On the Climatic Impact of Ocean Circulation

Integrations of coupled climate models with mixed-layer and fixed-current ocean components are used to explore the climatic response to varying magnitudes of ocean circulation. Four mixed-layer ocean experiments without ocean heat transports are performed using two different atmosphere–land components—the new GFDL AM2 and the GFDL Manabe Climate Model (MCM)—and two different sea ice components, one dynamic and one thermodynamic. Both experiments employing the dynamic sea ice component develop unstable growth of sea ice while the experiments with a thermodynamic sea ice component develop very large but stable ice covers. The global cooling ranges from modest to extreme in the four experiments. Using the fixed-current climate model, a trio of 100-yr integrations are made with control currents from a GFDL R30 ocean simulation, same currents reduced by 50%, and same currents increased by 50%. This suite is performed with two coupled models again employing the two atmosphere–land components, AM2 and MCM, for a total of six experiments. Both models show a large sensitivity of the sea ice extent to the magnitude of currents with increased currents reducing the extent and warming the high latitudes. Low cloud cover also responds to circulation changes in both models but in the opposite sense. In the AM2-based model, low cloudiness decreases as ocean circulation increases, reinforcing the sea ice changes in reducing the planetary reflectivity, and warming the climate. This cloudiness change is associated with a reduction in lower-atmospheric stability over the ocean. Because the AM2-based model is able to simulate the observed seasonal low cloud–stability relationship and the changes in these quantities with altered ocean circulation are consistent with this relationship, the AM2 interpretation of the cloud changes is favored.