The Asian summer monsoon (ASM) in 1998 first broke out over the Bay of Bengal (BoB) on May 15, then the South China Sea (SCS) on May 25, and South Asia (the Indian summer monsoon) around June 10. However, the meridional temperature gradient in the upper troposphere (200-500 hPa) south of the Tibetan Plateau did not reverse its sign until the onset of the Indian summer monsoon. This suggests that some other mechanism must be responsible for the monsoon onset over the BoB, which is often referred to as the first transition of the ASM. The main objective of this study is to explore such a mechanism through an observational analysis. The thermal condition over the Tibetan Plateau was dominated by both sensible and latent heat fluxes, hut the sensible heat flux (SHF) shows a substantial increase at the end of April, which is much earlier than the time of the first transition of the ASM. In contrast, changes in the thermal condition over the tropical Indian Ocean is mainly contributed by latent heat fluxes (LHF), the drastic increase of which in mid May coincided with the onset of the ASM. Such an increase occurred concomitantly with a migration of warm water from the southern Indian Ocean and across the Arabian Sea, as well as a strengthening of westerlies associated with the development of a tropical cyclone over the Bay of Bengal. The dynamics of the tropical cyclone allowed the latent heat to be realized through deep convection, which led to the onset of the ASM over the BoB.
