Summertime anomalous warming over the midlatitude western North Pacific and its relationships to the modulation of the Asian monsoon

An anomalously warm sea-surface temperature (SST) developed to the east of Japan in the midlatitude western North Pacific (WNP) during three consecutive years, from 1999 through to 2001. The anomalies in those years simultaneously reached a maximum deviation (>2.5 °C) during the boreal summer. Prior to this seasonal peaking, convective activity was significantly enhanced around the Philippine Sea, while Japan and the adjacent ocean were characterized by weak convection. Almost concurrently with this, the lower troposphere in the subtropical WNP exhibited an anomalous cyclonic flow slightly to the northwest of the Philippine convection. In contrast, anticyclonic anomalies prevailed over the midlatitude WNP in the area centred around 35°N, 15°E. The many changes observed in the convection and circulation fields can be explained by wave trains related to the stationary Rossby wave. In situ heat budget analysis at the ocean surface indicates that, in association with the midlatitude anticyclone, the combined effect of reduced evaporative cooling and intensified downward shortwave radiation flux can be responsible for the salient increase in SST. Furthermore, analysis of the subsurface water reveals that the anomalous state in the midlatitude WNP was limited in the near-surface layer, which is consistent with the result of the heat exchange at the ocean surface. Finally, these anomalous years correspond to a cold episode of El Niño, suggesting the potential predictability of the Asian summer monsoon. In view of this, we also examine precursor signals in the pre-monsoon season over the tropical Indian Ocean and their possible linkage to summertime climate conditions in the midlatitude WNP.