中層大気における定常惑星波の全球構造

Abstract

Global structures of stationary planetary waves in the middle atmosphere are studied by using the linearized primitive equations in spherical coordinates and by imposing observed heights as the lower boundary condition under realistic zonal wind configurations at the winter solstice and the vernal equinox of the northern hemisphere.The results for winter season show that the calculated amplitudes of WN=1 (wavenumber 1) in the global primitive model are about one half of the observed ones, whereas those of WN=2 nearly same. In the phase profiles for the two wavenumbers, no remarkable differences are seen between the calculated and observed ones. Further it is noted that maximum geopotential amplitudes of WN=1 and 2 are located about 10* latitude northward of the jet maximum, mainly owing to no artificial rigid boundary at the equator. Such characteristics are in agreement with Simmons (1978), but not mainly owing to a-geostrophy as he mentioned.In the results for equinox season, inter-hemispheric coupling through the waves is shown. In the case of WN=1 where the wave forcings at the lower boundary are given in the both hemispheres, cross-equatorially propagating waves from one hemisphere play a role of the energy source in the other hemisphere, and there the wave phase changes with height eastward. In the case of WN=2 where the wave forcings are given only in the northern hemisphere, the amplitudes in the northern mesosphere decrease with decreasing latitude and attain its minimum at the equator. After cross-equatorial propagation, the amplitudes in mid-latitudes of the southern hemisphere, despite of no wave forcings at the lower boundary, recover its magnitude as large as those in the northern hemisphere. It is found that stationary planetary waves in the tropics propagating from mid-latitudes behave like the equatorial Rossby waves with smaller geopotential amplitude and shorter vertical wave-length compared with those in higher latitudes.Zonal flow acceleration due to the waves is investigated in relation to the semi-annual oscillation. At its westerly phase in equinox, WN=2 could cause an easterly acceleration of zonal flow, by -10cm/sec/day, near 45km height in the tropics; this acceleration is comparable order as the observed one. Therefore it may be said that planetary waves could play an important role in the initiation of the easterly phase of the semi-annual oscillation.