The Relationship between Persistent Cold Spell in Southern China and the Variation Mode of East Asian Winter Monsoon with Opposite Signs in the North and South

In this paper, atmospheric circulation features for persistent cold spells in southern China are investigated and their connection with the opposite variation mode of East Asian Winter Monsoon (EAWM) in the south and north is discussed. Compared with cold waves, the persistent cold spells in southern China persist longer and occur more frequently. During the cold waves, the ridges and troughs form a typical wave train propagating through the Eurasian continent. During the cold spells, the major East Asian trough retreats to the north. According to locations and orientations of tilted ridges and associated troughs in the large-scale circulation at high-mid latitudes, zonal flows prevail in the mid latitudes over Asia, leading to the so called "western path" of cold air, i. e. cold airmass mainly come zonally from the west. In addition, the southern part of Japan is dominated by positive height anomalies, which block the cold air in the west and prolong the control of cold air over southern China, and thus lead to a persistent extreme low temperature spell in southern China. Besides, active perturbations in the southern branch of the jet stream also play an indispensable role in the event. In most of the previous studies, the Asian Winter monsoon was taken as a whole. However, recent works have shown that there mainly exist two modes of EAWM anomalies, i. e. the mode with the same sign of anomalies throughout the eastern part of China and that with opposite signs of EAWM anomalies in the south and north. With respect to these cold spells, the anomalous surface wind field is featured by variations with opposite signs, i. e. the north is under the control of anomalous southerly wind, while the south is under the control of anomalous northerly winds. This pattern corresponds to the second type of EAWM variability mode, which is different to the mode with the same sign of variation throughout the eastern part of China. For this mode, the northerly winds that lead to the drop of temperature in southern China are not caused by strong cold air intrusion from the high latitudes. Case studies also confirm the above results.