Observations of the Kuroshio's barotropic and baroclinic responses to basin-wide wind forcing

Observations show that the Kuroshio in the East China Sea (ECS-Kuroshio) responds to the large-scale wind stress curl field at two time scales. It is argued that these two responses are related to barotropic and baroclinic modes that reach the ECS via different waveguides. Variability in the ECS-Kuroshio is assessed by comparing satellite altimetry, historical hydrography, and the Pacific Decadal Oscillation (PDO) index with the latter used as a proxy for the large-scale wind stress curl forcing. Sea level difference across the ECS-Kuroshio is positively correlated with PDO at zero lag and negatively correlated at 7 year lag. In contrast, pycnocline steepness and PDO are uncorrelated at zero lag and negatively correlated at 7 year lag. These signals in the ECS-Kuroshio, considered together with wind stress curl anomalies in the open ocean, are consistent with a barotropic response to the wind at zero lag. The barotropic response is likely forced in the central North Pacific by wind stress curl anomalies of opposite sign, one of which is centered at ECS latitudes (similar to 27 degrees N) while the other sits further north. This suggests that, in general, the absolute transport at a given latitude is not simply that predicted by the Sverdrup balance along the latitude. This is a consequence of waveguides that can steer the barotropic mode across latitude lines. In contrast, the signals that lag PDO by 7 years are consistent with a baroclinic mode, which represents the ocean's time-integrated response to the wind stress curl along a single latitude band between 24 degrees N and 27 degrees N.