The role of eddies in the isopycnic transfer of nutrients and their impact on biological production

Eddies provide a systematic, large-scale transfer of tracers and nutrients along isopycnals through a combination of eddy-induced diffusion and advection. The nutrient distribution is controlled by advection, rather than diffusion, when the nutrient lifetime exceeds a timescale given by kappa /V-2 where kappa is the lateral diffusivity and V is the characteristic transport velocity. Idealized, eddy-resolving experiments are conducted using an isopycnic model configured for a zonal channel. Whether the eddies act to enhance or inhibit biological production depends on the location of the nutrient source and the nutrient lifetime. For a subtropical gyre and the Southern Ocean, the eddy-induced diffusion and advection are likely to oppose each other over the euphotic zone but reinforce each other at depth. Over the euphotic zone, the combination of eddy diffusion and Ekman advection of nutrients should dominate the opposing eddy-induced advection. This lateral transfer becomes more important for long-lived nutrients, such as dissolved organic nitrogen, within the euphotic zone.