FIG. 2 Magnitude of the climatological-mean SST gradient (℃/100km) over the Northwestern Pacific for boreal (a) winter and (b) summer. Shaded for ＞ 0.8 and contoured for every 0.4 (heavy lines for 2.0). Tropics are excluded where the gradient ＜ 0.8 in both seasons. (c) Decadal SST variability as measured by its standard deviation (σ, contoured for σ= 0.3, 0.5, 0.7, 1.1 and 1.3℃; heavy lines for σ= 0.5℃) for boreal winter. Shading is the same as in (a). (d) As in (c) but for summer, but shading is as in (b). All statistics are based on the JMA data. After NY98.
To extract dominant modes of the wintertime decadal SST variability associated NP-DICEs, a conventional EOF analysis was applied to the whole domain of the JMA data for 1967-1991. We focus on the 3 leading EOFs, whose basinwide structure and associated atmospheric anomalies are shown in Fig. 3. The first mode accounts for 41% of the decadal SST variance over the NW Pacific. It represents the fluctuations in SAFZ, which is in seesaw with those in the Gulf of Alaska but exhibit no significant simultaneous correlation with the tropical and subtropical fluctuations (Fig. 3d). Our first EOF is almost identical to that of NLY97 based on the COADS data. The decadal SST anomalies in SAFZ tend to be in conjunc-tion with the anomalous Aleutian low at the surface (Fig. 3c) and the “genuine” PNA pattern aloft (Fig. 3a). Polarities of these anomalies are such that, during warm periods in SAFZ, the westerlies at the surface and aloft tend to be weakened over the extratropics in accordance with the weak Aleutian Low, and it tends to be wamer above SAFZ (Fig. 3b). The opposite is the case during cool periods. The second EOF, which accounts for 〜 17% of the total variance, captures an in-phase relationship in the decadal SST variability between the equatorial Pacific and the Kuroshio off Japan (Fig. 3h). In a basin-wide view, the anomalies form a seesaw between the tropics and STFZ, just like in EOF2 of NLY97. This mode, which represents decadal SST fluctuations within the subtropical gyre and tropics, is accompanied by the anomalous surface subtropical high (Fig. 3g) and seesaw-like anomalies aloft over the NE Pacific (Fig. 3e). The SST anomalies in STFZ tends to have the same sign as air temperature anomalies aloft (Fig. 3f). The third mode accounts for 〜15% of the wintertime decadal SST variance over the NW Pacific. Significant, coherent SST anomalies associated with this mode can be seen only over the East China Sea and the southern Sea of Japan (Fig. 3l). This mode, trapped near the East Asian coast, was not identified in NLY97. Associated upper-level atmos-pheric anomalies include the Eurasian (EU) pattern (Fig. 3i), and in a hemispheric view it bears some resemblance to the “Arctic Oscillation” (Thompson and Wallace 1998). During warm periods in the East China Sea, the surface Siberian high tends to be weakened (Fig. 3k) with the warmer air-temperatures over the Far East. (Fig. 3j). This mode may be interpreted as the one representing influence of the decadal modulation of the East Asian winter monsoon upon the relatively-shallow marginal seas.
