The reversing roles of local and remote circulations on the Indian summer monsoon under a warming scenario

The simulated response of the Indian summer monsoon to increasing greenhouse gases has been fairly robust, with a majority of the climate models showing an increase in the seasonal mean rainfall and a decrease in the intensity of the associated monsoon circulation. The background global warming signal produces an increased surface and lower tropospheric meridional temperature gradient over the Indian region, while also producing anomalously positive sea surface temperatures (SSTs) in the tropical Pacific. Our analysis reveals that the monsoon outflow associated with the local circulation driven by the differential (solar plus convective) heating explains more monsoon rainfall variance than that explained by the Walker circulation driven by the remote tropical Pacific SST anomalies under a warming scenario. In the simulated present-day climate, Walker circulation explains more rainfall variance than that explained by the local circulation, consistent with the well-documented conventional relationships between the monsoon rainfall and ENSO. Thus, remote and local circulations reverse their corresponding roles on influencing the monsoon rainfall. Climatologically, the mean ascending branch of the Walker circulation shifts eastward, resulting in an anomalous upper level convergence and corresponding anomalous lower level divergence over the eastern tropical Indian Ocean. The resulting pressure gradients at lower levels favor the northwestward displacement of low-level monsoon flow. At upper levels an anomalous descending motion produces an anomalous adiabatic compression over the south of the equator, reducing the upper level meridional temperature gradient. Changes in the mean local circulation reinforce the northwestward displacement of the low-level monsoon flow and also promote upper level meridional divergent flow toward the equator.