Persistent severe drought in southern China during winter-spring 2011: Large-scale circulation patterns and possible impacting factors

Severe drought persisted in southern China from January to May in 2011. In this study, a statistical analysis is carried out to discuss the multiple possible impacting factors including La Nina, the North Atlantic Oscillation (NAO), and the thermal condition of the Tibetan Plateau (TP). The La Nina event in 2010-11 excited a lower-tropospheric anomalous cyclone over the northwestern Pacific, weakening the northwestern Pacific subtropical high and caused an eastward shift of the high. As a result, transportation of wet and warm moisture from tropical oceans to southern China decreased. The La Nina event also strengthened the upper-tropospheric East Asian jet stream and deepened the East Asian trough, favoring a southward intrusion of dry northerly flow from the Siberia. The La Nina condition in the previous two seasons also seemed to provide precursory signals for the drought. Moreover, in January-May 2011, the NAO was in a positive phase and it tended to excite stationary Rossby waves that were distributed along the sub-polar and subtropical waveguides, respectively. The sub-polar one induced an anomalous anticyclone over the Siberia, favoring a southward intrusion of high-latitude northerly flow to southern China. The subtropical one, associated possibly with the enhanced convection over the broad region from the Mediterranean to Sahara, was favorable for an influence of upper-tropospheric flow on southern China. The TP might also exert an influence on the drought by weakening the westerly flow to the southern flank of TP and reducing water vapor transport from the Bay of Bengal to southern China.