Triggering effect of an unusual northwestward-moving tropical cyclone over the Bay of Bengal on the extremely early Indian summer monsoon onset

The onset of Indian summer monsoon (ISM) in 1990 is ascertained to be the earliest in recent decades. Analysis attributes this extreme event to an unusual northwestward-moving tropical cyclone (TC) in the Bay of Bengal (BOB). The TC released strong latent heat over the western BOB, which enhanced the upper-tropospheric South Asian High with divergent easterlies to the south. Meanwhile, the TC-heating intensified the lower-level cross-equatorial southwesterlies in the Arabian Sea, prompting a downstream moisture convergence. Under the concurrent upper- and lower-level atmospheric Gill-type responses, convective activities over the equatorial Arabian Sea moved northward, triggering an early ISM onset. This study highlights the direct and more efficient promotion of ISM onset by the strong convective heating over the western BOB. It further reveals that relatively weak heating over the western BOB (corresponding to the weak northwestward-moving TCs) presents limited influence and may be damped by other factors, while strong heating over the eastern BOB (corresponding to the strong northeastward-moving TCs) cannot induce significant atmospheric conditions for the ISM onset, emphasizing the locations and intensities of the BOB convection deserve more attention.