Maturing process of the summer monsoon over the Western North Pacific - A coupled ocean/atmosphere system

The abrupt northward shift of the active convective region from 10 degrees N to 25 degrees N around 150 degrees E on July 25-29 (Pentad 42) seen in the seasonal cycle of the summer monsoon over the western North Pacific (convection jump) and other associated phenomena preceding this convection jump are investigated using data of equivalent black body temperature derived from GMS and sea surface temperature (SST) for the period of 1980-1994 together with circulation field data objectively analyzed by the ECMWF. The mechanism of the maturing process of the summer monsoon between middle June and late July are examined by comparing 8-year remarkable convection jump Sears (typical cases) and other years (atypical cases). Tongue-shaped warm-SST areas (warmer than 29 degrees C) are observed in early July around 20 degrees N, 150-160 degrees E, preceding the typical convection jump. This warming of SST is closely related to the appearance of a weak-wind region (weakening of easterlies) around 25 degrees N, 140 degrees E-160 degrees E in late June. These weak easterlies are likely to be associated with the propagation of a Rossby wave induced by the occurrence of the active convection near the Philippine Islands in middle to late June. Most of the atypical cases appear in the El-Nino years.