Seasonal Variation of Circulation in the Monsoon Area and Its Anomalies in El Nino Years

By using the EOF analysis method, the seasonal variation and its anomalies of circulation over the Indian Ocean﹑West pacific monsoon area during the period from May to September, 1981~1983 are studied. The first eigenvector field represents the monsoon component. The comparisons of the corresponding time coefficient curve for 1983 with those for 1981 and 1982 indicate that the summer monsoon circulation in E1 Nino years is weak, and the circulation transition from winter monsoon pattern to summer monsoon pattern is later than the normal, which is reflected by the time lag of the seasonal northward shift of the subtropical high over West Pacific. The corresponding numerical experiments in terms of the IAP GCM confirm to the above results.