Causes of Interannual and Interdecadal Variations of the Pacific-Japan Pattern in northern Summer

A Pacific-Japan (PJ) pattern index is defined based on the singular value decomposition (SVD) analysis of summertime 500-hPa height in East Asia and precipitation in the tropical western North Pacific (WNP). The time series of this PJ index shows clearly the interannual and interdecadal variations since 1948. Idealized atmospheric general circulation model (AGCM) experiments were carried out to understand the remote and local SST forcing in causing the interannual and interdecadal variations of the PJ pattern. It is found that the PJ interannual variation is closely related to the El Niño Southern Oscillation (ENSO). A basin wide warming occurs in the tropical Indian Ocean (TIO) during El Nino mature winter. The TIO warming persists from El Nino peak winter to succeeding summer. Meanwhile, a cold SST anomaly (SSTA) appears to the eastern of WNP and persists from El Nino mature winter to succeeding summer. Idealized AGCM experiments that separate the TIO and WNP SSTA forcing effects show that both the remote TIO forcing and local WNP SSTA forcing are important in affecting atmospheric heating anomaly in WNP monsoon region, which further impacts the PJ teleconnection pattern over East Asia. In contrast to the interannual variation, the interdecadal change of the PJ pattern is primarily affected by the interdecadal change of SST in TIO, not affected by the local SSTA in the WNP.