Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate

We test the climate effects of changes in the tropical ocean by imposing th ree different patterns of tropical SSTs in ice age general circulation mode l simulations that include water source tracers and water isotope tracers. The continental air temperature and hydrological cycle response in these si mulations is substantial and should be directly comparable to the paleoclim atic record. With tropical cooling imposed, there is a strong temperature r esponse in mid- to high-latitudes resulting from changes in sea ice and dis turbance of the planetary waves; the results suggest that tropical/subtropi cal ocean cooling leads to significant dynamical and radiative feedbacks th at might amplify ice age cycles, The isotopes in precipitation generally fo llow the temperature response at higher latitudes, but regional delta O-18/ air temperature scaling factors differ greatly among the experiments. In lo w-latitudes, continental surface temperatures decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, O-18/O-16 ratios in low-latitude precipitation show no change from modern values. How ever, the experiments with additional cooling of SSTs produce much lower tr opical continental delta O-18 values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation is reduced). The water isotopes are especially sensitive to continental ari dity, suggesting that they represent an effective tracer of the extent of t ropical cooling and drying. Only one of the tropical cooling simulations pr oduces generalized low-latitude aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier co ntinents, and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean.