Can eddies make ocean deserts bloom?

An eddy-resolving coupled ecosystem-circulation model of the North Atlantic is used to investigate the impact of mesoscale variability on the nitrate supply to the light-lit euphotic zone. The focus is on the oligotrophic subtropical gyre where eddies have been suggested to reconcile apparently contradictory observational estimates of nutrient supply and export production. Comparison with observations indicates that the numerical model provides a realistic description of the subtropical eddy field and its interaction with biogeochemical tracers. The model results illustrate that the eddy-induced nitrate flux into the euphotic zone is largest near the margins of the oligotrophic gyre where both vertical and lateral nutrient supply by eddies are effective. Typical values of simulated eddy-induced nitrate supply are 0.05 mol m-2 yr-1, which is much lower than has been suggested previously. This new estimate of eddy-induced nitrate supply is not sufficient to reconcile seemingly contradictory observational estimates of biological production in the subtropics. Alternative sources of fixed nitrogen, deviations from standard elemental stoichiometry, and possible effects of interannual variability will have to be considered in order to resolve apparent observational discrepancies in the oligotrophic subtropical gyres