A diagnostic study on the fluctuations of atmospheric moisture over the Indian Ocean during the summer monsoon season is carried out for a 10-year period (1992-2001) using the SSM/I water vapour data. The global daily 10 C 10 regridded data sets of SSM/I for 1992 to 2001 have been used instead of the 0.250 C 0.250 data sets. We have analysed the normalized water vapour index (NWI) (as per the method suggested by Xubin and Lu, 2002) over different domains in the Indian Ocean. Time series of spatially averaged NWI over 4 different domains in the Indian Ocean, 3 domains in Arabian Sea (Central Arabian Sea (CAS), Somali Coast (SC) and Northern Arabian Sea (NAS)) and one domain in Central Bay of Bengal (C-BOB)), is analysed. The study reveals the prominence shown by C-BOB over the other domains in giving predictive signal for the gradual development of the NWI for predicting monsoon onset on a medium range scale. The NWI over CAS also indicated medium range predictive behaviour, but not to the same extent as that of Bay of Bengal domain. The study has also highlighted the effect of strong upwelling near the SC during the onset phase of the monsoon and the subsequent fall of NWI over this domain. Similarly the effect of intrusion of northerly desert air into NAS and the consequent reduction in this domain has also been brought out in this study.

The northward and eastward propagation of the moisture peak north of the equator has been analysed for the year 1994 with respect to the monsoon onset. The study has shown a noticeable relation of the moisture peak and advancement of the monsoon.