Impact of Mid-high Latitude Circulation and Surface Thermal Forcing on Drought Events in Central Asia

With the significant warming, Central Asia (CA) has suffered from frequent drought events and vegetation degradation. However, whether it is the large-scale circulation dynamics or the surface local thermal mechanism that plays the dominant role in the drought remains unknown. Here we used 3-month Standardized Precipitation Evaporation Index in August to identify the summer drought events for 1980-2022 and conducted a composite analysis. Results indicate that the drought related wave train, originating from mid-high latitude North Atlantic (NA), has a barotropic vertical structure and propagates eastward, featuring a positive geopotential height center in CA. The pronounced warm sea surface temperature (SST) over the middle-latitude NA and cold SST over the high-latitude NA contribute to the Rossby wave formation, which is verified by an analysis of the apparent vorticity anomaly and linear baroclinic model experiments. The anticyclone anomaly over CA, corresponding to strong vertical subsidence, enhances downward shortwave radiation and surface sensible heat flux, while significantly reducing surface latent heat flux. The maintenance of drought is usually associated with persistent precipitation deficits. By using the backward moisture tracking model, we further found that the recycled precipitation, induced by the local evapotranspiration, contributes to the 88.39% reduction of total precipitation during drought periods, whereas the inflow of external advected moisture shows no significant decrease. The above results highlight the dominated role of local land-atmosphere interactions responsible for the drought through reduced local evapotranspiration, with large-scale circulation anomalies providing a conducive background for the drought.