Quasi-Stationary Waves in Mid-Latitudes and the Baiu in Japan

Abstract

A close relationship between the variation of the quasi-stationary wave pattern and the transition of the early summer rainy season in Japan (the so-called Baiu) is described by using deviations of 5-day mean 500mb heights from zonal average in 1979 and nephanalyses based on GMS image. During the period from May to August the quasi-stationary troughs over East Asia and the western North Pacific intermittently moved westwards. Major cloud systems in the nephanalyses formed most frequently in the east side of the quasistationary trough, and the region of high frequency of appearance of the major cloud system moved west with the shift of the trough.
The seasonal transition of the Baiu goes through three stages; the Baiu in the Ryukyus, the Baiu in the Japanese Main Islands and the end of Baiu. From the view point of the quasi-stationary upper flows, the transition between the stages is connected with the westward shifts of the quasi-stationary trough as follows: During the first stage the trough stayed along 140°E, and the rainy zone at the Ryukyus could not extend north beyond 30°N. The northward advance of the Baiu (transition from the first to the second stage) was a result of the westward shift of the zone of baroclinic activity associated with the movement of the quasi-stationary trough to 120°E. The cloud systems in the Baiu of the Japanese Main Islands were organized mainly by baroclinic disturbances in the east side of the trough at 120°E. In the final stage, the trough moved further west and weakened. The decay of the trough caused a diminution of cyclonic activity over Japan and brought about fair summer weather.