Comparison of Two Persistent Extreme Cold Events in China during January 2018 and 2008: The Modulation Roles of Arctic Sea Ice Reduction and SSTA

In January 2018 and January 2008, extreme persistent cold events occurred in China (Fig. 1). They caused extremely low temperatures and blizzard in southern China in common. The two cases also shared some similarity in monsoonal atmosphere circulation and climate background. They consisted of a total of four synoptic processes, along with intensified winter monsoonal systems like the Siberian high, blocking over Urals and deepened East Asian grand trough. Besides, they have a similar climatological background, with reduced sea ice in the Arctic in preceding fall and La Nina SSTA in the tropical central-eastern Pacific as well as warmer SSTA in the North Atlantic. However, difference are evident. The 2008 case persisted for more days, caused more freezing rain in addition to snowstorm, and has stronger warm and moisture flow transported from the tropical oceans. In comparison, there are more strongly warmer North Atlantic SSTA in the Gulf Stream extension region in the 2018 case. In the study we analyzed the formation of their difference from intraseasonal perspective through the observational diagnostics and AGCM experiments. Our results suggest the substantial role of the tropical and mid-high latitudinal intraseasonal activity which can be modulated by lower boundary anomaly. The difference between the two cases was shaped by SSTA over the Gulf stream extension region to a considerable extent.