A synoptic-climatic study of the onset of southwest monsoon over Mumbai

Surface and upper air synoptic features prevailing at the time of onset of southwest monsoon over Mumbai have been examined for a period of 50 years f rom 1947 to 1996. These synoptic situations have been found to fall under t hree broad categories. The study shows that on about 66% of the occasions, the onset of monsoon over Mumbai is caused by low pressure area farming ove r southeast and east central Arabian Sea as they move northwards, while on 28% of the occasions, the onset is due to the low pressure systems forming over northwest/head Bay of Bengal and their movement towards northwest/west northwest. Based on the findings of the study and the climatological aspec ts, a criterion for declaring onset over Mumbai has been suggested.