INTERANNUAL VARIABILITY IN THE NORTHERN-HEMISPHERE WINTER MIDDLE ATMOSPHERE IN CONTROL AND PERTURBED EXPERIMENTS WITH THE GFDL SKYHI GENERAL-CIRCULATION MODEL

This paper reports on interannual variability of the Northern Hemisphere winter stratospheric circulation as simulated by the 40-level GFDL ''SKYHI'' general circulation model. A 31-year control simulation was performed using a climatological annual cycle of sea surface temperatures. The interannual variability of the stratospheric circulation in this model has some realistic features. In particular, the simulated variance of monthly mean, zonal-mean temperature and wind in the extratropical Northern Hemisphere agrees fairly well with observations. The day-to-day variability of the circulation also appears to be rather well simulated, with midwinter warmings of realistic intensity and suddenness appearing in the polar regions. The major deficiency is the absence of a realistic quasi-biennial oscillation (QBO) in the simulated winds in the tropical lower stratosphere. There is also an indication of long period (approximately 10 year) variability in the winter polar vortex. This appears not to be relatd to any obvious source of long-term memory in the atmosphere such as surface boundary conditions or the flow in the tropical stratosphere. The model has also been run through a large number of boreal winter simulations with imposed perturbations. In one set of experiments the Pacific sea surface temperatures have been changed to those appropriate for strong El Nino or La Nina conditions. The model is found to reproduce the observed extratropical stratospheric response to El Nino conditions quite well. Interestingly, the results suggest that including the interannual variations in SST would not greatly enhance the simulated interannual variance of the extratropical stratospheric circulation. Another set of integrations involved arbitrarily altering the mean flow in the tropical lower stratosphere to be appropriate for different extremes of the QBO. The effect of these modifications on the simulated zonal-mean circulation in the extratropical winter stratosphere is found to be quite modest relative to that seen in comparable observations. The model results do display a clear effect of the imposed tropical lower-stratospheric wind perturbations on the extratropical summer mesospheric circulation. This could reflect the influence of the mean flow variations on the gravity waves forced in the Tropics, propagating upward and poleward and ultimately breaking in the extratropical mesosphere. The model behavior in this regard may be related to reported observations of an extratropical mesospheric QBO. The equilibration of the stratospheric water vapor field in the long SKYHI control integration is examined. The results suggest that the mean residence time for upper-stratospheric air in the model is about 4 years.