Sub-tropical Teleconnection Drives ‘Onset’ of Indian Summer Monsoon over Northeast India (NEI) in May.

Abstract While recognizing that the length of the rainy season over northeast India (NEI) is much longer than June-September as over Central India (CI), the official onset of Indian summer monsoon over NEI is around 3rd June, and May rainfall is still considered ‘pre-monsoon’. We show that the climatological ‘onset’ over the NEI occurs around 11th May and is associated with 3–4 times increase in large-scale cyclonic vorticity over the region and a 5–6 times increase of northward moisture transport over North Bay of Bengal (BoB). Busting a myth, that May rainfall is ‘pre-monsoon’, we further show that the May rainfall is ‘monsoon’ rainfall and comes in spells longer than 5 days as in June and associated with large synoptic-scale low-level cyclonic vorticity. However, what drives the Onset of monsoon in May while the Intertropical Convergence Zone (ITCZ) is still close to the equator remained a puzzle. Here, we show that the early May ‘onset’ over NEI is forced by the winds at a low level associated with a climatological barotropic cyclonic vortex that interacts with the orography and sets up a large-scale heat source over NEI, accelerates northward moisture transport from Bay of Bengal (BoB), sustains the heat source and leads to ‘onset’ of monsoon over NEI. Delayed (early) onset over the NEI is due to the weakening (strengthening) of the climatological vortex through teleconnection with drivers like the El Nino and Southern Oscillation and Atlantic Nino. Our finding that a component of the Indian summer monsoon rainfall (ISMR) is unrelated to the ITCZ not only warrants a reexamination of the basic definition of South Asian Monsoon but also a revision of the prevailing notion of June-September as the Indian monsoon season over Northeast India (NEI) to May-September. The conclusions have important implications for prediction and predictability of the NEI climate.