Comparison of the Synoptic Conditions in Midlatitudes Accompanying Cold Surges over Eastern Asia for the Months of December 1974 and 1978. Part II: Relation of Surge Events to Features of the Longer Term Mean Circulation

Abstract

This is the second of two papers dealing with midlatitude initiation of East Asian cold surges. The effects of individual cold surges on the circulation an the East Asian coast are studied for the months of December 1974 and December 1978. These months represent a contrast with respect to the strength of cold surges occurring in that month. The surges occurring in December 1978 were weak, those in 1978 were strong. The circulation features and processes that were considered were: 1) the 200 mb zonal momentum budget, 2) the 400 mb frontogenesis forcing, 3) the low-level meridional eddy heat fluxes, and 4) the thermally direct circulation cell in the entrance region to the East Asian jet maximum. The times of the surge events are shown to be periods which dominate in computing the longer term monthly statistics of the aforementioned circulation features. The monthly mean features computed here are also typical of other studies using even much longer term winter averages. The implication is that surges also dominate these statistics, and long-term averages of the East Asian winter monsoon circulation at the midlatitudes and subtropics are dominated by the surge characteristics. During the cold surge event the balance of the 200 mb zonal momentum budget is between the zonal advection of momentum and the coriolis acceleration. Between the events the balance is somewhat mixed, with no terms clearly dominating. The climatological maximum in confluence at 400 mb (quasi-geostrophic frontogenesis) found over East Asia is seen to be, for the most part, the result of intensified confluence accompanying the surge event. This frontogenesis is necessary for the temperature field to remain in thermal-wind balance with the accelerating jet. There is also marked low-level frontogenesis taking place at lower levels as the cold air sweeps southward. The low-level eddy heat fluxes, which have a large maxima on the East Asian coast, are shown by means of time?longitude plots to be largely the results of the surge circulation. Cold air is continuously moving southward in the Asian winter monsoon, but the surge fluxes are intense and are focused on the coast. The thermally direct circulation cell in the entrance region to the East Asian jet maximum reaches its peak intensity during the cold surges. The two months show very similar patterns, only the intensity of the circulation is somewhat reduced in the month of the weaker surges (December 1978). Evidently the same processes are at work during the surges of each month only at reduced levels in December 1978.