ON THE NATURE OF THE 1994 EAST-ASIAN SUMMER DROUGHT

East Asian countries experienced record-breaking heat waves and drough t conditions during the summer monsoon season of 1994. This study docu ments the large-scale circulation associated with the drought and sugg ests a forcing mechanism responsible for the anomalous evolution of th e East Asian monsoon. The results, based on Goddard Earth Observing Sy stem (GEOS) global assimilated data for 1985-94, indicate that the abs ence of monsoon rainfall during July 1994 over central China and the s outhern parts of Korea and Japan is due to the unusually early develop ment of the climatological upper-level anticyclonic flow east of the T ibetan Plateau. The anomalous July anticyclonic circulation over the E ast Asian-northwestern Pacific region and the cyclonic circulation ove r the subtropical western Pacific, which are more typical of August, a cted to reduce the moisture supply from the western Pacific and the In dian Ocean leading to suppressed rainfall over East Asia. The similari ty of the July 1994 East Asian circulation anomalies to the climatolog ical July to August change in these quantities suggests that the anoma lies may be viewed as an acceleration of the seasonal cycle in which t he circulation transitions to August conditions earlier than normal. N either tropical nor middle latitude SST anomalies provide a viable for cing mechanism for the 1994 East Asian circulation anomalies: the trop ical anomalies are weak and the middle latitude anomalies, while stron ger, appear to be primarily a response to atmospheric forcing, though they may feed back to reinforce the atmospheric anomalies. It is sugge sted, instead, that the anomalous circulation is primarily the result of an orographic forcing associated with zonal wind changes over Tibet . The zonal wind change, characterized by an anomalous northward shift of the East Asian jet is, in turn, tied to unusually persistent stati onary waves extending from northern Europe, which developed prior to t he onset of the East Asian anticyclone. Several other occurrences of a tmospheric anomalies similar in structure (though weaker in amplitude) to the July 1994 anomalies are found in the previous nine summers, su ggesting the operative mechanism is not unique to 1994. Such a mechani sm appears to operate both for the climatological development of the r idge and for the occurrences of similar anomalies in previous summers: in the former the northward shift of the jet over Tibet is a reflecti on of climatological seasonal change in the zonal wind, while in the l atter, the shift is the result of anomalies similar in structure to th e 1994 European-Asian wave pattern. The indirect role of the Eurasian waves in the development of the East Asian circulation anomalies sugge sts that useful monthly and longer predictions of the monsoon rests, n ot only on our ability to predict the occurrence of these waves, but a lso on our ability to properly model their interaction with orography.