The role of mesoscale eddy stirring and microscale turbulence in sustaining biological production in the subtropical gyres

Sustaining phytoplankton primary production and organic carbon export requires the physical supply of nutrients to the sunlit ocean. In the extensive downwelling regions of the subtropical gyres, the pathways of this nutrient supply remain unclear. Vertical sinking of organic matter from the sunlit layer and its remineralization below cause net downward nutrient transfer in the upper subtropical ocean. Microscale mixing of nutrients across density surfaces and upwelling by mesoscale eddies and submesoscale fronts have been invoked to re-supply nutrients from the thermocline to the sunlit layer. However, a physical mechanism is required to replenish nutrients exported across the thermocline base and sustain a quasi-steady state upper-ocean nutrient budget on inter-annual timescales. Stirring along density surfaces by mesoscale eddies has emerged as a possible supply mechanism to close this nutrient budget. Here, we quantify the relative importance of mesoscale stirring and microscale mixing in supplying nutrients to the oligotrophic regions of the upper subtropical oceans, using global observationally based data sets for nutrients and diapycnal and isopycnal diffusivities. Mesoscale stirring dominates nutrient replenishment in the thermocline of subtropical gyres over microscale turbulence, contributing to 70%–90% of combined supply by the two processes. The stirring supply is most important along gyre flanks, where boundary currents and upwelling zones promote sharp nutrient gradients and vigorous mesoscale activity. Mesoscale fluxes provide sufficient nutrients to offset depletion in the thermocline due to upward microscale mixing into the sunlit layer. This analysis suggests that eddy stirring is significant in maintaining organic carbon export within subtropical gyres.