Local and global climate feedbacks: Application to global warming experiments in models with widely-varying global climate sensitivity

The climatic behaviour of three global coupled ocean-atmosphere general circulation models, the NCAR CCSM1 and CCSM2 and the CCCma CGCM3, is analyzed in climate change experiments forced with a switch-on increase in the solar constant. The radiative forcing of the models is readily available in this case. The climate sensitivity and the geographicalpattern of climate feedbacks, and of their shortwave, long-wave, atmosphere/surface (clear-sky) and cloud components are studied and compared as the climate evolves. After a short period of adjustment the both the magnitude and pattern of the feedbacks become reasonably stable with time, implying that they may be estimated from relatively short integrations. The global mean forcing is almost identical in the models but differs from that inferred by regressing top-of-atmosphere radiative imbalance against mean surface temperature change by about 25% for the NCAR models.

The climate sensitivities of the models differ by almost a factor of two. The magnitudes of the global mean atmosphere/surface (clear-sky) feedbacks and the long-wave component of the cloud feedbacks are very similar in the models and the geographic patterns of these feedbacks are reasonably congruent. The difference in climate sensitivity is due almost entirely to shortwave cloud feedback in the tropical region which is strongly negative in the NCAR models but weakly positive in the CCCma model.

All models exhibit a region of positive feedback in the equatorial Pacific sourrounded by broad regions of negative feedback and associated with a local maximum of temperature. However, the models differ in the zonal structure of this surface warming which ranges from a mean El Nino-like warming in the eastern Pacific in the CCCma model to a far western Pacific maximum of warming in the NCAR CCSM2 model. A simulation with the CCSM2 model in which these tropical Pacific zonal gradients of temperature are suppressed also suppresses the Pacific region of positive feedback. The global mean feedback is, however, essentially unchanged suggesting that the positive feedback in the tropical Pacific region may not contribute importantly to global mean climate sensitivity.