A Diagnosis of the Formation of Intermediate-scale Disturbances near Japan, the Western Pacific and Southeast Asia

Frequency of cyclogenesis of intermediate-scale disturbances is surveyed over the Far East. We take up the following two items as the definition of the intermediate-scale disturbance: (1) The characteristic wavelength lies between 1,000 km and 2,000 km. (2) At the initial stage of the formation, the disturbance does not couple with upper troughs and is mostly confined to the lower part of the troposphere. The disturbance is usually formed on an extended front. Five-year statistics for the period 1966-70 show that on the average the monthly frequency in the first half year is in excess of the total average number and that in the second half year is less than that (Fig. 1). However, features of the frequency distribution vary conspicuously from year to year (Table 1). Regardless of the year-toyear variation, we may describe the gross features of spatial distribution of cyclogenesis as follows : (a) In winter, most of the spots distribute over the sea off the southern coast of the Japan islands, (b) As the year advances to spring and early summer, intermediatescale disturbances appear in the lower latitudes. The horizontal distribution of cyclogenesis in July makes a zone centering on the Baiu front. (c) In autumn, the place of frequent cyclogenesis shifts northward and the disturbance disappears from the southwest Pacific. (d) The birthplace of the cyclone moves down to the south in winter (Fig. 2)'. Comparison of the distribution with the monthly mean 500 mb pattern suggests the close connection between the cyclogenesis of intermediate-scale disturbances and the general circulation of the atmosphere (Fig. 3). In short, we may say that in a year when the mean pattern shows a zonal or flat flow, the number of cyclones generated increases and that in a year when the upper long wave trough anchors in the west of Japan, the number descreases and the cyclogenesis of the ordinary baroclinic wave becomes active. * Concurrently appointed to the Meteorological Research Institute . 290 T. Nitta and J. Yamamoto Vol. XXIV No. 3