Contribution of mesoscale processes to nutrient budgets in the Arabian Sea

We examine the impact of mesoscale dynamics on the seasonal cycle of primary production in the Arabian Sea with an eddy-resolving (1/12 degrees) bio-physical model. Comparison with observations indicates that the numerical model provides a realistic description of climatological physical and biogeochemical fields as well as their mesoscale variability during the Southwest and Northeast Monsoons. We show that mesoscale dynamics favors biological production by modulating the nutrient supplies throughout the year. Different processes are involved depending on the blooming season. During the summer bloom period, we found that the main process is the export of nutrients from coastal upwelling regions into the central Arabian Sea by mesoscale filaments. Our model suggests that lateral advection accounts for 50-70% of the total supply of nutrients to the central AS. A less expected result is the major input of nutrients (up to 60-90%) supplied to upwelling regions during the early stage of the summer bloom period by eddy-induced vertical advection. During the winter bloom period, our model evidences for the first time how vertical velocities associated with mesoscale structures increase the supply of nutrients to the upper layer by 40-50% in the central Arabian Sea. Finally, the restratification effect of mesoscale structures modulates spatially and temporally the restratification that occurs at large-scale at the end of the Northeast Monsoon. Although this effect has no significant impact on the large-scale budget, it could be a source of uncertainty in satellite and in-situ observations.