Stable water isotopes in atmospheric general circulation models

Abstract

The water isotopes HDO and H 2 18 O are widely used as tracers in the global hydrological cycle. The isotope composition of modern precipitation is strongly linked to local temperatures in high latitudes and, less clearly, to the amount of precipitation in low latitudes. As the isotopic composition of precipitation can be reconstructed from many palaeo-archives these isotopes serve as quantitative climate proxies for the past. In this report we describe the different type of models that have been used to understand the global distribution of the isotopic composition of precipitation, today and in the past. The focus of this paper is on the results of atmospheric general circulation models (AGCMs) that have been fitted with water isotope diagnostics. A good correspondence between simulated and observed water isotope signals is demonstrated on an annual time-scale. In particular the regional and seasonal isotope-temperature relations estimated by the AGCMs were in good agreement with global observations. Furthermore, we discuss the results of the models when they are forced with the climatic boundary conditions of 21 kyr BP (last glacial maximum) and of 6 kyr BP (mid-Holocene). These numerical experiments allow us to compare directly the results of the AGCMs, that is the water isotopes, with the corresponding palaeo-observations. We confirmed that in general the water isotopes are valuable proxies of temperature variations in high latitudes. The calibration of the isotope-temperature thermometer', however, might be biased by local effects, such as the seasonal distribution of precipitation or the strength of the local temperature inversion. Moreover, comparable 'small differences' in the forcing compared with today's climate, such as in the mid-Holocene experiment, produce small responses of the water isotopes, which are often strongly affected by atmospheric circulation changes. Finally, future applications of isotopic AGCMs, such as the coupling with the carbon cycle or the study of cloud processes, are discussed also.