Physical drivers and trends of the recent delayed withdrawal of the Southwest Monsoon over Mainland Indochina

Abstract. The study investigates the key factors that cause the Mainland Indochina Southwest Monsoon (MSwM) to delay withdrawal, utilizing a spatial trend correlation between the monsoon index and various meteorological and oceanic variables such as sea surface temperature (SST), zonal winds, and moisture transport. A significant strengthening trend in the Subtropical Westerly jet (SWJ) and a weakening Tropical Easterly jet (TEJ) not only impacts regional wind patterns but also delays the monsoon departure. The anomalous South China Sea and the equatorial Indo-Pacific Ocean surface temperature (SSTA) further contribute to these delayed withdrawals, and there is a significant correlation between the MSwM withdrawal index and SSTA, moisture transport, and essential atmospheric factors. The results clarify MSwM dynamics, offering significant insights for future climate research associated with MSwM. The study also suggests that the variability of ocean-atmosphere interactions and local atmospheric circulation patterns is critical for understanding monsoon variability, which has a potential impact on climate predictions, water resource management, and agriculture practices over Mainland Indochina.