A 2-dimensional Numerical Model of the Semi-annual Zonal Wind Oscillation

[摘要] The semiannual oscillation of the mean zonal wind in the equatorial middle atmosphere is investigated by using a zonally averaged two-dimensional model. In the model, the following mechanisms are included: Easterlies just after the solstices at the stratopause level are due to meridional wind producing nonlinear advection of the easterly momentum in the summer hemisphere. Westerlies just after the equinoxes are due to the momentum deposit by Kelvin waves. The semiannual oscillation at the mesopause level is due to the selective transmission of internal gravity waves through the stratopause level semiannual oscillation (cf. Dunkerton, 1982a, b).We discuss the following two cases;(1) the case which does not include the acceleration due to gravity waves but includes Rayleigh friction in the mesosphere, (2) the case which does not include Rayleigh friction in the mesosphere but includes the acceleration due to gravity waves.In both cases, we can obtain a semiannual zonal wind oscillation at the 50km level over the equator. But, in Case (1), there appears upward propagation of the westerly wind at the equinox. In Case (2), we can obtain a semiannual oscillation with amplitude 7m/sec at the mesopause level, as well as that at the stratopause level over the equator.However, in both cases, there remain constant westerly winds at 30-40km level over the equator, and the amplitude of the semiannual oscillation at the stratopause level is small at 10° latitude. These results imply that the easterly acceleration due to planetary waves may be important in obtaining more realistic semiannual zonal wind oscillation.

[学科分类] 大气科学