Comparison of the Synoptic Conditions in Midlatitudes Accompanying Cold Surges over Eastern Asia for the Months of December 1974 and 1978. Part I: Monthly Mean Fields and Individual Events

Abstract

East Asian cold surges during two periods of contrasting intensity, December 1974 (strong) and December 1978 (weak), are studied. It is shown that the midlatitude mechanisms initiating cold surges during both months are quite similar in nature. Synoptic scale short waves passing through the long-wave trough position near the East Asian coast act to release the cold air southward from the main reservoir over eastern Siberia. Dynamic descent is initiated by these synoptic scale waves entering the northwesterly flow on the western side of the long-wave trough anchored on the coast. This is the descent center in the local direct circulation in the entrance region of the East Asian jet maximum at 200 mb. The frequency and intensity of the cold air surges for a given month depend on the large scale circulation pattern. The average flow over Asia for December, 1974 and 1978, presents a contrast with respect to ridging over the midcontinent. In December 1978 the flow was characterized by a low zonal index over Asia and the low-level development forced by the smaller scale waves is not well-focused with respect to position. Strong ridging in the middle of the Asian continent during December 1974 yielded a stable pattern for descent on the East Asian coast. The results indicate that the surge event is not the result of the expansion of an intensifying Siberian anticyclone but of a separate dynamically forced subsidence to the east and southeast of the Siberian anticyclone mean position; nor is the surge directly the result of cyclonic development off the East Asian coast. Such cyclogenesis follows the surge initiation and is triggered by the same synoptic short wave which initiated the surge as the wave passes to the eastern side of the long-wave trough position.