Blocking Features for Two Types of Cold Events in East Asia

Cold air outbreaks (CAOs) always hit East Asia during boreal winter and have significant impacts on human health and public transport. The amplitude and route of CAOs are closely associated with blocking circulations over the Eurasian continent. Two categories of CAOs are recognized, namely, the ordinary cold wave events (CWEs) and the extensive and persistent extreme cold events (EPECEs), with the latter having even stronger impacts. The blocking features associated with these two types of CAOs and their differences are investigated in this study on the intraseasonal timescale. What these two CAOs do have in common is that they are both preceded by the intensification and recurrence of a blocking high over the midlatitude North Atlantic. The difference between these events is primarily reflected on the spatial scale and duration of the corresponding blocking high. During the CWEs, blocking occurs around the Ural Mountains, and exhibits a regional feature. The resulting cold air temperature persists for only up to 8 days. In contrast, during the EPECEs, the blocking region is quite extensive and is not only confined around the Ural Mountains but also extends eastward into Northeast Asia in a southwest–northeast orientation. As a result, the cold air tends to accumulate over a large area and persists for a much longer time. The blocking activity is primarily induced by an increased frequency and eastward extension of the synoptic anticyclonic Rossby wave breaking (AWB). Compared with the CWEs, characterized by a regional and short-lived synoptic AWB frequency, the EPECEs tend to be accompanied by more recurrent and eastward extensions of the synoptic AWB.