What controls the spatio-temporal distribution of D-excess and O17-excess in precipitation? A general circulation model study
Combined measurements of H2O18 and HDO isotopic ratio in precipitation, leading to second-order parameter D-excess, have provided additional constraints on the water cycle and past climates compared to the H2O18 isotopic ratio alone. More recently, measurements of H2O17 have led to the definition of another second-order parameter: O17-excess. Recent studies suggest that O17-excess may provide information on evaporative conditions at the source of moisture in high latitudes, and on convective processes in the tropics. However, the processes controlling the spatio-temporal distribution of O17-excess are still far from being fully understood. Here we use the isotopic general circulation model LMDZ to better understand what controls D-excess and O17-excess in precipitation. The simulation of D-excess and O17-excess is evaluated against a set of measurements in meteoric water and water vapor and polar ice cores. A set of sensitivity tests and diagnostics are then used to quantify the relative effects of evaporative conditions (sea surface temperature SST and relative humidity RH), of precipitation reevaporation and of super-saturation during condensation at low temperature. In the tropics, simulations suggest that convective processes, in particular rainfall reevaporation, are important controls on D-excess and O17-excess. In the subtropics and mid-latitudes, simulated D-excess and O17-excess decrease with latitude, consistent with observations. The simulated decrease in D-excess is due the decrease in SST, and to a lesser extent to the increase in RH, with latitude. In contrast, the simulated decrease of O17-excess is mainly due to distillation processes. In high latitudes, LMDZ simulates the right sign of the D-excess and O17-excess seasonality. In Antarctica, the higher d-excess in winter is due to stronger distillation at colder temperature, while the lower O17-excess in winter is due to stronger super-saturation at colder temperature. At paleo time scales, LMDZ captures the lower D-excess and O17-excess recorded in polar ice cores during the last glacial climate. Half of the simulated D-excess shift is due to lower SST and higher RH at evaporation during glacial climate, while the other half of the dexcess shift and all of the O17-excess shift are due to stronger super-saturation at colder temperature. Results in high latitudes are however very sensitive to the super-saturation parameterization. Therefore, distillation effects, evaporative conditions and super-saturation control D-excess and O17-excess in different proportions. This suggests that these two parameters could be jointly used with H2O18 isotopic ratio to better constrain past climatic conditions, provided that the super-saturation parameterization could be better calibrated.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- 1655 GLOBAL CHANGE / Water cycles