Surface vertical PV fluxes and subtropical mode water formation in an eddy-resolving numerical simulation

Subtropical mode waters are characterized by low potential vorticity (PV) and so the mechanisms by which PV is extracted from the ocean by air-sea interaction are of great relevance to our understanding of how mode waters are formed. This study analyzes those mechanisms by comparing the magnitude and spatial patterns of surface PV fluxes of diabatic and frictional origin in a high resolution numerical simulation of the North Atlantic. The model resolves mesoscale eddies and exhibits realism in the volume and regional distribution of subtropical mode water, both in the annual-mean and seasonal cycle. It is found that the diabatic and mechanic fluxes of PV through the sea surface are of similar amplitude locally, but their spatial structures are very different. The diabatic PV flux has a large scale pattern that reflects that of air-sea heat fluxes directed from the ocean to the atmosphere along and to the south of the separated Gulf Stream. In contrast the mechanical PV flux, because of its dependence on horizontal surface density gradients, exhibits much smaller scales but embedded within a coherent large scale pattern. When mapped over the North Atlantic subtropical mode water (EDW) outcropping region, the diabatic PV flux pattern is found to be directed out of the ocean everywhere, whereas the mechanical PV fluxes exhibits small-scale patterns of both sign. The amplitude of the diabatic PV fluxes is found to be at least one order of magnitude larger than the mechanical PV fluxes demonstrating the overwhelming importance of diabatic processes in creating mode waters. Finally, we note that the large scale climatological patterns and magnitudes of both diabatic and mechanical PV flux mapped over the EDW outcropping region, are very similar to patterns obtained from coarse-grained ocean state estimates that do not resolve the eddy field.