Eddy Stirring and Diapycnal Transfer of Nutrients to the Euphotic Zone and Upper Thermocline of the North Atlantic Subtropical Gyre

There has been a long standing problem of how sufficient nutrients are supplied to the euphotic zone at mid latitudes in order to sustain biological productivity. Here we explore the potential role of eddy stirring along neutral surface in supplying nutrients to the euphotic zone in the North Atlantic subtropical gyre. In summer 2016 an observational campaign was performed consisting of 67 stations of CTD, nutrient, and microstructure profiles, in the vicinity of the mid Atlantic ridge. This work was also supported by a year-long deployment of a mooring with full water column velocity data. With these data we calculate the supply of nutrients to the euphotic zone driven by mesoscale turbulence acting along neutral surfaces: (i) the along neutral diffusivity is taken from the excess temperature variance dissipation compared to the turbulent kinetic energy dissipation and invoking a scale separation between across and along neutral surface mixing, and (ii) the along neutral surface nutrient gradients are estimated from both the observed nutrient field and the WOA climatology. This estimate of the eddy stirring is also supported by an independent estimate using eddy statistics from satellite observations which are then extended through depth using mooring data. Both of these techniques provide comparable estimates of the contribution of mesoscale eddies to the diffusivity and nutrient flux directed along neutral surfaces. For both nitrate and phosphate, there are comparable contributions to the nutrient transfer over the upper ocean from the transfers along and across neutral surfaces. Hence, both eddy stirring and diapycnal transfers are jointly important in supplying nutrients to the euphotic zone during summer. On denser surfaces below the euphotic zone, the nutrient transfer is stronger along neutral surfaces, indicating that eddy stirring is playing a leading role in resupplying nutrients in the upper thermocline.