Annual variability of the subtropical recirculations in the North Atlantic and North Pacific: A Topex/Poseidon study

Using sea surface height data collected by Geosat and Topex/Poseidon, the seasonal( annual) gyre circulations in the regions of the Gulf Stream and the Kuroshio Extension were studied. The seasonal gyre circulation is roughly confined to the regions where the annual mean subtropical recirculations exist. Associated with this seasonal gyre circulation, the surface transports of the Kuroshio and the Gulf Stream are found to be maximum in the late fall and minimum in the late spring. Using historical data, the authors demonstrated that these seasonal gyre circulations are mostly confined to the mixed layer. A simple diagnostic calculation of the buoyancy balance associated with the seasonal gyre circulations shows that they are driven primarily by local buoyancy flux (heating and cooling), while contribution from advection by large-scale ocean circulation is negligible. Even though the seasonal gyre circulation is primarily driven by local buoyancy forcing, it is in the opposite sense to that originally proposed by Worthington for the annual mean subtropical recirculation. The buoyancy balance within the study region suggests that dynamics associated with the mean recirculation and the seasonal gyre are fundamentally different.