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2. Existence of the layered disturbances

A typical example of the layered disturbances is shown in Figure 1. Figure 1a shows original (i.e. unfiltered) meridional wind $v$ at Naha located in the south part of Japan. Strong and shallow northward winds are observed below the upper tropopause continuously from 25 December to 10 January. To see the layered structure of meridional winds more clearly, we extracted fluctuations using a bandpass filter in the vertical with cutoff lengths of 1.5 and 6 km and a lowpass filter in time with a cutoff length of 2 days, which hereafter we refer to as $v'$ component. Similar disturbances are seen simultaneously at other stations of Ishigakijima (Figure 1c) and Chichijima (Figure 1d).

Figure 1. Time-height sections for the time period of December 20, 1995 to January 10, 1996 of (a) $v$ at Naha (26.2N, 127.7E, station No. 47936), (b) $v'$ at Naha, (c) $v'$ at Ishigakijima (24.3N, 124.2E, 47918), and (d) $v'$ at Chichijima (27.1N, 142.2E, 47971). Contour intervals are 5{\ms} ($\cdots$, -10, -5, 0, 5, 10, $\cdots$) for (a) and {\ms} ($\cdots$, -5, -3, -1, 1, 3, $\cdots$) for (b), (c), and (d). Thick contours show 0{\ms} for (a). Dots indicate the tropopause levels.

Such layered disturbances frequently appear at many stations mostly in winter. Thus, further examination is made for winter periods from 1 December through 10 March in each year. The total number of analyzed vertical profiles is 802 at each station. It should be noted that such small vertical-scale atmospheric disturbances have been analyzed mostly in terms of gravity waves so far. However, it may be needed to consider another possibility of inertial instability particularly for small-scale disturbances in low latitude regions.