Decadal Background for Active Extreme Drought Episodes in the Decade of 2010-19 over Southeastern Mainland Asia

Abstract

Severe and extreme drought in southeastern mainland Asia (SEMA) worsened drastically in 2010-19, occurring more than twice as frequently as in the preceding decade. It is found that the spring rainfall has undergone a concordant positive-to-negative transition with the turning point at 2010, and can explain 43% of the overall regime shift toward exaggerated severely dry condition. Associated with the decadal precipitation change, the anomalous northeasterlies prevail over SEMA, resulting in weakened eastward moisture propagation from the Indian Ocean as well as enhanced divergence. Meanwhile, there is downward motion over SEMA. This circulation pattern is remotely forced by a teleconnection from the tropical western Indian Ocean (TWI) SST. TWI SST is negatively correlated with SEMA precipitation and highlights a regime shift around 2010, after which the TWI has persistent warm SST helping to maintain deficient SEMA precipitation. In terms of the physical mechanism, the heating in the TWI warms the troposphere aloft and emanates wedge-shaped Kelvin waves with their northeast flank traversing SEMA, where friction-driven northeasterly lowlevel wind and divergence emerge to block moisture penetration from the Indian Ocean. The low-level divergence is followed by descending motion in SEMA, suppressing convection and rainfall. Further, the simulated structure forced by TWI SST alone bears a close resemblance to the observed evidence, confirming the critical role of the TWI. Finally, it is shown that ENSO and its diversity have a modulating effect on SEMA precipitation as well as on the coupling between TWI SST and SEMA precipitation, during both the previous winter and the concurrent spring.