Delayed response of the onset of the summer monsoon over the Bay of Bengal to land–sea thermal contrast

The mechanisms involved in the onset of the Bay of Bengal summer monsoon (BOBSM) were studied using reanalysis data and numerical model experiments. Results revealed that the weak meridional land–sea thermal contrast (LSTC) over the northern BOB in early spring enhances the lower-tropospheric easterly belt along 10°–15°N, which is unfavorable for the BOBSM onset. The BOBSM onset is driven by the cumulative impact of this LSTC along with the LSTC in the meridional direction across the equator and in the zonal direction across the tropics, together with air–sea interactions. While the LSTC intensifies over the northern BOB, a near-surface northward cross-equatorial flow develops south of India, inducing springtime zonal flow and surface sensible heating over the southern BOB and a pair of cyclones straddling the equator over the central Indian Ocean at 700 hPa. The zonal LSTC in the tropics generates near-surface cyclones over land and anticyclones over the sea. This induces a zonal SST warm pool around 10°N, which produces vertical westerly wind shear to the north and weakens the wintertime easterly aloft and the anticyclone to its north. As the cyclone over southern India develops eastward, the cyclone below 700 hPa develops northward over the eastern BOB in response to the enhancing tropical westerly and surface sensible heating. The wintertime anticyclonic belt and easterly belt split, and the southerly carries water vapor northward over the eastern BOB, heralding the onset of the BOBSM and presenting a delayed response to the springtime LSTC changes.