Implications of Eddy Cancellation for Nutrient Distribution Within Subtropical Gyres

Plain Language Summary 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. Subtropical gyres are large oceanic circulations that span thousands of kilometers in the Pacific and Atlantic oceans, where very little grows. Phytoplankton, the microscopic plants that form the basis of the oceanic food chain, need nutrients, which are in short supply. However, some phytoplankton do live in these gyres and how they get their nutrients has been a mystery for decades. The textbooks state that winds push the surface waters into the center of the gyres, and then downward, removing nutrients from the upper sunlit waters and preventing phytoplankton from thriving. However, some recent works showed that this might not be correct. We build an idealized model that takes account of this new result. With our idealized model we find that the growth of phytoplankton at the surface is affected by a layer of water well below the surface called mode water; thicker mode waters help to support more phytoplankton growth near the surface. Using data from satellites, autonomous ocean robots, and ships, we find support for the result from our idealized model: thicker mode water enhances phytoplankton growth. This is a fundamental change in our understanding of how nutrients are supplied to the surface waters of subtropical gyres.