Comprehensive simulation of the middle atmospheric climate: some recent results

This study discusses the results of comprehensive time-dependent, three-dimensional numerical modelling of the circulation in the middle atmosphere obtained with the GFDL SKYHI troposphere-stratosphere-mesosphere general circulation model (GCM). The climate in a long control simulation with an intermediate resolution version (3° in horizontal) is briefly reviewed. While many aspects of the simulation are quite realistic, the focus in this study is on remaining first-order problems with the modelled middle atmospheric general circulation, notably the very cold high latitude temperatures in the Southern Hemisphere (SH) winter/spring, and the virtual absence of a quasi-biennial oscillation (QBO) in the tropical stratosphere. These problems are shared by other extant GCMs. It was noted that the SH cold pole problem is somewhat ameliorated with increasing horizontal resolution in the model. This suggests that improved resolution increases the vertical momentum fluxes from the explicitly resolved gravity waves in the model, a point confirmed by detailed analysis of the spectrum of vertical eddy momentum flux in the winter SH extratropics. This result inspired a series of experiments with the 3° SKYHI model modified by adding a prescribed zonally-symmetric zonal drag on the SH winter westerlies. The form of the imposed momentum source was based on the simple assumption that the mean flow drag produced by unresolved waves has a spatial distribution similar to that of the Eliassen-Palm flux divergence associated with explicitly resolved gravity waves. It was found that an appropriately-chosen drag confined to the top six model levels (above 0.35 mb) can lead to quite realistic simulations of the SH winter flow (including even the stationary wave fields) through August, but that problems still remain in the late-winter/springtime simulation. While the imposed momentum source was largely confined to the extratropics, it produced considerable improvement in the simulation of the equatorial semiannual oscillation, with both the easterly and westerly phases being somewhat more intense than in the control simulation. A separate experiment was conducted in which the SKYHI model was simplified so that it had no topography and so that the seasonal cycle was frozen in perpetual equinox conditions. These changes result in a model that has much reduced interhemispheric asymmetry. This model spontaneously produces a long period mean flow oscillation of considerable amplitude in the tropical upper stratopause. The implication of this result for the general issue of obtaining a QBO in comprehensive GCMs is discussed.