A STUDY OF GENERAL-CIRCULATION MODEL CLIMATE FEEDBACKS DETERMINED FROM PERTURBED SEA-SURFACE TEMPERATURE EXPERIMENTS

The response of a general circulation model (GCM) to global perturbati ons in sea surface temperatures (SSTs) is examined. The feedback stren gths in the model are diagnosed by the response of top of atmosphere ( TOA) radiative fluxes determined after substitution of fields from the ''perturbed'' climate into the ''control.'' Total feedback is divided into terms due to water vapour, lapse rate, sui face temperature, and clouds (in turn analysed in terms of cloud amount, height and types). The ''standard experiment'' prescribes a globally uniform SST perturb ation with fixed soil moisture. Four additional experiments vary the n umber of model vertical levels, the pattern of SST changes, the convec tion scheme, and the soil moisture. The SST pattern change chosen foll ows that of an equilibrium 2xCO(2) experiment, which shows polar ampli fication of the surface warming. Variations in the clear sky sensitivi ty of the model are shown to depend primarily on changes in the long w ave response due to competing (positive) water Vapor and (generally ne gative) lapse rate feedbacks. Results here indicate that these feedbac ks may be very different for differing experimental boundary condition s. The long wave feedback due to cloud amount changes is negative in a ll experiments, due to avery consistent decrease in high and middle cl oud fractions. Conversely, cloud height feedback is positive due to a general increase in the altitude of (particularly high) cloud. Cloud h eight feedback is very sensitive to the choice of the convection schem e and to the change in vertical resolution. Greatest changes in the st rength of the short wave cloud feedback results from modifications to the soil moisture specification and the convection scheme. The results here indicate that large differences in cloud feedback may be diagnos ed from a single model, even without changes being made to the cloud p arametrization. The value of the sensitivity can thus be expected to b e a function not only of the physical parametrizations chosen for the model (e.g. the penetrative convection scheme), but also of the detail s of the manner in which the experiment was performed (e.g. SST and so il moisture specifications). The TOA radiation perturbation analysis m ethod proves to be a powerful technique for diagnosing and understandi ng the physical processes responsible for the range in climate sensiti vity found between the experiments.