The onset of a bloom after deep winter convection in the northwestern Mediterranean sea: mesoscale process study with a primitive equation model

The importance of mesoscale processes for primary production predictions is examined in a process study concerning the onset of the spring bloom after deep winter convection in the northwestern Mediterranean sea. Winter deep convection brings nutrient to the enlightened surface layer, but inhibits photosynthesis; phytoplankton biomasses are very low. As soon as restratification occurs, vertical mixing is blocked and a strong bloom onsets. Coastal Zone Color Scanner images have emphasized a strong mesoscale signal in the sea surface chlorophyll during this period. Mesoscale heterogeneity of the mixed-layer depth, due to the baroclinic instabilities associated with the process of deep water formation, is indeed responsible for the mesoscale variability of primary 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 of deep mixing is embedded in a three-dimensional primitive equation model 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 mixing. In the first experiment, the horizontal diffusion is set to a low level so that mesoscale activity can be explicitly resolved. Surface density meanders of 50 km wavelength develop at the periphery of the chimney. These meanders, and the associated vertical motions, induce the sinking and spreading of the chimney, and subsequent surface restratification. Upward motions are responsible for mesoscale mixed layer shallowing, leading to an enhancement of primary production. Maxima of productivity are obtained at the edge of the chimney, where mesoscale activity is the most intense, in agreement with in situ data. In the second experiment, the horizontal diffusion is set to a high level so that lateral mixing occurs primarly through those terms: explicit mesoscale activity is completely damped. The initial structure of the chimney progressively disappears due to the horizontal diffusion of density across the isopycnals instead of three-dimensional redistribution. Mixed-layer depth and productivity are homogeneous. It is shown that instantaneous primary production can be underestimated by a factor of 4 when mesoscale eddies are not explicitly solved. This finding questions the evolution of large-scale coarse resolution clima