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

Abstract

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 disturbances in the western tropical Pacific-south China Sea (WTP-SCS) region may result in a corresponding change in the formation frequency of monsoon depressions over the Bay of Bengal. Two such possible mechanisms are interannual and intraseasonal variations of large-scale summer circulation in the WTP-SCS region induced by 1 ) the interannual variation of the sea surface temperature (SST) in the eastern tropical Pacific and 2) the northward migration of the 30-60-day monsoon trough/ridge. The National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data and the 6-hourly tropical cyclone track collected by the Japan Meteorological Agency for the period of 1979-94 were analyzed to substantiate the aforementioned hypothesis. The findings are as follows. 1) Interannual variation. Based upon the SST averaged over the National Oceanic and Atmospheric Administration NINO3 region (150°-90°W, 5°S-5°N), the summers of 1982, 1983, 1987, and 1991 and 1981, 1984, 1985, 1988, 1989, and 1994 are defined as warm and cold, respectively. A clear interannual variation can be seen in the frequency of monsoon depressions in the Bay of Bengal: an enhancement (reduction) of monsoon depression activity occurs during cold (warm) summers. This interannual variation of monsoon depression activity is traceable to the corresponding variation of the combined tropical cyclone and 12-24-day monsoon low frequency in the south China Sea. The latter interannual variation results from the development of an anomalous anticyclonic (cyclonic) circulation between 15° and 30°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 occurrence of tropical cyclones and of 12-24-day monsoon lows over the south China Sea, which is followed by a corresponding variability of monsoon depressions over the Bay of Bengal. The formation frequency of these depressions is dependent on the penetration role of the residual lows of these two types of disturbances across Indochina. These residual lows lead to an intraseasonal change in monsoon depression formation in connection with a deepening/filling of the monsoon trough over northern India and the Bay of Bengal.