The Numerical Simulation of Ice-Age Climate with a Global General Circulation Model

Abstract The global distribution of July climate has been simulated with a two-level atmospheric general circulation model using the surface boundary conditions of sea-surface temperature, ice-sheet topography and surface albedo assembled by CLIMAP for 18 000 years before present. These conditions respresent an approximate doubling of the ice-covered surface area of the earth, a 1°C decrease of the average sea-surface temperature, and an increase of the average surface albedo from 0.14 to 0.22. Compared with the simulation of present July conditions, the ice-age atmosphere is found to have been substantially cooler and drier, especially over the continents of the Northern Hemisphere, corresponding to an enhanced anticyclonic circulation over the major ice sheets and a general weakening of the summer monsoonal circulation. The midlatitude westerlies are strengthened and systematically displaced southward in the vicinity of the major ice sheets, along with an equatorward shift in the zones of maximum eddy a...