The evolution of mean conditions of surface meteorological fields during active/break phases of the Indian summer monsoon

Abstract

The intra-seasonal variability of the Indian summer monsoon (ISM) is an important aspect that has been studied by various researchers, but still requires further examination to advance our understanding. A break monsoon situation, which is nothing more than a considerable reduction in rainfall over most parts of the Indian sub-continent, lasting from a few days to a few weeks, is one of the very significant periods of intra-seasonal variability of the ISM. In the present study, we examine the relationship between the break monsoon situation and changes in surface meteorological parameters using the National Centers for Environmental Prediction (NCEP) reanalysis datasets. Break monsoon situations persisting for a period of 3 days or more during 1968-1996 have been considered. An examination of average pentad data, commencing two pentads prior to the onset of the break situation to two pentads after the cessation of the break monsoon, is carried out in a sequential mode. The regions with statistically significant differences between prior and present break pentad are delineated by a Student's t-test at the 95% confidence level. The aim is to examine the evolution of the break monsoon and its relationship with surface meteorological parameters and to identify possible precursors, that can assist in the identification of the initiation or cessation of the break monsoon situation over India. It is noted that the onset of the break is a gradual process, whereas the cessation of the break is much more rapid. Prior to the occurrence of the break over India, the surface meteorological fields and derived surface heat fluxes clearly indicate significant changes over the South China Sea and South-East Asia. The precursor signals over the South China Sea can be used as potential predictors for the forecasting of the break period in the summer monsoon over India.