The impact of solar variability on the middle atmosphere in present-day and pre-industrial atmospheres

A series of experiments with a 2D radiative-photochemical-transport model has been performed in order to assess the impact of solar activity, and the effects of increased concentrations of industrial gases, on the composition and thermal structure of the middle atmosphere. It is found that in the pre-industrial unpolluted atmosphere a decrease of solar irradiance of 0.18%, representative of the difference from 1790 to 1690, leads to more than 3% depletion of stratospheric ozone (O-3) concentration and more than I K decrease of temperature (T). Repeating the same experiments with a much drier background atmosphere (with a 30% reduction in the pre-industrial atmospheric H2O vapour profile) we find an increased ozone response throughout most of the stratosphere. This is due to the activation of the NOx (NO + NO2) ozone destructive cycle but also because the NOx increases are themselves larger in the drier atmosphere. Near the stratopause the ozone reduction is slightly smaller in the drier stratosphere due to the stronger temperature dependence of the drier atmosphere. We do not, however, find the ozone increase at these levels reported by Shindell et al. (2001). By analysing separately the contributions of UV and temperature variations to the ozone concentration we show that both change in a drier atmosphere but that the net effect is always dominated by the direct (UV) term. By assessing both independently and simultaneously the effects of changing solar irradiance and atmospheric composition, we show that the ozone response is not a linear combination of the two factors. The key to this behaviour is again the NO, field which shows in a contemporary atmosphere, unlike in the pre-industrial atmosphere, a decrease in the lower stratosphere in response to a reduction in solar activity. The removal of NO, depends on the background humidity (via the reaction NO2 + OH (M)--> HNO3), which is higher in the contemporary atmosphere. This result may help to explain the negative response of the lower stratospheric ozone to enhanced solar activity in the contemporary atmosphere, found in some analyses of satellite measurements. (C) 2004 Elsevier Ltd. All rights reserved.