Abstract
The NCAR global circulation model has been used to simulate global atmospheric conditions using boundary conditions representing those of the present day and those of the Würm/Wisconsin glacial maximum at about 20,000 years ago, for January and July cases.
The mean zonal wind strength in the July ice age case in the middle latitudes of the Northern Hemisphere was comparable with present winter conditions. Also in the ice age cases, the upper westerlies were not apparently displaced south of the Laurentide ice sheet. The Icelandic and Aleutian lows in January were displaced 10° southward, the Siberian high remained unchanged from the control situation, and a new low center was found over eastern Europe and the European USSR. In July high pressure developed over most of Asia. Maps of cyclone frequency in a 30-day period showed the influence of major ice sheets and sea ice in displacing zones of cyclone activity southward in January. Frequent cyclones occurred over central North America and there was a continuation of cyclone activity in the North Atlantic and from eastern Europe into Asia. There was virtually no cyclonic activity near the Laurentide ice sheet in July.
Cloud cover and precipitation were also analyzed. Changes in precipitation for the glacial maximum cases are mainly quantitative rather than affecting its spatial distribution. The zonal averages show small changes for the Southern Hemisphere. In the Northern Hemisphere precipitation was decreased slightly in winter with most pronounced effects between 0–10N and 55–70N. The summer shows a dramatic reduction of precipitation north of 10N.
There is broad agreement between these paleo-climatological reconstructions and those of other studies using different models.
