THE ROLE OF STRATOSPHERIC OZONE IN MODULATING THE SOLAR RADIATIVE FORCING OF CLIMATE

MANY recent studies have reported an apparent correlation between sola r activity and the Earth's climate on the timescale of the ii-year sol ar cycle(1-4) and over longer periods(5-10), but to date no physical m echanism has been proposed that can satisfactorily explain the observa tions. In general, it has been assumed that changes in total solar irr adiance outside the atmosphere will be reflected in proportionately eq ual changes at the tropopause (from where the influence on climate is determined). Here I present results from a two-dimensional radiative-c hemical-transport model which show that the spectral composition of th e solar variations and the photochemical production of stratospheric o zone together lead to a highly nonlinear relationship between the extr aterrestrial and cross-tropopause solar radiative flux. Because of thi s relationship, at middle to high latitudes in the winter hemisphere l ess solar radiation reaches the troposphere during periods of higher s olar activity. The consequent change in latitudinal temperature gradie nt also affects infrared radiative forcing and potentially planetary-w ave activity. The general mechanism proposed here may explain some fea tures of the observed correlations between solar variability and clima te.