Physical mechanism of winter precipitation variations in southern arid Central Asia

The climate of southern arid central Asia (SACA, 35.25°-45°N，46.25°-80°E) is controlled by the subtropical high and it exhibits typical Mediterranean characteristics. Given the lack of systematic research on the physical mechanism of precipitation variations in the major precipitation seasons of SACA, we analyzed the physical mechanism of winter precipitation variations during 1979-2017. The results suggest that two water vapor pathways influence the winter precipitation in SACA and that they are closely related to a low-latitude high-pressure anomaly and the mid-high latitude North Atlantic Oscillation (NAO). Specifically, at low latitudes, the northern Indian Ocean heated by El Nino  causes the anomalous intensification of the subtropical high throughout the low latitude region, especially over the Indian subcontinent, resulting in increased water vapor transport from the northern Indian Ocean to SACA. At middle and high latitudes, the negative NAO phase leads to the southward movement of the water vapor pathway, which causes it to pass over a greater number of upwind water bodies, resulting in the transport of more westerly-associated water vapor to SACA. Further analysis showed that there is a northwest-southeast teleconnection wave train, from the North Atlantic to Central Asia and to the Indian subcontinent, which allows wave fluxes originating in the North Atlantic and the northern Indian Ocean to propagate from high and low latitudes, respectively, to the study area. At the same time, high-latitude cold air advection, brought by the low-pressure system in Central Asia, converges with the flow of warm water vapor from the low-latitude northern Indian Ocean, generating an ascending motion and reducing atmospheric static stability, which results in increased precipitation in SACA. Therefore, the key to determining the origin of precipitation variations in SACA is understanding the interaction of large-scale circulation systems at low and mid-high latitudes. In the future, with continued global warming, strong El Nino events will occur more frequently, and the subtropical high is like to intensify and move northward. As a result, there will be a tendency for the NAO and the Arctic Oscillation (AO) to remain in a negative phase; consequently the Arctic vortex in the eastern hemisphere is likely to provide favorable water vapor and dynamic conditions promoting increased winter precipitation in SACA. Overall, our findings are valuable for understanding the regional response of precipitation in arid areas against the background of ongoing global warming.