Characteristics of Transient Eddy Fluxes during Blocking Highs Associated with Two Cold Events in China

Abstract

In Eastern Asia, two categories of cold air outbreaks, ordinary type and extensive and persistent type, usually occur during boreal winter, with obviously different circulation backgrounds. Blocking highs, which play an important role in cold events, are closely related to the evolution of transient eddies in their dynamical mechanism. This paper investigates the characteristics of thermal-dynamical transport of transient eddies associated with blocking events in two cold air events: a low-temperature, snowfall, and freezing event in early 2008 and a cold wave in January 2016. The results suggest that there is a convergence of transient eddy momentum and heat transport around blocking high locations during both cold events, which are beneficial for establishing and maintaining the blocking events. Moreover, the convergence of transient eddy momentum and heat transport is stronger in the 2016 event than in the 2008 event, with a northward shift of the convergence centers. Further analysis shows that the blocking high location is controlled by the convergence of Eliassen-Palm (E-P) flux in both events, which contributes to the westerly wind deceleration and provides a beneficial background for the development of blocking events. Moreover, the anticyclonic Rossby wave breaking (RWB) event occurs two days prior to the onset of blocking and weakens along with the collapse of blocking in both cold events; however, the RWB has a longer duration, stronger intensity, and more north-eastward location in the 2016 event than for the 2008 event. These results indicate that the effect of a synoptic-scale eddy is one important factor influencing different characteristics of the blocking highs and is related to different weather phenomena during the two cold events.