CLOUD ALBEDO FEEDBACK AND THE SUPER GREENHOUSE-EFFECT IN A GLOBAL COUPLED GCM

Two competing cloud-radiative feedbacks identified in previous studies i.e., cloud albedo feedback and the super greenhouse effect, are exam ined in a sensitivity study with a global coupled ocean-atmosphere gen eral circulation model. Cloud albedo feedback is strengthened in a sen sitivity experiment by lowering the sea-surface temperature (SST) thre shold in the specified cloud albedo feedback scheme. This simple param eterization requires coincident warm SSTs and deep convection for uppe r-level cloud albedos to increase. The enhanced cloud albedo feedback in the sensitivity experiment results in decreased maximum values of S ST and cooler surface temperatures over most areas of the planet. Ther e is also a cooling of the tropical troposphere with attendant global changes of atmospheric circulation reminiscent of those observed durin g La Nina or cold events in the Southern Oscillation. The strengthenin g of the cloud albedo feedback only occurs over warm tropical oceans ( e.g., the western Pacific warm pool), where there is increased albedo, decreased absorbed solar radiation at the surface, stronger surface w esterlies, enhanced westward currents, lower temperatures, and decreas ed precipitation and evaporation. However, the weakened convection ove r the tropical western Pacific Ocean alters the large-scale circulatio n in the tropics such that there is increased upper-level divergence o ver tropical land areas and the tropical Indian Ocean. This results in increased precipitation in those regions and intensified monsoonal re gimes. The enhanced precipitation over tropical land areas produces in creased clouds and albedo and wetter and cooler land surfaces. These a dditional contributions to decreased absorbed solar input at the surfa ce combine with similar changes over the tropical oceans to produce th e global cooling associated with the stronger cloud albedo feedback. I ncreased low-level moisture convergence and precipitation over the tro pical Indian Ocean enhance slightly the super greenhouse effect there. But the stronger cloud albedo feedback is still the dominant effect, although cooling of SSTs in that region is less than in the tropical w estern Pacific Ocean. The sensitivity experiment demonstrates how a re gional change of radiative forcing is quickly transmitted globally thr ough a combination of radiative and dynamical processes in the coupled model. This study points to the uncertainties involved with the param eterization of cloud albedo and the major implications of such paramet erizations concerning the maximum values of SST, global climate sensit ivity, and climate change.