Month-To-Month Evolution of Persistent Extreme Drought Over Southwest China in January–May 2023: Roles of Eurasian Snow Cover and Sea Surface Temperature Anomalies in the Indian Ocean

Southwest China (SWC) experienced a persistent extreme drought event from January to May 2023, with extensive and severe drought conditions peaking in January, April, and May. During these peak months, the standardized precipitation evapotranspiration index anomalies exceeded −1.5 standard deviations, indicating the extreme seasonal drought. Analysis revealed that anomalous descending motion and suppressed moisture transport resulted in precipitation deficit and enhanced potential evapotranspiration, further causing the prolonged drought. Specifically, in January 2023, reduced snow cover in southern Europe induced mid-upper tropospheric high-pressure anomalies over the Tibetan Plateau and SWC and enhanced Ural blocking high, contributing to anomalous northerly cold winds and subsidence. Simultaneously, cold sea surface temperature (SST) anomalies in the tropical western Indian Ocean weakened the south branch trough (SBT), further limiting moisture supply. These factors resulted in cold and dry conditions across SWC. In April, decreased snow cover in northern Europe excited upper-level Rossby waves, favoring positive geopotential height anomalies and anomalous descending motions over SWC. These conditions, combined with a weakened SBT linked to warm SST anomalies in the Arabian Sea, suppressed precipitation in SWC. Additionally, strong local land-atmosphere interactions and subsidence-induced diabatic warming increased surface air temperature, exacerbating high temperature and drought conditions in April. In May, mid-upper tropospheric high-pressure anomalies over SWC linked to dry soil moisture in western Siberia and a weakened SBT associated with southern Indian Ocean SST anomalies, together with local land-atmosphere coupling, sustained dry and hot conditions in SWC. Numerical experiments further confirmed the above physical mechanism.