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 Str eam and the Kuroshio Extension were studied. The seasonal gyre circula tion is roughly confined to the regions where the annual mean subtropi cal recirculations exist. Associated with this seasonal gyre circulati on, the surface transports of the Kuroshio and the Gulf Stream are fou nd to be maximum in the late fall and minimum in the late spring. Usin g historical data, the authors demonstrated that these seasonal gyre c irculations are mostly confined to the mixed layer. A simple diagnosti c calculation of the buoyancy balance associated with the seasonal gyr e 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 th e opposite sense to that originally proposed by Worthington for the an nual mean subtropical recirculation. The buoyancy balance within the s tudy region suggests that dynamics associated with the mean recirculat ion and the seasonal gyre are fundamentally different.