Physical Drivers and Future Risks of the 2014-like Southeast Asia Drought

The Western Malay Archipelago undergoes a severe drought in early 2014, with Singapore experiencing its longest recorded drought period of 62 days from January 13 to March 15, a record dating back to 1929. Here we conduct in-depth analysis to examine the physical drivers of this unprecedented drought. We find that the 2014 drought is primarily due to anomalously high pressure over Southeast Asia. This condition induces the convergence of mid-to-upper level airflows, which then intensifies the subsidence. Simultaneously, the dry and cold airflows from the western and northern continents further exacerbated the subsidence. Anomalous geopotential heights are closely related to the North Atlantic Oscillation (NAO) and the Madden-Julian Oscillation (MJO): during the drought, Atlantic sea temperatures exhibit an abnormal tripole pattern, with the MJO in phases 7 and 8. The wave activity flux analysis show that, the NAO induces an eastward-propagating wave train at mid to low latitudes, leading to suppressed convection over the tropical Indian Ocean and a positive anomaly in geopotential height over Southeast Asia. In addition, we find that the seasonal averaged vertical motion (Omega) and relative humidity (RH) anomaly during 2014 Jan-Mar is unprecedented in the observational record from 1980 to 2020, with a return period of Omega and RH likely (>66% probability) in the range of 43~98 years with a median of 147 years. Climate projections based on the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) models indicate that dynamical component under global warming is the main driver increasing the frequency of 2014-like droughts in the future.