SENSITIVITY OF A GENERAL-CIRCULATION MODEL TO CHANGES IN NORTHERN-HEMISPHERE ICE SHEETS

Sensitivity experiments with a general circulation model demonstrate t he role of ice sheet size on the local, regional, and global climate. Model experiments isolate the effects of albedo, height, and area of t he ice sheets and show how the National Center for Atmospheric Researc h Community Climate Model 1 responds to changes in the size of norther n hemisphere ice sheets. A flat ice sheet with full glacial areal exte nt but no elevation is used to study albedo effects. A full ice sheet with full glacial areal extent and elevation is used to represent heig ht effects, An ice sheet with half the glacial area of the others but the full glacial elevation is used to represent area effects. All of t he sensitivity experiments have (1) interactive sea surface temperatur es calculated by a slab ocean and (2) modern boundary conditions excep t for the ice sheets. The experiments show that both the full and flat ice sheets lower the global mean surface temperatures (GMT) by 2.5 de grees C and that the GMT is dependent upon the area, rather than the h eight, of the ice sheets, High ice sheets maintain colder temperatures than lower ice sheets over the ice sheets themselves, but compensatin g warmer temperatures occur downstream from the high ice sheets. The d ownstream warmer temperatures are the result of (1) glacial anticyclon es that cause subsidence and reduced cloud cover during summer as well as reduced soil moisture and (2) increased southwesterly flow across the Atlantic Ocean that results in increased southerly advection of wa rm air during winter. A dynamical effect of the high ice sheets during summer is to change the wave number of the planetary waves in the mid latitudes, whereas a thermodynamic effect of the flat ice sheets durin g summer is to lower the geopotential heights throughout the northern hemisphere. In general, northern hemisphere ice sheets induce both a l ocal response over the ice sheets and a regional response downstream f rom the ice sheets but have little impact on the southern hemisphere e xcept where sea ice expands.