VARIATION IN THE RELATIONSHIP OF THE INDIAN-SUMMER MONSOON WITH GLOBAL FACTORS

Utilizing data for the long period 1871-1990, variation in the relatio nships between Indian monsoon rainfall (IMR) and tendencies of the glo bal factors. Southern Oscillation Index (SOI) and the sea surface temp erature (SST) over eastern equatorial Pacific Ocean has been explored. The periods for which relationships exist have been identified. Tende ncies from the season SON (Sept-Oct-Nov) to season DJF (Dec-Jan-Feb) a nd from DJF to MAM (Mar-Apr-May) before the Indian summer monsoon are indicated respectively by SOIT-2/SSTT-2 and SOIT-1/SSTT-1, current ten dency from JJA (June-July-Aug) to SON, by SOIT0/SSTT0, tendencies from SON to DJF and DJF to MAM following monsoon, by SOIT1/SSTT1 and SOIT2 /SSTT2 respectively. It is observed that while the relationships of IM R with SSTT-1, SSTTO and SSTT2 exist almost throughout the whole perio d, that with SOIT-1 exists for 1942-1990, with SOIT0 for 1871-1921 and 1957-1990 and with SOIT2, for 1871-1921 only. The relationships that exist with SOIT-1, SOIT2, SSTT-1, SSTT2 and with SSTT0 (for period 193 1-1990) are found to be very good and those that exist with SOIT0 for periods 1871-1921 and 1957-90 and for SSTT0 for the period 1871-1930 a re good. It is thus seen that the relationships of SOIT-1, SOIT0 and S OIT2 with IMR do not correspond well with those of SSTT-1, SSTTO and S STT2 with IMR respectively, even though SOI and SST are closely relate d to each other for all the seasons. SOIT-1 and SSTT-1 can continue to be used as predictors for IMR. During the whole period, IMR is found to play a passive, i.e. of being influenced or anticipated by SSTT-1 a s well as an active role, i.e. of influencing or anticipating SSTT2. T his implies a complex and perhaps non-linear interaction between IMR a nd SST tendency from DJF to MAM. Possibly, this is a part of the large r interaction between Asian monsoon rainfall and the tropical Pacific. A possible physical mechanism for the interaction is indicated.