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

Abstract

Results Of experiments with a GCM involving changes in UV input (±25%, ±5% at wavelengths below 0.3 µ) and simulated equatorial QBO are presented, with emphasis on the tropospheric response. The QBO and UV changes alter the temperature in the lower stratosphere/upper troposphere, altecting tropospheric/stratospheric vertical stability. When the extratropical lower stratosphere/upper troposphere warms, tropospheric eddy energy is reduced, leading to extratropical tropospheric cooling of some 0.5°C on the zonal average, and surface temperature changes up to ±5°C locally. Opposite effects occur when the extratropical lower stratosphere/upper troposphere cools. Cooling or warming of the comparable region in the Tropics decreases/increase static stability, accelerating/decelerating the Hadley circulation. Tropospheric dynamical changes are on the order of 5%. The combined UV/QBO effect in the troposphere results from its impact on the middle atmosphere. in the QBO east phase, more energy is refracted to higher latitudes, due to the increased horizontal shear of the zonal wind, but with increased UV, this energy propagates preferentially out of the polar lower stratosphere, in response to the increased vertical shear of the zonal winds; therefore, it is less effective in warming the polar lower stratosphere. Due to their impacts on planetary wave generation and propagation, all combinations of UV and QBO phase affect the longitudinal patterns of tropospheric temperatures and potential heights. The modeled perturbations often agree qualitatively with observations and are of generally similar orders of magnitude. The results are sensitive to the forcing employed. In particular, the nature of the tropospheric response depends upon the magnitude (and presumably wavelength) of the solar irradiance perturbation. The results of the smaller UV variations (±5%) are more in agreement with observations, showing clear differences between the UV impact in the cast and west QBO phase. However, since the UV magnitudes have been exaggerated relative to observed solar UV variations during the last solar cycle, the results cannot be used to prove an actual solar forcing of the troposphere. The results will also likely be sensitive to the model, particularly its planetary longwave energy, and may be influenced by other processes that have not been included, such as changes in stratospheric ozone. The dynamical changes are accompanied by changes in cloud cover and snow cover that differ between maximum and minimum UV, and affect the radiative balance of the planet. As these influences do not cancel in the extreme phases of the UV variations, a net radiative forcing may result from solar cycling in conjunction with the QBO. An assessment of the solar impact on climate change must include these dynamically driven forcings.