Wintertime Extensive and Persistent Low-Temperature Events of China and Anomalous Precipitation over Southern China

The possible causes of anomalous precipitation over southern China during the wintertime extensive and persistent low-temperature events(EPLTEs) are analyzed using long-term(1951–2009) reanalysis and observational data.The precursory circulation signals for the two types of EPLTEs with below-normal and above-normal precipitation levels over southern China are also investigated. The mid-tropospheric tilted ridge and trough with a continental scale was observed to be the key common circulation feature among the EPLTEs. The high and mid-latitude circulation features associated with the two types of EPLTEs differ significantly. When the large-scale tilted ridge shows a larger zonal extent and its positive height anomaly center attains stronger amplitude, the tilted trough appears to be close to the climatological axis of the East Asian major trough. In this case, the Siberian High is amplified extensively and the southward cold air activity is strengthened, causing below-normal precipitation over southern China. By contrast, when the large-scale tilted ridge shows a smaller zonal extent, the tilted trough is located inland, away from the climatological axis of the East Asian major trough. In this circumstance, the extent of the amplified Siberian High is smaller, the southward cold air activity is moderate, and the EPLTE is accompanied by above-normal precipitation over southern China. Moreover, it was discovered that the precipitation anomalies over southern China during EPLTEs are determined by the roles of the western Pacific subtropical high and the southern branch trough over the Bay of Bangladesh, which when intensified causes warm and wet water vapor to be transported from the western Pacific Ocean, South China Sea,and the Bay of Bangladesh to southern China to create abundant precipitation over southern China and vise versa. Further,with respect to the two types of EPLTEs, the precursory signals of the atmospheric circulation occurring 1–3 pentads before the occurrence of EPLTE, also differ. Prior to the EPLTEs with above-normal precipitation over southern China,Rossby waves originating from the North Atlantic propagated toward East Asia. For the other type of EPLTEs, the polar vortex is considerably weakened over the sub-arctic coast of the Eurasian continent, and its structure shows two poles extending toward the regions of South Europe/Mediterranean and toward the Aleutian Islands. On medium and extended time scales, this result relevant to the EPLTE may provide a useful index to the prediction of precipitation over southern China.