EQUATORIAL OSCILLATIONS MAINTAINED BY GRAVITY-WAVES AS DESCRIBED WITHTHE DOPPLER SPREAD PARAMETERIZATION - I - NUMERICAL EXPERIMENTS

Gravity Waves (GW) as described with the Doppler Spread Parameterizati on (DSP) of Hines have been shown to produce significant Quasi-biennia l 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 variati ons in solar differential heating, and the conclusions are summarized here. (1) As reported earlier, two kinds of oscillations are then gene rated spontaneously: one at lower altitudes in the stratosphere which has a long period approaching that of the QBO, the other one al 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 independ ent solar healing is turned off, the oscillations continue but with si gnificantly reduced period in the mesosphere where the vertical winds driven by solar differential heating produce Doppler shifting. The ass ociated 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 t he short-period oscillation, the long-period oscillation below continu es with enhanced amplitude and period. When the source is turned off b elow 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 (decr easing) K, and above a certain value of viscosity, both oscillations c ompletely cease. Lowering K thus helps in generating a larger and long er period QBO at lower altitudes, but can be detrimental to the SAO wh en the natural oscillation period increase significantly above six mon ths. (5) Under oscillation conditions, the height variations in the GW vertical wave numbers reveal a staircase pattern reflecting the inter mittence characteristic of the wave interaction. It reveals the nonlin earity of the GW interaction to be of third (odd) order, which has the distinctive property that it can feed and maintain the fundamental ha rmonic 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. (C) 1998 Elsevier S cience Ltd. All rights reserved.