资源环境科技发展态势分析平台

The extreme drought in Southwest China (SWC) during the winter 2009/2010 brought catastrophic losses to the state and people. In this article, evidence has been presented that this drought event was not a single one but part of a decade-long drying period after 2000. Based on observed data from meteorological stations, we identified two distinct modes of winter precipitation in SWC: one is spatially in-phase change pattern and the other is east-west dipole pattern, both of which experienced a sharp decadal decrease since the late 1990s. The two modes are closely linked to two atmospheric circulation patterns induced by Arctic Oscillation (AO) and El Nino, respectively. The sustained precipitation deficit in SWC after 2000 was associated with the anomalous atmospheric circulation patterns mainly caused by the decadal transition of AO towards its negative phases and frequent El Nino, especially the central Pacific El Nino. The model experiments forced by sea surface temperature anomaly patterns in the North Atlantic (NA) and tropical Pacific (TP), respectively, could robustly reproduce precipitation anomaly in SWC and large-scale circulation pattern. The NA SST could excite a series of quasi-stationary wave trains, which propagate eastward to the south of the Himalayas, similar to the AO-related teleconnection. The SWC is just beneath the anti-cyclonic circulation anomaly with dry conditions. In the TP experiment, the simulated ascending motion locates over the central Pacific, while the anomalous descending branch of the western Walker cell appears over the SWC, being not conducive to the formation of precipitation.