Implications of eddy cancellation for nutrient distribution within subtropical gyres

The role of mesoscale eddies within the nutrient budget of subtropical gyres remains poorly understood and poorly constrained. We explore a new mechanism by which mesoscale eddies may contribute to these nutrient budgets, namely eddy cancellation. Eddy cancellation describes the rectified effect of mesoscale eddies acting to oppose the Eulerian-mean Ekman pumping. We present an idealized axisymmetric two-layer model of a nutrient in a wind-driven gyre and explore the sensitivity of this model to variations in its parameter values. We find that the residual Ekman pumping velocity has a substantial impact on nutrient concentration, as does mode water thickness. These results suggest the response to both residual Ekman pumping and mode water thickness is nonmonotonic: for small values of these parameters the nutrient concentration decreases as the parameter increases. However, beyond a critical value, further increases in Ekman pumping or mode water thickness increase nutrient concentration throughout our highly idealized model. A thin mode water layer promotes vertical diffusion of nutrients from the abyss, while a thicker mode water layer increases productivity by reducing the parametrized particulate flux through the thermocline. The impact of mode water thickness is modulated by the residual Ekman pumping velocity: strong Ekman pumping suppresses the influence of mode water thickness on nutrient concentrations. We use satellite and in situ measurements to assess the influence of mode water thickness on primary productivity and find a statistically significant relationship; thicker mode water correlates with higher productivity. This result is consistent with a small residual Ekman pumping velocity.