Abstract
This paper shows statistical properties about the interannual variations of seasonal change of quasi-stationary ultra-long waves in the troposphere. Especially, interannual variations of the trough positions over the Far East region are expressed by means of the comPosit diagram of their seasonal movements. The data used in this study are 5-day mean height values at the 500 mb level which are defined at every 10° latitude and longitude intervals between 30°N and 60°N taking each longitudinal circles during the period from 1946 to 1975.
Short period variations which do not take part in the seasonal change of ultra-long waves are included in the variation of 5-day mean height field observed at the 500 mb level. Thus, these variations must be removed from the observed height for the purpose of this study. In this paper, the quasi-stationary ultra-long waves along longitudinal circles are represented by the following procedures. First, for each 5-day mean observed 500 mb height, the variations of the height along longitudinal circles are separated into Fourier harmonics by means of zonal harmonic analysis. Secondly, in order to remove the oscilations whose periods are shorter than semi-annual periods, a low-pass time filter method is applied to each time series of Fourier harmonics. Finally, the quasi-stationary ultra-long waves along longitudinal circles are obtained by composition of the filtered Fourier harmonics with the wave numbers of 1 through 4.
The normal seasonal change of the quaisi-stationary ultra-long wave along each longitudinal circle is illustrated in Fig. 2. The interannual variation of the seasonal change of the quasi-stationary ultra-long waves are expressed by calculating the standard deviation of the filtered 500 mb height for each 5-day mean for the period of 28 years from 1947 to 1974. Apparently, the interannual variation of the filtered 500 mb height is larger in the winter season than in the summer season. In Fig. 4, it is shown by the distribution map of the standard deviation in the winter season that the interannual variations are relatively larger in higher latitude than in the lower latitude. In particular, the maxima of standard deviations are found over the Aleutian region, the Baffin island, and the Scandinavia peninsula.
Around the Far East, the interannual variations of the trough positions are scattered within the range of 30° difference in longitude at 30°N in each seasons. On the other hand, they are scattered within the range of 90° difference in longitude at 50°N in the summer season (Fig. 5). Furthermore, it has been found that the trough positions at 50° N are closely connected with the 500 mb height over the Aleutian region. When the 500 mb height values over the Aleutian region show positive anomally from normal height, the trough positions over the Far East region displace to the west. On the contrary, when the values show negative anomally from normal height, the trough positions displace to the east (Fig. 6).
As a result, since the 500 mb height is strongly related to the heat budget of the lower troposphere, it is considered that the trough position over the Far East region depends on the heat budget of the lower troposphere over the Aleutian region.
