NONLINEAR CLIMATE FEEDBACK ANALYSIS IN AN ATMOSPHERIC GENERAL-CIRCULATION MODEL

A method is described for evaluating the 'partial derivatives' of glob ally averaged top-of-atmosphere (TOA) radiation changes with respect t o basic climate model physical parameters. This method is used to anal yse feedbacks in the Australian Bureau of Meteorology Research Centre general circulation model. The parameters considered are surface tempe rature, water vapour, lapse rate and cloud cover. The climate forcing which produces the changes is a globally uniform sea surface temperatu re (SST) perturbation. The first and second order differentials of mod el parameters with respect to the forcing (i.e. SST changes)are estima ted from quadratic least square fitting. Except for total cloud cover, variables are found to be strong functions of global SST. Strongly no n-linear variations of lapse rate and high cloud amount and height app ear to relate to the non-linear response in penetrative convection. Gl obally averaged TOA radiation differentials with respect to model para meters are also evaluated. With the exception of total cloud contribut ions, a high correlation is generally found to exist, on the global me an level, between TOA radiation and the respective parameter perturbat ions. The largest non-linear terms contributing to radiative changes a re those due to lapse rate and high cloud. The contributions of linear and non-linear terms to the overall radiative response from a 4 K SST perturbation are assessed. Significant non-linear responses are found to be associated with lapse rate, water vapour and cloud changes. Alt hough the exact magnitude of these responses is likely to be a functio n of the particular model as well as the imposed SST perturbation patt ern, the present experiments flag these as processes which cannot prop erly be understood from linear theory in the evaluation of climate cha nge sensitivity.