On the Role of Successive Downstream Development in East Asian Polar Air Outbreaks

Abstract

A composite of 16 strong East Asian polar outbreak occurrences, widely separated in time, reveals a clear sequence of events: beginning over the western North Atlantic six or seven days in advance of the key day (as defined by the cold frontal passage over Korea) troughs and ridges are seen to form, develop and decay successively downstream of one another across the Eurasian continent until the polar outbreak occurs. These troughs and ridges reach their maximum amplitude in much the same location and at the same time relative to the key day in the majority of the 16 cases. The center of the wave packet moves along a curved trajectory approximating the mean 300 mb flow at a nearly constant rate of 30° longitude per day. The perturbation moves as an essentially barotropic dispersive wave across most of Eurasia, but its evolution becomes highly baroclinic as it approaches the East Asian coast. The wavetrain nature of this perturbation breaks down as it propagates out over the Pacific Ocean. This breakdown coincides in 8 of the 16 cases with the formation of a large amplitude ridge of great meridional extent.