A Study of East Asian Cold Surges during the 2004/05 Winter: Impact of East Asian Jet Stream and Subtropical Upper-Level Rossby Wave Trains

Cold surges were unusually active in subtropical East Asia during January - February 2005. These cold surges were preceded by upstream wave trains, which originated in the Mediterranean-Sahara region and propagated eastward along the subtropical jet stream over the Eurasian continent. The northerly of the upper-level cyclonic anomaly in East Asia coupled with the low-level northerly upon the arrival of wave activity, and this was followed by a quick southward penetration of cold air mass and surface anticyclone. Diagnostic and numerical results suggest that the anomalously active wave activity affecting the East Asian cold surges may be attributed to an anomalously enhanced jet stream over the Middle East and an anomalously westward extension of the East Asian Jet Stream. The configuration of these two subtropical jet streams established a strong waveguide through which wave activity forced in the Mediterranean-Sahara region could efficiently propagate to East Asia, resulting in above average cold Surge events in subtropical East Asia. Wave-like perturbation tended to be amplified at the entrance to the East Asian jet through barotropic energy conversion from the mean flow.