesoscale signal in the sea surface chlorophyll during thi s period. Mesoscale heterogeneity of the mixed-layer depth, due to the baroclinic instabilities associated with the process of deep water fo rmation, is indeed responsible for the mesoscale variability of primar y production. To ascertain interactions between hydrological processes and primary production occurring at mesoscales, a primary production model with a parameterization of production inhibition in situations o f deep mixing is embedded in a three-dimensional primitive equation mo del with explicit mixed-layer physics. The model is initialized with a circular chimney of dense water surrounded by a stratified ocean. Two experiments are performed using different treatments of lateral mixin g. In the first experiment, the horizontal diffusion is set to a low l evel so that mesoscale activity can be explicitly resolved. Surface de nsity meanders of 50 km wavelength develop at the periphery of the chi mney. These meanders, and the associated vertical motions, induce the sinking and spreading of the chimney, and subsequent surface restratif ication. Upward motions are responsible for mesoscale mixed layer shal lowing, leading to an enhancement of primary production. Maxima of pro ductivity are obtained at the edge of the chimney, where mesoscale act ivity is the most intense, in agreement with in situ data. In the seco nd experiment, the horizontal diffusion is set to a high level so that lateral mixing occurs primarly through those terms: explicit mesoscal e activity is completely damped. The initial structure of the chimney progressively disappears due to the horizontal diffusion of density ac ross the isopycnals instead of three-dimensional redistribution. Mixed -layer depth and productivity are homogeneous. It is shown that instan taneous primary production can be underestimated by a factor of 4 when mesoscale eddies are not explicitly solved. This finding questions th e evolution of large-scale coarse resolution climatic models of the oc eanic carbon cycle. (C) 1998 Elsevier Science B.V. All rights reserved .