General Circulation Model Simulation of the Semiannual Oscillation of the Tropical Middle Atmosphere

Abstract

A study has been made of the evolution of the zonal-mean zonal wind and temperature in a multiyear integration of the 40-level, 3° ? 3.6° resolution ?SKYHI? general circulation model (GCM) that has been developed at GFDL. In the tropical upper stratosphere the mean wind variation is dominated by a strong semiannual oscillation (SAO). The peak SAO amplitude in the model is almost 25 m s?1 and occurs near the 1 mb level. The phase of the SAO near the stratopause is such that maximum westerlies occur shortly after the equinoxes. These features are in good agreement with the available observations. In addition the meridional width of the stratopause SAO in the GCM compares well with observations. A diagnostic analysis of the zonal-mean momentum balance near the tropical stratopause was performed using the detailed fields archived during the GCM integration. It appears that the easterly accelerations in the model SAO are provided by a combination of (i) divergence of the meridional component of the Eliassen-Palm flux associated with quasi-stationary planetary waves and (ii) mean angular momentum advection by the residual meridional circulation. The effects of the residual circulation dominate in the summer hemisphere, while the eddy contributions are more important in the winter hemisphere. The westerly accelerations in the model SAO result from the convergence of the vertical momentum transport associated with gravity waves that have a broad distribution of space and time scales. Thus, in contrast to some simple theoretical models, large-scale equatorial Kelvin waves appear to play only a very minor role in the dynamics of the SAO in the SKYHI GCM. A second equatorial SAO amplitude maximum was found in the tropical upper mesosphere of the GCM. This apparently corresponds to the mesopause SAO that has been identified in earlier observational studies. While the observed phase of this oscillation is reproduced in the model, the simulated amplitude is unrealistically small. The model integration included the computation of the concentration of N2O. The results show a fairly realistic simulation of the semiannual variation of tropical stratospheric N2O mixing ratio seen in satellite observations.