Interannual Fluctuations of the Tropical Easterly Jet and the Summer Monsoon in the Asian Region

Abstract

Synoptic analysis was conducted of interannual variations of the tropical summer monsoon circulation during the 17-year period from 1964 to 1980. The 150 mb wind speed at 10°N (40°E-110°E) were read from the analyzed maps and used to represent the fluctuations of the tropical easterly jet stream (TEJ). The strength of the TEJ was used as an indicator of the strength of the tropical summer monsoon over Asia (10-30°N, 40-130°E). Evidence has been shown that the interannual fluctuations of the summer monsoon are under the strong influence of the middle latitude circulation of the northern hemisphere. When the monsoon is strong, the circulation near 50°N is zonal. On the other hand when the monsoon is weak, a blocking high develops to the north of the Caspian Sea and a trough develops near 50°N, 110-120°E. The Walker circulation plays a smaller role compared to the winter monsoon. However major El Nino years (1965, 1972, 1976) were found to be the years with weak monsoon. The 1979 MONEX summer was one with a weak monsoon. Only 3 of the past 17 summers (major El Nino years) had weaker monsoon compared to the MONEX summer.