Interannual and intraseasonal variations in monsoon depressions and their westward-propagating predecessors

The majority of monsoon depressions develop from the regenesis of westward- propagating residual lows from the east. Most of these residual lows can be traced to weather disturbances in the south China Sea, including tropical cyclones and 12-24-day monsoon lows. Hypothetically, any mechanism causing a variation in the occurrence frequency of these two types of weather distu rbances in the western tropical Pacific-south China Sea (WTP-SCS) region ma y result in a corresponding change in the formation frequency of monsoon de pressions over the Bay of Bengal. Two such possible mechanisms are interann ual and intraseasonal variations of large-scale summer circulation in the W TP-SCS region induced by 1) the interannual variation of the sea surface te mperature (SST) in the eastern tropical Pacific and 2) the northward migrat ion of the 30-60-day monsoon trough/ridge. The National Centers for Environ mental Prediction-National Center for Atmospheric Research reanalysis data and the 6-hourly tropical cyclone track collected by the Japan Meteorologic al Agency for the period of 1979-94 were analyzed to substantiate the afore mentioned hypothesis. The findings are as follows. 1) Interannual variation. Based upon the SST averaged over the National Oce anic and Atmospheric Administration NINO3 region (150 degrees-90 degrees W, 5 degrees S-5 degrees N), the summers of 1982, 1983, 1987, and 1991 and 19 81, 1984, 1985, 1988, 1989, and 1994 are defined as warm and cold, respecti vely. A clear interannual variation can be seen in the frequency of monsoon depressions in the Bay of Bengal: an enhancement (reduction) of monsoon de pression activity occurs during cold (warm) summers. This interannual varia tion of monsoon depression activity is traceable to the corresponding varia tion of the combined tropical cyclone and 12-24-day monsoon low frequency i n the south China Sea. The latter interannual variation results from the de velopment of an;anomalous anticyclonic (cyclonic) circulation between 15 de grees and 30 degrees N in the WTP-SCS region in response to the warm (cold) SST anomalies in the eastern tropical Pacific. 2) Intraseasonal variation. There is an intraseasonal variability in the oc currence of tropical cyclones and of 12-24-day monsoon lows over the south China Sea, which is followed by a corresponding variability of monsoon depr essions over the Bay of Bengal. The formation frequency of these depression s is dependent on the penetration role of the residual lows of these two ty pes of disturbances across Indochina. These residual lows lead to an intras easonal change in monsoon depression formation in connection with a deepeni ng/filling of the monsoon trough over northern India and the Bay of Bengal.