Water isotope module of the ECHAM atmospheric general circulation model: A study on timescales from days to several years

Abstract

Results are presented of a global simulation of the stable water isotopes H 218 O and HD 16 O as implemented in the hydrological cycle of the ECHAM atmospheric general circulation model. The ECHAM model was run under present‐day climate conditions at two spatial resolutions (T42,T21), and the simulation results are compared with observations. The high‐resolution model (T42) more realistically reproduced the observations, thus demonstrating that an improved representation of advection and orography is critical when modeling the global isotopic water cycle. The deuterium excess (d=δD−8*δ 18 O) in precipitation offers additional information on climate conditions (e.g., relative humidity and temperature) which prevailed at evaporative sites. Globally, the simulated deuterium excess agrees fairly well with observations showing maxima in the interior of Asia and minima in cold marine regions. However, over Greenland the model failed to show the observed seasonality of the excess and its phase relation to δD reflecting either unrealistic source areas modeled for Greenland precipitation or inadequate description of kinetics in the isotope module. When the coarse resolution model (T21) is forced with observed sea surface temperatures from the period 1979 to 1988, it reproduced the observed weak positive correlation between the isotopic signal and the temperature as well as the weak negative anticorrelation between the isotopic signal and the precipitation. This model simulation further demonstrates that the strongest interannual climate anomaly, the El Niño Southern Oscillation, imprints a strong signal on the water isotopes. In the central Pacific the anticorrelation between the anomalous precipitation and the isotope signal reaches a maximum value of−0.8.