Impacts of Mega-ENSO on the rainy season onset over the Southeast Asia low-latitude highlands

The Southeast Asia Low-Latitude Highland (SEALLH), mainly comprising the Yunnan–Guizhou Plateau in Southwest China and its extension into the Indochina Peninsula, is a region where rainfall anomalies exert substantial socioeconomic impacts. This study investigates the linkage between the rainy season onset (RSO) over the SEALLH and Pacific sea surface temperature (SST) anomalies. A delayed RSO is closely associated with a mega-ENSO pattern, characterized by triangular warming in the central–eastern Pacific and K-shaped cooling in the western and subtropical Pacific. These SST anomalies affect the RSO through both tropical and extratropical pathways. Tropically, warming in the central–eastern Pacific induces anomalous ascent and compensating subsidence over the Maritime Continent, where cold SST anomalies and suppressed convection further excite equatorial Rossby wave responses, generating easterly wind anomalies over the tropical Indian Ocean. Extratropically, the mega-ENSO pattern triggers a quasi-stationary Rossby wave train that induces tropospheric cooling over India, weakening the land–sea thermal contrast and monsoonal circulation. The combined tropical and extratropical influences lead to persistent easterly anomalies over the North Indian Ocean, suppressing moisture transport into the SEALLH and ultimately delaying the RSO. These results provide implications for the short-term prediction of the RSO over the SEALLH.