GCM experiments to test a proposed dynamical stabilizing mechanism in the climate system

Abstract

A dynamical mechanism of sufficient strength to maintain the stability of the observed annualmean global climate against the destabilizing influence of the positive lower tropospheric watervapour/infrared radiative (WVIR) feedback on SST perturbations has recently been proposed(Bates, 1999). The mechanism consists of an evaporative negative feedback associated with theatmospheric angular momentum (AM) cycle and the surface winds which it induces. The mechanismwas found from an analysis of a simple two-zone model of the climate system. Numericalexperiments with an atmospheric GCM aimed at testing the conclusions of the simple modelare here described. Aquaplanet boundary conditions with a prescribed latitudinally-varyingSST distribution are adopted. An equinoctial distribution of solar radiation is assumed. Anequilibrium climate for the GCM is established and perturbation experiments with a uniform2 K increase in the SST are carried out. In the equilibrium and basic perturbation experiments, the GCM is run with all fields evolving freely. Further perturbation experiments are then carriedout with the cloud fields held fixed, and with both the cloud and water vapour fields held fixed, in the GCM’s radiation package. These experiments allow the relative influence of cloud andwater vapour in the infrared and solar radiative feedbacks at the surface to be estimated. Theresults of the experiments are compared with those predicted by the simple model for a similarSST perturbation. The GCM results are found to be consistent with the basic assumptions andconclusions of the simple model. In particular, they indicate that a positive WVIR feedbackexists in both the tropical and extratropical zones and confirm the existence of a negativeevaporative feedback associated with the AM cycle that is of sufficient strength to overcome it.The experiments support the conclusion based on the simple model that it is necessary toconsider both the wind factor and the humidity factor in evaluating the evaporative feedback.The cloud/infrared radiative feedback in the GCM is found to be small compared with theWVIR feedback. The experiments suggest that the feedback resulting from the absorbtion ofsolar radiation by water vapour (which was not included in the simple model) provides asignificant additional stabilizing influence on SST perturbations.