Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime

Using a protocol of numerical experiments where horizontal resolution is pr ogressively increased, we show that small-scale (or sub-mesoscale) physics has a strong impact on both mesoscale physics and phytoplankton production/ subduction. Mesoscale and sub-mesoscale physics result from the nonlinear equilibration of an unstable baroclinic jet. The biogeochemical context is oligotrophy. The explicitly resolved sub-mesoscales, at least smaller than one fifth of the internal Rossby radius of deformation, reinforce the mesoscale eddy fie ld and contribute to double the primary production and phytoplankton subduc tion budgets. This enhancement is due to the reinforced mesoscale physics a nd is also achieved by the small-scale frontal dynamics. This sub-mesoscale physics is associated with density and vorticity gradients around and betw een the eddies. It triggers a significant small-scale nutrient injection in the surface layers, leading to a phytoplankton field mostly dominated by f ine spatial structures. It is believed that, depending on wind forcings, th is scenario should work alternately with that of Abraham (1998) which invok es horizontal stirring of nutrient injected at large scales. Results also r eveal a strong relationship between new production and negative vorticity, in the absence of wind forcing and during the period of formation of the ed dies.