Equatorial oscillations maintained by gravity waves as described with the Doppler Spread Parameterization: I. Numerical experiments

Details Equatorial oscillations maintained by gravity waves as described with the Doppler Spread Parameterization: I. Numerical experiments Equatorial oscillations maintained by gravity waves as described with the Doppler Spread Parameterization: I. Numerical experiments

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jastp..60..181m&link_type=abstract

Journal of Atmospheric and Solar-Terrestrial Physics, Volume 60, Issue 2, p. 181-199.

Physics

17

Scientific paper

Gravity Waves (GW) as described with the Doppler Spread Parameterization (DSP) of Hines have been shown to produce significant Quasi-biennial and Semi-annual oscillations (QBO and SAO) in the zonal circulation at equatorial latitudes. To elucidate the process involved, we discuss computer experiments performed without the external, seasonal variations in solar differential heating, and the conclusions are summarized here. (1) As reported earlier, two kinds of oscillations are then generated spontaneously: one at lower altitudes in the stratosphere which has a long period approaching that of the QBO, the other one at higher altitudes in the mesosphere which has a shorter period close to that of the SAO. The amplitudes of these oscillations are weaker, however, than those generated with seasonal heating. (2) When the time independent solar heating is turned off, the oscillations continue but with significantly reduced period in the mesosphere where the vertical winds driven by solar differential heating produce Doppler shifting. The associated temperature variations are then small, consistent with the one dimensional prototype calculations discussed by Lindzen and Holton. (3) When the GW momentum source is turned off above 40 km to suppress the short-period oscillation, the long-period oscillation below continues with enhanced amplitude and period. When the source is turned off below 35 km, the amplitude of the short-period oscillation increases at higher altitudes, indicative of increased acceleration. (4) The wind oscillations change systematically and dramatically with changing eddy diffusivity/viscosity (K). The amplitudes and periods of both short- and long-period oscillations decrease (increase) with increasing (decreasing) K, and above a certain value of viscosity, both oscillations completely cease. Lowering K thus helps in generating a larger and longer period QBO at lower altitudes, but can be detrimental to the SAO when the natural oscillation period increase significantly above six months. (5) Under oscillation conditions, the height variations in the GW vertical wave numbers reveal a staircase pattern reflecting the intermittence characteristic of the wave interaction. It reveals the nonlinearity of the GW interaction to be of third (odd) order, which has the distinctive property that it can feed and maintain the fundamental harmonic of the oscillation-considered a prerequisite for sustaining the oscillations without external time dependent source. In the companion paper, an analytical model of the equatorial oscillation is presented which describes some of the trends reported here.

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