ABSTRACT

Recent coupled ocean-atmospheric modeling experiments have suggested a dynamic role for ocean western boundary currents in decadal climate variability. Indeed, the largest SST variability on the interannual- to-decadal time scale in the North Pacific resides in the outflow regions of the Kuroshio and the Oyashio, the two prominent ocean western boundary currents. Analyzing sea surface height data from multi-year satellite altimetry missions reveals that the large-scale, interannual changes of the Kuroshio Extension are characterized by the oscillation between an elongated state and a contracted state. In the elongated state, the Kuroshio Extension has a larger eastward surface transport, a greater zonal penetration, and a more northerly zonal-mean path. All these characteristics are closely connected to the presence of an intense, zonally-elongated southern recirculation gyre. The large-scale, interannual changes in the Kuroshio Extension system have a significant impact on the regional wintertime SST anomaly field: the warm (cold) wintertime SST anomalies tend to persist in years when the Kuroshio Extension is in its elongated (contracted) state. A diagnostic analysis of the surface ocean heat balance indicates that the nonseasonal geostrophic advection by the ocean circulation works to reduce (increase) the wintertime SST anomalies when the Kuroshio Extension changes from an elongated (contracted) state to a contracted (elongated) state. The SST anomalies associated with the large-scale changes of the Kuroshio Extension have an area-averaged, peak-to-peak amplitude > 1°C and appear independent of the interannual SST changes in the tropical Pacific Ocean.