Impacts of the Madden–Julian Oscillation on the Flash Drought Occurrence over the Lancang–Mekong River Basin

Abstract This study explores the potential impacts of the Madden–Julian oscillation (MJO) on flash drought occurrence over the Lancang–Mekong River basin (LMRB). The flash drought events are identified across individual grid cells from gridded datasets, based on the rapid decline rate in soil moisture and dry persistency of flash droughts. The onset of flash drought is typically accompanied by precipitation deficit and abnormal increase in surface air temperature (SAT). Both the onset and persistency risks of flash drought vary significantly depending on the MJO phase. Generally, the onset and persistency risks of flash droughts over the LMRB increase when the MJO convection is active over the Indian Ocean (phases 2–3), while decrease when the MJO is active over the Pacific Ocean (phases 6–7). The MJO primarily influences flash droughts over the lower reaches of the LMRB by modulating SAT and precipitation. During MJO phases 2–3, there exists significant increased SAT and preceding precipitation deficit over the lower reaches, leading to decline in soil moisture. During MJO phases 6–7, there are decreased SAT and preceding increased precipitation. Diagnosis results revealed that the impact of the MJO on SAT over the lower reaches is dominated by large-scale temperature advection associated with the MJO-induced vertical and meridional circulation anomalies, while its impact on precipitation is mainly modulated by the MJO-induced zonal wind gradients. This study emphasizes the importance of considering the effects of the MJO on precipitation and SAT for predicting flash droughts over the LMRB.