アジア・西太平洋の中緯度にみられる夏季の季節内変動について

Abstract

In the large scale circulation over Asia Continent and North-western Pacific Ocean in 1983 summer season (from May to October), we found very clear and systematic wave activity with intraseasonal time scales on the subtropical jet stream.The intraseasonal wave activity is best seen at 200mb and 300mb pressure level.It is stationary wave with about 30-day period, and seems to have fixed nodes at about 60, 90, 120, 150 and 175ﾟE (longitudinal wavelength is about 60 degrees). It has barotropical vertical structure. In other years, large power areas of intraseasonal time scales are found on the subtropical jet stream, also. These facts imply that there should be intraseasonal variations on the subtropical jet stream, generally.By computing energy cycle of intraseasonal wave activity, it is shown that westerly wave accepts energy from zonally-varying basic flow. This feature corresponds to barotropic unstable mode shown in Simmons et al. (1983). The variation in energy conversion with time from basic flow to westerly wave well corresponds rather to that of kinetic energy (KE) than to the time change (dKE/dt).Furthermore, it is found that the energy cycle of wave activity has relation to the Indian monsoon activity. KE increases several days after active phase of Indian monsoon, and comes to its maximum value in break phase.