DOMINANT MODES IN THE NP-DICE
We focus on the period 1967-1991, during which quality of the SST data is higher particularly in the low latitudes than before (Ostrovskii and Piterbarg 1995). During that period the decadal SST variability within the North Pacific basin was concentrated along the two major oceanic fronts (Fig. 1a). The strongest variability is found in the subarctic frontal zone (SAFZ), and the second maximum is in the subtropical frontal zone (STFZ). In contrast, the SST variability with shorter periods tends to be confined in the frontal zones to much lesser degree, especially over the eastern half of the Pacific (Fig. 1b), where the distribution is determined primarily by the atmospheric response to ENSO. In SAFZ, the decadal variability dominates over the shorter-period variability. Except in the tropics, the strength of the decadal SST variability and its seasonality markedly reflect even fine details of the oceanic frontal structure and its seasonal variability, respectively (Fig. 2). In specific, extratropical fronts tend to be more intense in winter than in summer, and so are the decadal fluctuations along them. This tendency most clearly appears in the Kuroshio/Oyashio inter-frontal zone (KOIZ), where an intense SST front extends to 〜35°N in winter associated with the Oyashio penetration along the Japanese coast but the front retreats back in summer to 〜40°N. The tendency also appears in SAFZ, the East China Sea, an intense frontal zone in the Sea of Japan and the western end of STFZ.
