A trend analysis of the Webster and Yang (1992) index shows that the monsoonal zonal circulation has changed during the last 50 years. Mean value between years 1948-1971 is 0.8, while mean value between 1972–2003 is -0.6, indicating a weakening of the circulation during the last 30 years. Additionally, observed sea surface temperature (SST) show an increasing trend in the Indian Ocean for all the seasons. Since the monsoon is a coupled ocean – atmosphere system, changes in the distribution and intensity of SST should affect the variability and dynamics of this system.

To study the response of the Asian monsoon to changes in the distribution of SST during the last century we used the Simplified Parameterization Primitive Equation Dynamics (SPEEDY) model, which was forced by four different configurations of SST. The sets of SST we consider are: 1) Mean SST from 1930 to 2000 (control run). 2) Mean SST from 1950 to 1960 (cold run). 3) Mean SST from 1990 to 2000 (warm run). 4) Mean SST from 1930 – 1940 (colder run). From these experiments, dynamic and precipitation variables were analyzed during the summer months from May to September. Also, several monsoonal indexes previously defined in the literature, indicative of the strength of the monsoon, were estimated from the ensemble means.

Precipitation indexes defined in the Bay of Bengal, the Indian continent, the Arabian Sea and the Indian Ocean show that the behavior of precipitation for each experiment changes between the beginning and the end of the summer, suggesting that different mechanisms compete in the generation of rainfall.

Comparison of circulation fields show that convergence and divergence patterns change with changes in SST distribution. Specifically, the distribution of SST in the Pacific is going to determine the strength and location of the Walker circulation which in turn is going to affect the subsidence and convergence patterns in the monsoonal system through teleconnections.

Finally, a trend analysis of Indian rainfall data by regions, in conjunction with the analysis of the circulations variables suggest that the mechanisms that control precipitation in the western India and the north east India and Bay of Bengal are different. Results show that the precipitation in the Indian peninsula in controlled by conditions that favor stronger monsoonal winds. Further, SST intensity plays an important role in the generation of precipitation in the Bay of Bengal and north east India, implying that warmer SST favors more intense precipitation.