Interannual Variability of Autumn Precipitation Over the Greater Mekong Subregion

This study investigates the interannual variability in autumn precipitation over the Greater Mekong Subregion (GMS) during the period 1981-2019 and its possible causes. Statistical analysis indicates that GMS autumn precipitation showed a seesaw pattern between the central-southern and northeastern GMS over interannual timescales. The tropical sea surface temperature anomalies (SSTAs), reflected in the tropical northeastern Pacific SST index (TNEPI), significantly modulate anomalous GMS autumn precipitation. When the TNEPI is higher than normal, the SSTAs pattern prevails during autumn, with warmer SSTAs in the tropical eastern Pacific and western Indian Ocean, and colder SSTAs in the tropical western Pacific and southeastern Indian Ocean. This SSTAs pattern excites an anomalous anticyclone in the lower troposphere over the central-southern GMS and an anomalous cyclone over the northeastern GMS by the Gill-Matsuno mechanism. Subsequently, the anomalous anticyclone with anomalous descending motion results in below-normal autumn precipitation over the central-southern GMS via a weakening of the convergence of water vapor from the tropical western Pacific and the Bay of Bengal. The anomalous cyclone on the northern flank of anomalous anticyclone, benefiting from water vapor supply from the Bay of Bengal, leads to above-normal autumn precipitation over the northeastern GMS. The TNEPI, incorporating signals from the eastern-Pacific (EP) type of ENSO, Indian Ocean dipole (IOD), and SSTAs associated with the Trans-Nino index (TNI-like), shows a stronger relationship with GMS autumn precipitation than individual indexes that describing the EP-type ENSO, IOD or TNI-like pattern. Therefore, the TNEPI can be considered an efficient predictor of GMS autumn precipitation.