The Fujita-Ziegler model: a new semi-quantitative technique for estimating paleoclimate from paleogeographic maps

Abstract

The Fujita-Ziegler model combines approaches developed by T.T. Fujita and A.M. Ziegler of the University of Chicago to produce semi-quantitative retrodictions of paleoclimates based on paleogeographic maps that include the sizes and positions of land and sea areas and the positions and extents of upland areas. Based on such paleogeographic maps, the Fujita model extrapolates past values of surface air pressure from present-day relationships among air pressure, land area, latitude and season. Zonal mean air pressure for each latitude and season is hypothesized to depend on the amount of land area at that latitude and deviations from this mean over major bodies of land or water are hypothesized to depend on the area of the relevant body at that latitude. Fujita diagrams both illustrate these hypottheses and permit application of the model to paleogeographic maps. Surface air pressure maps based on these diagrams can be used to qualitatively retrodict seasonal windflow patterns and ocean currents. Tests of the Fujita model on the Recent geography suggest that most major air pressure cells are positioned accurately and that the values at the centers of cells are within 2–3 mb of measured values at low and middle latitudes and within 5 mb at the poles. Because the Fujita model extrapolates from a partly glaciated world, its retrodictions for ice-free times are suspect, particularly near the poles. The Ziegler model infers the areal and seasonal distribution of precipitation from reconstructed patterns of winds and ocean currents on paleogeographic maps. Retrodictions are based on (1) mechanisms that lift air, including global patterns of atmospheric circulation (Intertropical Convergence Zone), regional storm systems (extratropical cyclones) and relatively local lifting mechanisms (orographic effects, land-sea breezes and summer convective thunderstorms); (2) sources of moist air (i.e., bodies of warm water upwind); and (3) mechanisms that interfere with convection (e.g., upwelling zones and trade wind inversions). Tests of the Ziegler model on the Recent geography accurately retrodicted seasonal precipitation at low and middle latitudes but did less well near the poles. Comparison with maps of the seasonality of precipitation in the Recent shows that the model accurately retrodicted most ever-dry and ever-dry regions. Results emphasize the importance of accurately retrodicting the effects of orography, moisture source and upwelling zones on precipitation.