Regime shift of Indian summer monsoon rainfall to a persistent arid state: external forcing versus internal variability

Abstract

The high propensity of deficient monsoon rainfall over the Indian sub-continent in the recent 3 decades (seven deficient monsoons against 3 excess monsoon years) compared to the prior 3 decades has serious implications on the food and water resources in the country. Motivated by the need to understand the high occurrence of deficient monsoon during this period, we examine the change in predictability of the Indian summer monsoon (ISM) and its teleconnections with Indo-Pacific sea surface temperatures between the two periods. The shift in the tropical climate in the late 1970s appears to be one of the major reasons behind this. We find an increased predictability of the ISM in the recent 3 decades owing to reduced ‘internal’ interannual variability (IAV) due to the high-frequency modes, while the ‘external’ IAV arising from the low-frequency modes has remained largely the same. The Indian Ocean Dipole–ISM teleconnection has become positive during the monsoon season in the recent period thereby compensating for the weakened ENSO–ISM teleconnection. The central Pacific El-Nino and the Indian Ocean (IO) warming during the recent 3 decades are working together to realise enhanced ascending motion in the equatorial IO between 70°E and 100°E, preconditioning the Indian monsoon system prone to a deficient state.