Water isotope modeling in the Asian monsoon region

The isotopic composition of paleoprecipitation is widely used as proxy data for temperature and precipitation. In order to investigate the validity of these transfer functions for water isotopes, oxygen 18 and deuterium, we installed a water isotope model into the ECHAM Atmospheric General Circulation Model (AGCM). Such an isotope model calculates all fractionations between the stable water isotopes in the hydrological cycle of the AGCM. We ran the model under present day conditions and under the boundary conditions of the Last Glacial Maximum (LGM) for four years each. On a global scale the model satisfactorily reproduces the main features of the water isotopes in the hydrological cycle (temperature effect, amount effect). The analysis focusses on the Asian monsoon area where atmospheric circulation strongly affects the seasonal cycle of water isotopes in precipitation. The model successfully simulates the influence of circulation, temperature and precipitation amount in this region under present day conditions. Under LGM conditions the model shows a weakened summer monsoon causing a strong positive isotope signal over East Asia although the continent is about 2-4°C cooler than today. This result underlines the difficulties to use the isotopic composition of paleoprecipitation simply as a measure of temperature. Furthermore, it demonstrates the importance of circulation changes and of changing moisture sources for the interpretation of a given isotope signal