An Analysis of the Large-Scale Climate Anomalies Associated with the Snowstorms Affecting China in January 2008

Extraordinarily frequent and long-lasting snowstorms affected China in January 2008, causing above-normal precipitation, below-normal temperature, and severe icing conditions over central-southern China. These snowstorms were closely linked to the change in the Middle East jet stream (MEJS), which intensified and shifted southeastward. The change in MEJS was accompanied by southeastward shifts of the ridge and the trough over Europe and western Asia. The intensified MEJS also strengthened the trough embedded in the southern branch of the subtropical westerlies over the southern Tibetan Plateau, enhancing the water vapor transport from western Asia and the Bay of Bengal to China. In the meantime, the subtropical western Pacific high (SWPH) was stronger and its ridgeline was farther north than normal. The anomalous high slowed down the eastward propagation of weather systems to the Pacific and favored convergence of water vapor over central-southern China. The MEJS is usually strong when the Arctic Oscillation (AO) is positive and the SWPH is farther north than normal in La Nina winters. Compared to the SWPH and the Nino-3.4 sea surface temperature (SST), the MEJS and the AO exert stronger influences on the temperature and the precipitation over central-southern China, despite the fact that these possible impacting factors are not completely independent from each other. Although the La Nina event might contribute to the climate anomalies through its relation with the SWPH in January 2008, an analysis of historical events indicates that La Nina conditions alone can hardly cause severe and persistent snow conditions over central-southern China. In addition, compared to the Nino-3.4 SST and the SWPH, the conditions of December MEJS and AO exhibit stronger precursory signals of the variability of January temperature over central-southern China.