全球スペクトルモデルT42の4月積分に見られるPJ的な南北振動

Abstract

The relationship between precipitation over the tropical western Pacific and sea level pressure of the subtropical high to the south of Japan was investigated by making use of the global spectral model. Two experiments in 4-month numerical integration starting from the initial value on 1 May 1984 without and with an SST warm anomaly of 1.5°C over the tropical western Pacific were examined. In both integrations, the subtropical high with a barotropic structure was formed around Japan in midsummer.Clear positive correlation was found between sea level pressure of the subtropical high and precipitation over the tropical western Pacific in the case of the integration with the SST anomaly. The relationship between the subtropical high and the tropical western Pacific could be also ascertained by EOF analyses of the precipitation. The EOF analyses indicate a close relationship of precipitation among the middle latitudes around Japan, the subtropics and the tropics over the western Pacific more clearly in the integration with the SST anomaly than without the SST anomaly. This relationship of precipitation has a resemblance in many respects to the intraseasonal PJ (Pacific-Japan) oscillation found by Nitta (1987), which is the north-south oscillation of cloud amount over the West Pacific and the Far East.It was also found in the integration with the SST anomaly that Rossby waves propagate along a great circle from the south of the Japan Islands to the west coast of North America, accompanied by an increase in precipitation over the tropical western Pacific. The wave propagation is similar to the phenomena reported by Nitta (1987). It took about 5 days for Rossby waves to propagate from Japan to North America. The increase in precipitation over the tropical western Pacific in the experiment caused not only the northeastward propagation of Rossby waves but also the westward propagation of fluctuations in precipitation along the south Asian monsoon region.