IMPACT OF EL-NINO ON THE EAST-ASIAN MONSOON - A DIAGNOSTIC STUDY OF THE 86 87 AND 91/92 EVENTS/

The East Asian monsoon circulation during the El Nino episodes of '86/ 87 and '91/92 is studied diagnostically. It is found that a southerly wind anomaly appeared in the lower troposphere along the coast of the East Asia during the mature phases of these two El Nino events. In the case of '86/87, the mature phase covered the boreal summer and the Ea st Asian summer monsoon was intensified. On the other hand, in the cas e of '91/92, the northern winter was within the mature phase and the E ast Asian winter monsoon was weakened. Examination of the circulation features suggests that the effects of the El Nino events on the East A sian monsoon are felt through the variation of convective activities o ver the western equatorial Pacific. Convections in this area are stron gly influenced by the evolution of sea surface temperature anomalies i n the equatorial Pacific and are strongly suppressed during the mature phase, which exerts significant influences on the direct monsoonal ci rculation over the western tropical Pacific and the East Asia. The rel ationship between the East Asian monsoon and El Nino in its mature pha se is also confirmed by a historical 850 hPa wind dataset that covers 6 events prior to the '86/87 event. The dataset also suggests that an inverse relationship does not hold during the La Nina periods.