THE LATE CRETACEOUS - SIMULATION WITH A COUPLED ATMOSPHERE-OCEAN GENERAL-CIRCULATION MODEL

Results are presented for the climate of the late Cretaceous period (s imilar to 75-65 Ma) as simulated by a global climate model that is int eractively coupled to a primitive equation global ocean model. Increas ed values of atmospheric CO2 and altered land surface albedos are invo ked to produce the warm Cretaceous temperatures that have been propose d from biogeographic reconstructions. For comparison, a control simula tion of the present climate is performed. The globally averaged atmosp heric temperature in the Cretaceous simulation stabilizes after 20 yea rs 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 eliminat ion of the Antarctic and Greenland ice sheets. Nevertheless, equatoria l surface temperatures are raised by similar to 5 degrees C above thos e of the control simulation and offset somewhat the reduction in near- surface baroclinicity caused by the absence of the high-latitude ice s heets. In the Cretaceous simulation, global precipitation is approxima tely 10% greater than in the present day, with the only region of redu ced precipitation occurring beneath the south Eurasian monsoon. Additi onally, the amplitude of the seasonal cycle in near-surface temperatur es is smaller in the Cretaceous and, in conjunction with increased mea n annual temperatures, precludes the presence of any year-round snow o r ice in the simulation. In high latitudes, however, there are regions that seasonally drop below freezing. The temperatures in these region s are warmer than have been previously observed in atmosphere-only sim ulations as a result of poleward heat transport by the ocean's surface currents.