Development of monsoon breaks due to heat source over northwest India

Abstract

A synoptic climatological analysis of pentad rainfall anomalies for 52 blocks over India for southwest monsoon season for the years 1901 to 1980 has been carried out to identify events of anomalous heavy rainfall over the western part of the monsoon trough area and study the change in rainfall distribution over central parts of the country in subsequent three pentads. This analysis is based on the results of a diagnostic study by Bhide et al. (1997) which showed that the events of development of diabatic heat sources due to convective heavy rainfall over the western part of the monsoon trough area were followed by the occurrence of a weak or break phase during monsoon 1979.

An event of positive pentad rainfall anomaly (SA) for a test area TA on the western part of the monsoon trough is considered as a basic synoptic event. SA is categorised into five classes C1 to C5 having ranges between –1 and 9. The change in rainfall anomalies for subsequent three pentads is studied. The pattern of rainfall anomalies for class C5, having the largest values of SA indicates development of a break phase over India and confirms the earlier observation for monsoon 1979. The development of a weak phase is indicated for class C4.