An assessment of thermal, wind, and planetary wave changes in the middle and lower atmosphere due to 11‐year UV flux variations

Abstract

A two‐dimensional radiative equilibrium calculation with fixed dynamic heating is used to calculate variations in atmospheric temperature due to solar UV flux variations which are assumed to be associated with the 11‐year solar cycle. Solar flux variations considered are those predicted by recently published solar models (Lean et al., 1982; Lean, 1984; Cook et al., 1980) and one similar to those used in recent photochemical studies (see, for example, Natarajan et al., 1980/81) which is assumed to represent a reasonable upper limit to such variations. The solar flux variations suggested by Lean et al. and Cook et al. have been used in a radiative‐convective‐photochemical model to derive related ozone perturbations which are included in the calculations. In addition to these ozone perturbation profiles, similar large and small ozone perturbation profiles are defined and used with the upper limit solar flux change to determine the sensitivity of the results to ozone changes alone. For the case with the upper limit solar flux change and the large ozone perturbation, calculations suggest maximum temperature increases of up to 7°K from solar minimum to solar maximum near the tropical stratopause, decreasing with increasing latitude and decreasing altitude. Maximum changes in the mean zonal wind of up to 15 m/s are found at mid‐latitudes near the stratopause but become negligible below 30 km. These wind field changes are used with a linear stationary quasi‐geostrophic model to estimate changes in the structures of planetary wave numbers 1 and 2 in the geopotential height field. When compared with the model reference state at 60°N, these results indicate that over the solar cycle, the amplitude of wave number 1 changes by 2% or less at 500 mbar 7% or less from 200 to 10 mbar, and 20% at 2 mbar. When compared to the extreme interannual variability, the changes at 500, 200, and 10 mbar are 2, 7, and 4%, respectively. Changes associated with wave number 2 are smaller than those found for wave number 1. Geopotential height phase changes for wave numbers 1 and 2 were found to be negligible at all latitudes and altitudes. Calculations carried out with the Lean and Cook et al. solar flux perturbations and related ozone changes resulted in significantly smaller changes than those noted above for wave number 1 in the geopotential height field. For wave number 2, larger changes were found using the results of Lean. The larger changes for the Lean case are felt to be associated with the reduced trapping of wave number 2 compared with the large ozone case and the Cook case. This, in turn, is due to the relative magnitudes of the zonal wind changes calculated for the different cases. For all cases, the results suggest that tropospheric and lower stratospheric changes associated with the altered planetary wave configurations and caused by long‐term UV flux changes would be small and difficult to detect. Middle and upper stratospheric effects, however, are larger and should be discernible given a long enough series of meteorological data.