30-50日周期波動の経年変動

Abstract

In this observational study the 30-50 day motion fields, at 850mb and 200mb, for 1980 to 1984 are analysed using the synoptic approach. The present analysis examines their interannual variability.A similar analysis of the 30-50 day motion fields during the FGGE (1979) year showed, a) eastward propagating planetary scale divergent waves around the globe throughout the FGGE year at 200mb and b) a family of northward propagating trough and ridge lines over the MONEX region (30E-150E, 30S-40N) during the summer of 1979 at 850mb.We have performed a careful analysis of the sensitivity of the calculations of low frequency waves to the width of the time filter. Our results show that the phenomenon is quite robust and is not very sensitive to the width of the filters.The 850mb streamline-isotach analysis of the 30-50 day time scale waves over the MONEX region for the years 1980 to 1984 shows the northward propagating trough and ridge systems during May, June and July of 1982 and during May, June and July of 1983. The speed of northward propagation is approximately 0.7 degree latitude/day during the summers of 1979, 1982 and 1983. During the summer seasons of 1980, 1981 and 1984 the trough and ridge lines do not exhibit a steady northward propagation. The direction and the speed of propagation are found to be variable from month to month during these years. The trough and ridge systems during 1980 and 1981 are either stationary or show some northward propagation. During the summer of 1984 the propagation of the trough and ridge system is northward in May and June and southward in July and August.Interannually the onset and break periods of the Indian summer monsoon exhibit a close relationship to the passage of the 30-50 day trough and ridge systems over India.A global analysis of velocity potential at 200mb shows eastward propagating waves during 1979 and between October 1982 to October 1983. During the remaining period the direction of propagation is less well defined. The amplitude of the divergent wave is found to be variable from one year to the next. The 30-50 day wave motions exhibit a similar behaviour during 1979, 1982 and 1983, both at the 200 and the 850mb levels. 1982-83 was an El Nino year and 1979 was characterized by weak warm sea surface temperature anomalies of the order of 1°C over the eastern equatorial Pacific Ocean. Both of these years were characterized by deficient rainfall over India, with somewhat above normal rainfall over the region south of India.