Surface thermal conditions in the western North Pacific during the ENSO events

In order to clarify the surface thermal conditions during the ENSO events, composite analyses of the SST anomaly fields were performed over the western North Pacific, and for the mixed layer in the Kuroshio current region at sections along the 137ºE line and a line over the Izu Ridge. Each winter during the 25 years from 1961 to 1985 was classified into one of four categorized winters, i.e., ENSO-1 year, ENSO year, ENSO+1 year and the other year winters. For example, for the 1982/83 ENSO event, the 1983 winter (January through March) was regarded as the ENSO year winter. It was found that in the ENSO year winter, a well-ordered positive SST anomaly appears in a wide zonal band along the 30ºN line, extending from the Asian coast to near 170ºE. On the other hand, the distribution of SST anomalies in the ENSO+1 year winter is quite similar to that in the ENSO year winter with its sign reversed. During the ENSO year winter, the mixed layer in the Kuroshio current region south of Japan was thinner and warmer than those in the other three categorized winters. One of possible causes for these differences was attributed to the strength of the east Asian winter monsoon. It is relatively weaker in the ENSO year winter, compared with the other three categorized winters.