The late Cretaceous: Simulation with a coupled atmosphere-ocean general circulation model

Results are presented for the climate of the late Cretaceous period (similar to 75-65 Ma) as simulated by a global climate model that is interactively coupled to a primitive equation global ocean model. Increased values of atmospheric CO2 and altered land surface albedos are invoked to produce the warm Cretaceous temperatures that have been proposed from biogeographic reconstructions. For comparison, a control simulation of the present climate is performed. The globally averaged atmospheric temperature in the Cretaceous simulation stabilizes after 20 years of integration at a value that is 4 degrees C greater than that of the present day. The lower troposphere in high latitudes contributes a majority of the globally averaged warming as a result of the elimination of the Antarctic and Greenland ice sheets. Nevertheless, equatorial surface temperatures are raised by similar to 5 degrees C above those of the control simulation and offset somewhat the reduction in near-surface baroclinicity caused by the absence of the high-latitude ice sheets. In the Cretaceous simulation, global precipitation is approximately 10% greater than in the present day, with the only region of reduced precipitation occurring beneath the south Eurasian monsoon. Additionally, the amplitude of the seasonal cycle in near-surface temperatures is smaller in the Cretaceous and, in conjunction with increased mean annual temperatures, precludes the presence of any year-round snow or ice in the simulation. In high latitudes, however, there are regions that seasonally drop below freezing. The temperatures in these regions are warmer than have been previously observed in atmosphere-only simulations as a result of poleward heat transport by the ocean's surface currents.