Can we determine what controls the spatio-temporal distribution of d-excess and ¹⁷O-excess in precipitation using the LMDZ general circulation model?

Abstract. 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 (GCM) 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, mixing between vapors from different origins, precipitation re-evaporation and supersaturation during condensation at low temperature. In LMDZ, simulations suggest that in the tropics convective processes and rain re-evaporation are important controls on precipitation D-excess and ¹⁷O-excess. In higher latitudes, the effect of distillation, mixing between vapors from different origins and supersaturation are the most important controls. For example, the lower d-excess and ¹⁷O-excess at LGM simulated at LGM are mainly due to the supersaturation effect. The effect of supersaturation is however very sensitive to a parameter whose tuning would require more measurements and laboratory experiments. Evaporative conditions had previously been suggested to be key controlling factors of d-excess and ¹⁷O-excess, but LMDZ underestimates their role. More generally, some shortcomings in the simulation of ¹⁷O-excess by LMDZ suggest that general circulation models are not yet the perfect tool to quantify with confidence all processes controlling ¹⁷O-excess.