Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate |
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Authors: | C D Charles D Rind R Healy R Webb |
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Institution: | (1) Scripps Institution of Oceanography, La Jolla, CA 92093-0224, USA E-mail: ccharles@ucsd.edu, US;(2) Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA, US;(3) Woods Hole Oceanographic Institution, Woods Hole, MA, USA, US;(4) NOAA/OAR/CDC, 325 Broadway, Boulder, CO 80303, USA, US |
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Abstract: | We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice
age general circulation model simulations that include water source tracers and water isotope tracers. The continental air
temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic
record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes
in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant
dynamical and radiative feedbacks that might amplify ice age cycles. The isotopes in precipitation generally follow the temperature
response at higher latitudes, but regional δ18O/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures
decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, 18O/16O ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling
of SSTs produce much lower tropical continental δ18O 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 aridity, suggesting that they represent an effective
tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude
aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents,
and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean.
Received: 26 July 1999 / Accepted: 10 July 2000 |
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