General circulation model simulations of Triassic climates: Preliminary results

The climates of two realistic geographic representations of the Triassic earth, corresponding in age to the Scythian (245 Ma) and the Carnian (225 Ma), are explored using a new atmospheric general circulation model (AGCM) called GENESIS. The GENESIS AGCM is coupled to a slab ocean 50 m thick, with prescribed heat transport; it also incorporates three types of cloud cover and new models for vegetation effects, soil hydrology, snow cover, and sea-ice formation and melting. Boundary conditions prescribed in the separate Scythian and Carnian experiments include realistic paleogeography and estimates of paleotopography, solar insolation, atmospheric C02 concentration, vegetation and soil types, and oceanic heat flux. Seasonal simulations of Triassic climate were performed using a horizontal spectral resolution of R15 (4.5 degrees latitude by 7.5 degrees longitude) and 12 levels in the vertical for the atmosphere and 2° × 2 for the surface. Results for both time intervals suggest that most of the seasonal precipitation fell on major highland areas of Pangea. Dry continental climates with very large seasonal temperature ranges (>45°C) were modeled in the dominantly lowland interior of Pangea. Carnian continental climates predicted by the AGCM were wetter than those of the Scythian; however, both time intervals were characterized by strongly monsoonal circulation. Comparison of these results with lithologic and fossil proxy climatic indicators suggests reasonably good correlations. However, the extreme temperature variations predicted for both Scythian and Carnian are somewhat difficult to reconcile with the fossil record, although accurate interpretation of fossil proxy climatic indicators is not a simple matter. Additional AGCM sensitivity studies may be necessary to resolve this problem.