Insights into Indian summer monsoon rainfall variability: early twentieth century warming vs. mid-twentieth century cooling

The north–south asymmetry in Indian Summer Monsoon Rainfall (ISMR) anomalies of the 1930s provides critical insights into the effects of Early Twentieth Century Warming (ETCW) on regional climate dynamics. In ETWC, central and northeast India experienced a significant increase in rainfall, while south peninsular India recorded a notable decline. After the 1930s, this trend reversed during the Mid-20th Century Cooling (MTCC), with decreasing rainfall trend in central and northeast India and increasing rainfall trend in the south peninsular India, which were significant at the 0.05 level. The heightened rainfall in central and northeast India during the ETCW is attributed to more frequent monsoon depressions in the Bay of Bengal (BoB), driven by increased low-level vorticity and reduced vertical wind shear. The contrasting temperatures between the warmer land and cooler ocean intensified the low-level jet and moisture transport over the northern Arabian Sea, increasing wind speed and moisture transport into the BoB. Additionally, positive phases of the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation suppressed upper-level winds over the BoB, further reducing vertical wind shear and promoting monsoon depression formation. During the transition to the MTCC, land cooling and Indian Ocean warming reduced the land-sea thermal contrast, resulting in decreased rainfall in central and northeast India and increased rainfall in south peninsular India. The dipole-like ISMR trend is linked to global teleconnection through the internal variability of ISMR, particularly the variation of synoptic-scale systems.