RECENT EXTRAORDINARY COOL AND HOT SUMMERS IN EAST-ASIA SIMULATED BY AN ENSEMBLE CLIMATE EXPERIMENT

An ensemble of three 40-year parallel simulations was performed using a T42 AGCM version of the Japan Meteorological Agency global model to answer the question why extraordinary cool and hot summers in East Asi a, especially Japan and Korea, tend to occur very frequently in recent years from the late 1970s to the early 1990s. Three independent long- term integrations from January 1955 to December 1994 were forced by th e same SST boundary condition observed on the global scale. Our AGCM s imulations employing prescribed observed SSTs were successful in repro ducing extratropical circulation anomalies that bring about the decada l-scale amplitude modulation of interannual variations of summer mean temperatures in the vicinity of Japan. During the period from the begi nning of 1980s to the early 1990s, the interannual variability of the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines became a ppreciably large, was accompanied by anomalous cumulus convection arou nd the Philippines, and its phases coincided quite well with those of model-simulated lower-tropospheric geopotential height variations near Japan. The anomalous convective heating substantially affected summer time lower tropospheric circulation anomalies in East Asia through the dynamic process of the excitation of PJ teleconnection pattern (Nitta , 1987). The anomalous SST forcing from the tropics is crucially respo nsible for the frequent occurrence of extreme cool and hot summers in Japan and Korea from the late 1970s to the early 1990s. The presence o f strong east-west gradient of SST anomalies across the Philippines is primarily attributed to the significant coupling of weak (strong) Sou th Asian summer monsoon and the warm (cold) episode of ENSO. The warm episodes that occurred during the period from the late 1970s to the ea rly 1990s are appreciably different from a typical model of El Nino ev ent exemplified by Rasmusson and Carpenter (1982) in terms of seasonal evolution. It is anticipated that both unusually persistent ENSO sign als from the preceding winter until summer and the associated South As ian summer monsoon activity strongly regulate the formation of the eas t-west SST gradient near the Philippines in boreal summer.