THE PHYSICS OF ENSO-MONSOON CONNECTION

The physical mechanism through which Ei-Nino and Southern Oscillation (ENSO) tends to produce deficient precipitation over Indian continent is investigated using both observations as well as a general circulati on model. Both analysis of observations and atmospheric general circul ation model (AGCM) study show that the planetary scale response associ ated with ENSO primarily influences the equatorial Indian Ocean region . Through this interaction it tends to favour the equatorial heat sour ce, enhance precipitation over the equatorial Indian Ocean and indirec tly cause a decrease in continental precipitation through induced subs idence. This situation is further complicated by the fact the regional tropospheric quasi biennial oscillation (QBO) has a bimodal structure over this region with large amplitude over the Indian continent. Whil e the ENSO response has a quasi-four year periodicity and tends peak d uring beginning of the calendar year, the QBO mode tends to peak durin g northern summer. Thus, the QBO mode exerts a stronger influence on t he interannual variability of the monsoon. The strength of the Indian monsoon in a given year depends on the combined effect of the ENSO and the QBO mode. Sines the two oscillations have disparate time scales, exact phase information of the two modes during northern summer is imp ortant in determining the Indian summer monsoon. The physical mechanis m of the interannual variations of the Indian monsoon precipitation as sociated with ENSO presented here is similar to the physical process t hat cause intraseasonal 'active', 'break' oscillations of the monsoon.