Response of the NCAR Climate System Model to Increased CO2and the Role of Physical Processes

The global warming resulting from increased CO2 is addressed in the context of two regional processes that contribute to climate change in coupled climate models, the “El Niño–like” response (slackening of the equatorial Pacific SST gradient) and sea-ice response at high latitudes. The National Center for Atmospheric Research (NCAR) Climate System Model (CSM) response is compared with results from a coupled model that produces comparatively greater global warming, the NCAR U.S. Department of Energy (DOE) global coupled model. In an experiment where atmospheric CO2 is increased 1% yr−1 compound, globally averaged surface air temperature increase near the time of CO2 doubling for the CSM is 1.43°C (3.50°C for the DOE model). Analysis of a simple coupled model shows the CSM equilibrium sensitivity to doubled CO2 is comparable to that from the slab ocean version (about 2.1°C). One process that contributes to global warming (estimated to be about 5% in one slab ocean model), as well as to significant Pacific region climate effects, is the El Niño–like response. It is a notable feature in the DOE model and some other global coupled models but does not occur in the CSM. The authors show that cloud responses are a major determining factor. With increased CO2, there are negative net cloud-forcing differences in the western equatorial Pacific in the CSM and DOE models, but large positive differences in the DOE model and negative differences in the CSM in the eastern equatorial Pacific. This produces asymmetric cloud radiative forcing contributing to an El Niño–like response in the DOE model and not in the CSM. To remove the amplifying effects of ocean dynamics and to identify possible parameter-dependent processes that could contribute to such cloud forcing changes, the authors analyze slab ocean versions of the coupled models in comparison with a slab ocean configuration of the atmospheric model in the CSM Community Climate Model Version 3 (CCM3) that includes prognostic cloud liquid water. The latter shows a change in sign (from negative to positive) of the net cloud forcing in the eastern equatorial Pacific with doubled CO2, similar to the DOE model, in comparison with the CCM3 version with diagnostic cloud liquid water. Atmospheric Model Intercomparison Project (prescribed SST) experiments show that all three atmospheric models (DOE, CCM3 with diagnostic cloud liquid water, and CCM3 with prognostic cloud liquid water) perform poorly relative to observations in terms of cloud radiative forcing, though CCM3 with prognostic cloud liquid water is slightly superior to the others. Another process that contributes to climate response to increasing CO2 is sea-ice changes, which are estimated to enhance global warming by roughly 20% in the CSM and 37% in the DOE model. Sea-ice retreat with increasing CO2 in the CSM is less than in the DOE model in spite of identical sea-ice formulations. Results from the North Atlantic and Greenland–Iceland–Norwegian (GIN) Sea region show that the surface energy budget response is controlled primarily by surface albedo (related to ice area changes) and cloud changes. However, a more important factor is the poleward ocean heat transport associated with changes in meridional overturning in the GIN Sea. With increased CO2, the transport of warmer water from the south into this region in the DOE model is greater in comparison with that of the CSM. This leads to a larger ice reduction in the DOE model, thus also contributing to the enhanced contribution from ice albedo feedback in the DOE model in comparison with the CSM.