Nutrient streams and their induction into the mixed layer

Nutrients are transferred from the nutricline into the winter mixed layer through a combination of vertical and lateral advection, referred to here as induction, the reverse of the subduction process. This advective supply of nutrients maintains the high productivity at high latitudes over timescales of several years and longer. Climatological diagnostics over the North Atlantic reveal that the induction of nutrients into the winter mixed layer is dominated by lateral advection even in the subpolar gyre where there is significant Ekman upwelling. The induction flux of nutrients is itself sustained by nutrient streams, strong nutrient transport associated with the western boundary currents. In the North Atlantic basin, the integrated induction flux accounts for typically 40% of the nitrate flux in the nutrient stream at 36 degrees N, while the remaining fraction is recirculated in the thermocline of the subtropical gyre. The relationship between the nutrient stream and induction flux is illustrated using a numerical model of the circulation and biogeochemistry of the North Atlantic. The model studies suggest that the nutrient stream preferentially supplies nutrients to isopycnals outcropping in the subpolar gyre. The nutrient sources for the nutrient stream are either mode waters originating from the Southern Ocean or the tropics, where the eastern upwelling circulation and particle fallout focuses nutrients onto the lighter isopycnal surfaces of the nutrient stream.