Characteristics of Coupled Atmosphere-Ocean CO₂ Sensitivity Experiments with Different Ocean Formulations

The Community Climate Model at the National Center for Atmospheric Research has been coupled to a simple mixed-layer ocean model and to a coarse-grid ocean general circulation model (OGCM). This paper compares the responses of simulated climate to increases of atmospheric carbon dioxide (CO₂) in these two coupled models. Three types of simulations were run: (1) control runs with both ocean models, with CO₂ held constant at present-day concentrations, (2) instantaneous doubling of atmospheric CO₂ (from 330 to 660 ppm) with both ocean models, and (3) a gradually increasing (transient) CO₂ concentration starting at 330 ppm and increasing linearly at one percent per year, with the OGCM. The mixed-layer and OGCM cases exhibit increases of 3.5°C and 1.6°C, respectively, in globally averaged surface air temperature for the instantaneous doubling cases. The transient-forcing case warms 0.7°C by the end of 30 years. The mixed-layer ocean yields warmer-than-observed tropical temperatures and colder-than-observed temperatures in the higher latitudes. The coarse-grid OGCM simulates lower-than-observed sea surface temperatures (SSTs) in the tropics and higher-than-observed SSTs and reduced sea-ice extent at higher latitudes. Sensitivity in the OGCM after 30 years is much lower than in simulations with the same atmosphere coupled to a 50-m slab-ocean mixed layer. The OGCM simulates a weaker thermohaline circulation with doubled CO₂ as the high-latitude ocean-surface layer warms and freshens and the westerly wind stress decreases. Convective overturning in the OGCM decreases substantially with CO₂ warming. Geographical distributions of surface air temperature change in the transient case show regional climate anomalies different from those in the instantaneous CO₂ doubling OGCM case, particularly in the North Atlantic and northern Europe. These two sets of experiments demonstrate that different ocean models and types of CO₂ forcing in the climate system result in different CO₂ climate responses. Earlier studies with energy-balance climate models confirm that instantaneous CO₂ doubling simulations respond differently than transient simulations.