Mechanism of the late summer monsoon onset in Bay of Bengal and South China Sea during 2021: Impacts of intraseasonal oscillation and local thermal contrast

Abstract

The process of Bay of Bengal (BOB) summer monsoon (BOBSM)- and South China Sea (SCS) summer monsoon (SCSSM)-onset in 2021 is investigated by using the NCEP/NCAR reanalysis, NOAA OLR and ERA5 SST data from 1981 to 2021. Results show that BOBSM and SCSSM are built up two pentads later than climatology, on May 18 (pentad 28) and May 29 (pentad 30), respectively. For the BOBSM, the late onset is attributed to the late-originating 10–30-day intraseasonal oscillation (ISO), of which the wet phase strengthens the vortex and local convection over BOB in pentad 28, providing dynamical and thermodynamical conditions for the establishment of BOBSM. For the SCSSM, however, the onset is impeded by the anomalously weak Indochina Peninsula (ICP)-SCS thermal contrast before pentad 30 and the lack of the mid-latitude trough (a trigger of SCSSM confirmed in previous studies) in pentad 29. We propose a possible trigger of SCSSM in 2021, i.e., land wind effect, based on the sudden strengthening of ICP-SCS thermal contrast, which connects BOBSM convection and SCSSM onset. Specifically, a sub-vortex is formed over the ICP in pentad 29 following the vigorous BOBSM convection. This sub-vortex structure results in a drastic surface temperature decrease in ICP, causing fairly significant thermal contrast between ICP and SCS. Then, the dynamical response to the low-level baroclinicity, i.e., land wind effect, triggers the onset of SCSSM in pentad 30.