Composite analyses of wintertime wind stress vector fields with respect to SST anomalies in the western north Pacific and the ENSO events

Long-term wind stress vector (WSV) fields in winter over the North Pacific, which were calculated by Kutsuwada and Teramoto (1987), are analyzed by means of the composite method with respect to SST anomalies in the mid-latitudes of western North Pacific (SST composite). According to SST anomalies, two categorized winters, i.e., warm and cold winters are selected during 24 years from 1961 to 1984. The numbers of warm and cold winters are six and five, respectively. In order to examine whether or not the composite WSV fields are well-ordered and/or rigid ones, maps of stability of WSV anomalies are constructed, and the new parameter, Degree of similarity of WSV anomaly field of each winter to the composited WSV anomaly field is introduced and discussed. Both parameters show that the extracted patterns for two categorized winters are well-ordered. In warm (cold) winter, mid-latitudes westerly weakens (strengthens) and shifts northward (southward). As a result, the East Asian Winter Monsoon (Kisetsuhu) over Japan weakens (strengthens). It is also seen that in the equatorial region, in warm winter the region with SST higher than 28ºC extends to the central to eastern part and its anomaly fields are very similar to those in ENSO year winter. Actually, winters selected as warm (cold) winter include the ENSO (ENSO+1) year winters, i.e., winters during ENSO events, but not the ENSO+1 (ENSO) year winters. Time series of Degree of similarity for westerly region to warm-winter composited WSV anomaly fields are well in agreement with those of “Far East zonal index”, which is used by the Japan Meteorological Agency. From the composite map for sea level pressures, it is shown that shifts of westerly axis correspond to north (warm winter)-south (cold winter) shifts of the Aleutian Low.