What controls the spatio-temporal distribution of D-excess and 17O-excess in precipitation? A general circulation model study
Combined measurements of the H218O and HDO isotopic ratios in precipitation, leading to second-order parameter D-excess, have provided additional constraints on past climates compared to the H218O isotopic ratio alone. More recently, measurements of H217O have led to another second-order parameter: 17O-excess. Recent studies suggest that 17O-excess in polar ice may provide information on evaporative conditions at the moisture source. However, the processes controlling the spatio-temporal distribution of 17O-excess are still far from being fully understood.
We use the isotopic general circulation model LMDZ to better understand what controls D-excess and 17O-excess in precipitation at present-day (PD) and during the last glacial maximum (LGM). The simulation of D-excess and 17O-excess is evaluated against measurements in meteoric water, water vapor and polar ice cores. A set of sensitivity tests and diagnostics are used to quantify the relative effects of evaporative conditions (sea surface temperature and relative humidity), Rayleigh distillation, precipitation re-evaporation and supersaturation during condensation at low temperature.
Simulations suggest that in the tropics convective processes and rain re-evaporation are important controls on D-excess and 17O-excess. In higher latitudes, the effect of distillation, transport and mixing balance the effect of supersaturation. Since these terms are very large and near cancellation, results for both PD and LGM are very sensitive to the supersaturation function. The lower D-excess and 17O-excess at LGM simulated at LGM are dominated by the supersaturation effect. Evaporative conditions had previously been suggested to be key controling factors of D-excess and 17O-excess. In LMDZ, evaporative conditions are key in explaining the PD latitudinal and seasonal distributions, but play little role for 17O-excess and for LGM variations. However, the LMDZ may underestimate this role. More generally, some shortcomings in the simulation of 17O-excess by LMDZ suggest that GCMs are not yet the perfect tool to quantify with confidence all processes controlling 17O-excess.