Present and Future of the South Asian Summer Monsoon’s Rainy Season over Northeast India

Abstract An iconic aspect of the Indian Summer Monsoon Rainfall (ISMR), a rainy season that lasts longer than June to September over Northeast India (NEI) and considerably shorter over Northwest India, is expected to be significantly altered by climate change. The lack of a physically based objective definition of the monsoon rainy season over the NEI introduces significant uncertainty in impact assessment, variability, and predictability studies of ISMR. Here, we objectively define the ‘Length of the Rainy Season’ (LRS) for NEI and show that the El Nino and Southern Oscillation is a primary drivers of LRS over NEI. In contrast to a significant decreasing trend of LRS and LRS-rainfall during the historical period over NEI, the projected LRS under SSP5-8.5 continues to decrease while the LRS rainfall acquires a significant increasing trend. Implications of the findings on variability and predictability of NEI-rainfall and in the context of large-scale ISMR are discussed.