SYNOPTIC-SCALE DISTURBANCES OF THE INDIAN-SUMMER MONSOON AS SIMULATEDIN A HIGH-RESOLUTION CLIMATE MODEL

The Hamburg atmospheric general circulation model ECHAM3 at T106 resol ution (1.125 degrees lat./lon.) has considerable skill in reproducing the observed seasonal reversal of mean sea level pressure, the locatio n of the summer heat low as well as the position of the monsoon trough over the Indian subcontinent. The present-day climate and its seasona l cycle are realistically simulated by the model over this region. The model simulates the structure, intensity, frequency, movement and lif etime of monsoon depressions remarkably well. The number of monsoon de pressions/storms simulated by the model in a year ranged from 5 to 12 with an average frequency of 8.4 yr(-1), not significantly different f rom the observed climatology. The model also simulates the interannual variability in the formation of depressions over the north Bay of Ben gal during the summer monsoon season. In the warmer atmosphere under d oubled CO2 conditions, the number of monsoon depressions/cyclonic stor ms forming in Indian seas in a year ranged from 5 to 11 with an averag e frequency of 7.6 yr(-1), not significantly different from those infe rred in the control run of the model. However, under doubled CO2 condi tions, fewer depressions formed in the month of June. Neither the lowe st central pressure nor the maximum windspeed changes appreciably in m onsoon depressions identified under simulated enhanced greenhouse cond itions. The analysis suggests there will be no significant changes in the number and intensity of monsoon depressions in a warmer atmosphere .