Influence of eddies on vertical transport and nutrients in subtropical gyres
The presence of large-scale Ekman pumping associated with the climatological wind-stress curl is the textbook explanation for low biological activity in the subtropical gyres. Using an idealised model it is shown that Eulerian-mean Ekman pumping may be opposed by an eddy-driven circulation, analogous to the way in which the atmospheric Ferrel cell and the Southern Ocean Deacon cell are opposed by eddy-driven circulations. Potential vorticity fluxes, Lagrangian particle tracking, and depth-density streamfunctions are used to show that, in the model, the rectified effect of eddies acts to largely cancel the Eulerian-mean Ekman downwelling. To distinguish this effect from eddy compensation, it is proposed that the suppression of Eulerian-mean downwelling by eddies be called “eddy cancellation.”
Eddy cancellation highlights that the thermocline forms a barrier between the surface waters and the abyssal ocean. The presence of this barrier suggests that the nutrient budget of the subtropical mode water and euphotic zone should be considered together. An idealised two-layer axisymmetric model of nutrient concentration in subtropical gyres is developed. This model is used to explore the concept of a unified nutrient budget for the euphotic zone and mode water of subtropical gyres. The steady-state nutrient distribution and fluxes from the idealised model are compared with observations and previous studies. These comparisons show that the solutions of the idealised model are reasonable. Following this validation the sensitivity of the model to changes in parameter values is explored. The model predicts a non-monotonic response to changes in the residual Ekman pumping velocity, with a productivity minimum at 30 m year−1. The model also predicts a positive relationship between primary productivity and mode water thickness. The predicted relationship between primary productivity and mode water thickness is tested using observational datasets, which provide some evidence to support the conclusion that thicker mode waters lead to higher productivity in the oligotrophic gyres.