On the Role of the Cross Equatorial Flow on Summer MonsoonRainfall over India using NCEP/NCAR Reanalysis Data

Summary  The role of the cross equatorial flow from the southern Indian Ocean on the Indian Summer monsoon is examined using the National Centre for Environmental Prediction (NCEP)/National Centre for Atmospheric Research (NCAR) data for the period January 1982 to December 1994. A comparison of NCEP/NCAR data with the satellite data retrieved from the Special Sensor Microwave Imager (SSM/I) sensor onboard the Defense Meteorological Satellite Program (DMSP) exhibited a negative bias for the wind speeds greater than 4 m/s. whereas in the case of specific humidity, SSMI values exhibited a positive bias and the precipitable water derived from the satellite data exhibited a negative bias. The NCEP reanalysis is able to depict the mean annual cycle of both the cross equatorial flow and moisture flow into the Indian subcontinent during the monsoon season, but it fails to depict these differences during excess (1983, 1988, 1994) and deficit monsoon (1982, 1986, 1987) composites. Further, it is seen that inter hemispheric flow far exceeds the excess moisture available over the Arabian Sea indicating that it is the cross equatorial flow which decides the fate of the Indian summer monsoon. Received September 29, 1998 Revised May 20, 1999