Abstract: The physical mechanisms causing the anomalous activities of East Asian summer monsoon and related rainfall-belts in 1999 have been investigated from the viewpoint of air-sea interaction.The results have shown that during the period from 1998 spring to 1999,because of the appearance of the strongest warm sea surface temperature (SST) anomalies in the region from South China Sea to the tropical western Pacific in past 20 years,the local thermal forcing induced by the persistent anomalous strong air-sea interaction in this region became the most important external forcing mechanism.This made the monsoon activities and precipitation pattern this year not consistent with the normal statistical relationship.During the period from 1998 fall to 1999,because of the response of tropical atmosphere to local strong heating induced by the anomalous warm SST from the South China Sea to the western Pacific,the anomalous circulation pattern similar to Gill mode was observed in the tropical western Pacific.The easterlies and westerlies were respectively found on eastern and western flanks of heat sources owing to the response of Kelvin and Rossby waves and a cyclone twin appeared in heat source region.This anomalous circulation pattern persisted from the previous winter to the summer and thus caused a strong Local positive feedback between the atmosphere and ocean.This allowed the anomalous circulation pattern and the strong warm SST to maintain in this region and thus evolved into the strongest signal that year.With the seasonal progress from the winter to summer,the impact of atmospheric basic state on the above persistent anomalous circulation resulted in the different tonal asymmetry of anomalous circulation in winter and summer,thus inducing the teleconnection wavetrain from the East Asia to North America in mid-summer.Under the influence of anomalous cyclonic circulation over the East Asian coast,the persistence of anomalous easterly winds over the East Asian coast was unfavorable to the turning of monsoonal westerlies in the South China Sea,which led to the anomalous weak northward advance of summer monsoon in East China and eastward shift of turning position of monsoonal westerlies in low latitudes.The sensitivity experiments of numerical model NCAR/CCM3 further supported that the persistent anomalous SST in the tropical Pacific from 1998 spring to 1999 can lead to the anomalous circulation and rainfall in East Asia in 1999.For that year,the local air-sea interaction in the western Pacific was the most important affecting factor and the remote forcing in the eastern Pacific took the second place.