LOW-FREQUENCY VARIABILITY IN REGIONS OF THE KUROSHIO EXTENSION AND THE GULF-STREAM

Using sea, surface height data collected by the TOPES/POSEIDON altimet er, we studied the low-frequency variability in the Kuroshio Extension and Gulf Stream. The most significant signals are the basin-wide back ground seasonal cycle and low-frequency wave activity generated over m ajor bottom bathymetry and subsequent westward propagation in both reg ions. The spatial structures of the seasonal cycle in the two regions are quite different. In the Kuroshio Extension the seasonal cycle has a northeast-southwest orientation and lies closer to the western bound ary, whereas it has an east-west orientation and lies more offshore in the Gulf Stream. The close relationship between the low-frequency wav e activity and major bottom bathymetry features is discussed through b oth empirical orthogonal functional (EOF) analysis and calculation of eddy kinetic energy and Reynolds stress. The spatial structures of the se eddy statistics are fundamentally different from those of high-freq uency mesoscale variability. They ape not in the proximity of maximum time-mean flow, but rather, they lie mostly to the west of major botto m bathymetry. The close association of the clusters of local maximum e ddy properties with major bottom topographic features in both regions, wave generation over the major bottom topographic features and subseq uent westward propagation, and difference of the eddy statistics from that associated with high-frequency mesoscale variability all suggest that unlike in the case with high-frequency mesoscale variability gene ration in which internal dynamics of the background mean flow such as nonlinear instability plays a fundamental role, external forcing throu gh bottom topography must play a fundamental role im generating the lo w-frequency wave activity.