Modeling the Effects of UV Variability and the QBO on the Troposphere–Stratosphere System. Part I:. The Middle Atmosphere

Results of experiments with a GCM involving changes in UV input (±25%, ±10%, ±5% at wavelengths below 0.3 µm) and simulated equatorial QBO are presented, with emphasis on the middle atmosphere response. The UV forcing employed is larger than observed during the last solar cycle and does not vary with wavelength, hence the relationship of these results to those from actual solar UV forcing should be treated with caution. The QBO alters the location of the zero wind line and the horizontal shear of the zonal wind in the low to middle stratosphere, while the UV change alters the magnitude of the polar jet and the vertical shear of the zonal wind. Both mechanisms thus affect planetary wave propagation. The east phase of the QBO leads to tropical cooling and high-latitude warming in the lower stratosphere, with opposite effects in the upper stratosphere. This quadrupole pattern is also wen in the observations. The high-latitude responses are due to altered planetary wave effect, while the model's tropical response in the upper stratosphere is due to gravity wave drag. Increased UV forcing warms tropical latitudes in the middle atmosphere, resulting in stronger extratropical wen winds, an effect which peaks in the upper stratosphere/lower mesosphere with the more extreme UV forcing but at lower altitudes and smaller wind variations with the more realistic forcing. The increased vertical gradient of the zonal wind leads to increased vertical propagation of planetary warm altering energy convergences and temperatures. The exact altitudes affected depend upon the UV forcing applied. Results with combined QBO and UV forcing show that in the Northern Hemisphere, polar warming for the east QBO is stronger when the UV input is reduced by 25% and 5% as increased wave propagation to high latitudes(east QB0 effect) is prevented from then propagating vertically (reduced UV effect). The model results are thus in general agreement with observations associated with solar UV/QBO variations, although the west phase is not absolutely warmer with increased UV. Questions remain concerning the actual variation of stratospheric winds with the solar cycle as the magnitude of the variations reported in some observations cannot be associated with UV variations in this model (but do arise in the model without any external forcing). The model results actually come closer to reproducing observations with the reduced magnitude of UV forcing due to the lower altitude of west wind response, despite the smaller wind variations involved. An evaluation of the reality of the reported effects of combined QBO and solar UV variations on the middle atmosphere requires the use of proper UV solar cycle forcing and should include possible ozone variations.