RECENT ABNORMAL CHANGES IN WINTERTIME ATMOSPHERIC RESPONSE TO TROPICAL SST FORCING

An ensemble of climate experiments has been performed using a T42 GCM version of the Japan Meteorological Agency global model to examine abn ormal changes in the wintertime extratropical atmospheric response to anomalous sea surface temperature (SST) forcing in recent decades. Thr ee independent 39-year integrations for the period 1955-1993 are force d by the same observed SST boundary condition. Although the tropical P acific SST anomalies since the late 1980s indicate El Nine-like patter ns, the Pacific/North American teleconnection (PNA) pattern is found t o be hardly simulated, which stands in sharp contrast with that for th e period 1977-1983. The SSTs in the vicinity of the maritime continent (the Indonesian region), including the tropical Indian. Ocean, are ve ry high during the period 1987-1993 and associated model atmosphere do es undergo pronounced zonal heating over almost the entire tropics. Su ch tropical heating does not necessarily favor the barotropic Rossby w ave dispersion into the extratropics emanating out of convective forci ng since weakened Walker circulation is not capable of producing local ized strong divergence anomalies, hence inhibiting generation of appar ent vorticity sources in the subtropics. If the prevailing tendency of high SSTs in the vicinity of the maritime continent persists from now on, it is anticipated that a good correlation between EI Nino-Souther n Oscillation (ENSO) and PNA, which has been pointed out by previous n umerous studies, becomes insignificant. This anomalous feature may be viewed as a signature of global warming.