Low‐frequency variability in regions of the Kuroshio Extension and the Gulf Stream

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Abstract
Using sea surface height data collected by the TOPEX/POSEIDON altimeter, we studied the low‐frequency variability in the Kuroshio Extension and Gulf Stream. The most significant signals are the basin‐wide background seasonal cycle and low‐frequency wave activity generated over major bottom bathymetry and subsequent westward propagation in both regions. 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 boundary, whereas it has an east‐west orientation and lies more offshore in the Gulf Stream. The close relationship between the low‐frequency wave activity and major bottom bathymetry features is discussed through both empirical orthogonal functional (EOF) analysis and calculation of eddy kinetic energy and Reynolds stress. The spatial structures of these eddy statistics are fundamentally different from those of high‐frequency mesoscale variability. They are not in the proximity of maximum time‐mean flow, but rather, they lie mostly to the west of major bottom bathymetry. The close association of the clusters of local maximum eddy properties with major bottom topographic features in both regions, wave generation over the major bottom topographic features and subsequent 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 generation in which internal dynamics of the background mean flow such as nonlinear instability plays a fundamental role, external forcing through bottom topography must play a fundamental role in generating the low‐frequency wave activity.