Regional Circulation Characteristics Associated with a Cold Surge Event over East Asia during Winter MONEX

Abstract

A case study is presented of the cold surge over East Asia during 9–13 December 1978; using nine vertical levels of Winter MONEX data. The surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes (∼30°N) to the equator within three days. During the period studied, the lower tropospheric circulation dramatically changed in extent and intensity. With the onset of the surge event, the main divergent maximum began shifting from the South China Sea to southeast China. At the same time, the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere, and a direct vertical coupling between flows in the low and high troposphere was observed. The time-averaged meridional mass circulation between 100 and 126°E reveals a two-cell structure; the southern cell is located between the northern South China Sea and the equator, and the northern cell between midlatitudes and the northern South China Sea. Analysis of sensible heat transport indicates that the southern cell is associated with warm air in the south and cold air in the north; thus it resembles a thermally-direct local meridional circulation. Moisture transport analysis shows that the moisture source is found in the southern branch of this cell, and the sink in the northern branch. Conversely, the northern cell is thermally indirect. The time-mean zonal mass circulation between 32°N and 4°S is marked by two cells, linked by subsidence near the longitudes of the South China Sea. The eastern cell is accompanied by heat and moisture sources while the western cell is associated with heat and moisture sinks.