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

Abstract

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 low-level 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. Significance Statement In the last decade, a sequence of extreme droughts has swept southeastern mainland Asia (SEMA), resulting in tremendous losses. Instead of examining individual extreme cases, this study aims to advance the understanding of how the active extreme drought episodes can be modulated by the decadal background and to reveal the underlying physical mechanisms. The results suggest that decadal change can explain 43% of the overall regime shift toward more extreme dry conditions, and that the sea surface temperature in the tropical west Indian Ocean has a profound impact. The knowledge obtained here has benefits for estimation of overall risks to extreme drought hazards and highlights the future work direction that skillful decadal prediction is crucial for robust planning in an uncertain climate.