Large-scale situations over East Asia associated with the frontal zone extending from China to Japan are examined from late August to the beginning of November on the basis of 5-day averaged fields during 1981 to 1985, mainly in 1983.
As a result, the investigated period from late summer to autumn is divided into four stages according to the frontal positions, stagnating feature of fronts and characteristics of large-scale thermal, moisture and height fields. The relationship with the retreat of the Indian summer monsoon is also examined.
In Stage 1, the frontal zone was situated in the northern part of Japan. The center of a subtropical anticyclone was located to the south of Japan and an associated ridge extended over South Japan. This Stage is regarded as a part of mid-summer season. The beginning of this stage roughly coincided with the beginning of the retreat of the Indian summer monsoon from Northwest India. In Stage 2, the frontal zone shifted to the southern coast of Japan and stagnated there, although the stagnating nature was less pronounced than the Baiu frontal zone. Over the Eurasian Continent, the mid-latitude westerlies were meandering and a quasi-stationary trough was formed between 160°E and 160°W. The horizontal temperature gradient around the frontal zone over China in Stages 1 and 2 was small (-4°C/1000km), while the moisture gradient was large (-5g/kg/1000km). The temperature gradient was relatively large (6-8°C/1000km) over Japan during these stages. These characteristics are similar to those of the Baiu frontal zone. The frontal zone and this stage are regarded as the Shurin (Akisame) frontal zone and the Shurin season, respectively. The period of this stage was from September 6 to 26 on a five-year average.
In Stage 3, the position of the frontal zone was not changed from Stage 2. However, the stagnating nature of fronts was not pronounced compared with Stage 2. The height field of the westerlies over the Eurasia Continent abruptly changed to the zonal pattern at the beginning of this Stage and a quasi-stationary trough was formed between 100°E and 150°E. The temperature gradient along the frontal zone over China strengthened (-10°C/1000km). This stage is the early period of the autumn season. The period was on a five-year average from September 27 to October 21.
At the beginning of Stage 4, the frontal zone abruptly shifted southward to the southern part of Continental China and was located away from the Japanese Islands. At the same time, the subtropical jet stream suddenly shifted to the south of the Tibetan Plateau in its eastern part, and the northeast monsoon began in South India. This stage occurs in the late period of the autumn season.
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