On the Nature of the 1994 East Asian Summer Drought

Abstract

East Asian countries experienced record-breaking heat waves and drought conditions during the summer monsoon season of 1994. This study documents the large-scale circulation associated with the drought and suggests a forcing mechanism responsible for the anomalous evolution of the East Asian monsoon. The results, based on Goddard Earth Observing System (GEOS) global assimilated data for 1985?94, indicate that the absence of monsoon rainfall during July 1994 over central China and the southern parts of Korea and Japan is due to the unusually early development of the climatological upper-level anticyclonic flow east of the Tibetan Plateau. The anomalous July anticyclonic circulation over the East Asian?northwestern Pacific region and the cyclonic circulation over the subtropical western Pacific, which are more typical of August, acted to reduce the moisture supply from the western Pacific and the Indian Ocean leading to suppressed rainfall over East Asia. The similarity of the July 1994 East Asian circulation anomalies to the climatological July to August change in these quantities suggests that the anomalies may be viewed as an acceleration of the seasonal cycle in which the circulation transitions to August conditions earlier than normal. Neither tropical nor middle latitude SST anomalies provide a viable forcing mechanism for the 1994 East Asian circulation anomalies: the tropical anomalies are weak and the middle latitude anomalies, while stronger, appear to be primarily a response to atmospheric forcing, though they may feed back to reinforce the atmospheric anomalies. It is suggested, 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 stationary waves extending from northern Europe, which developed prior to the onset of the East Asian anticyclone. Several other occurrences of atmospheric anomalies similar in structure (though weaker in amplitude) to the July 1994 anomalies are found in the previous nine summers, suggesting the operative mechanism is not unique to 1994. Such a mechanism appears to operate both for the climatological development of the ridge and for the occurrences of similar anomalies in previous summers: in the former the northward shift of the jet over Tibet is a reflection of climatological seasonal change in the zonal wind, while in the latter, the shift is the result of anomalies similar in structure to the 1994 European?Asian wave pattern. The indirect role of the Eurasian waves in the development of the East Asian circulation anomalies suggests that useful monthly and longer predictions of the monsoon rests, not only on our ability to predict the occurrence of these waves, but also on our ability to properly model their interaction with orography.