Eddy Compensation in Wind-Driven Subtropical Gyres and Implications for Biological Productivity

Large-scale Ekman-induced downwelling is believed to suppress biological activity in subtropical gyres. Despite much work, the nutrient budget of these gyres remains poorly understood; estimates of primary productivity are consistently higher than estimates of nutrient supply. The existence of eddy compensation in the Southern Ocean, in which an eddy induced overturning circulation largely compensates the Ekman driven overturning circulation, is widely accepted. By analysing output from an idealised eddy-resolving model we show that eddy compensation also occurs within wind-driven gyres. By taking area integrals of vertical potential vorticity fluxes within the subtropical gyre, we show that Ekman induced downwelling is opposed and largely compensated by eddy induced upwelling. Using Lagrangian particle tracking techniques, we confirm that the Ekman-induced downwelling observed in the Eulerian-mean velocity fields, is suppressed when the particles are advected by the full time-varying velocity fields. The implications of these results for biological productivity are explored by the inclusion of a simple nutrient in our model. Our results may explain the unexpectedly high levels of biological productivity observed in subtropical gyres despite large-scale Ekman-induced downwelling.