An advection-condensation model for subtropical water vapor isotopic ratios

The processes controlling the joint distribution of water vapor specific humidity (q) and isotopic ratios (delta) at the subtropical relative humidity (RH) minimum are investigated through the development of an advection-condensation model that is applied to an idealized GCM and a GCM nudged by reanalysis data. The first-order characteristics of the joint distribution of delta and q values from the subtropical RH minimum can be satisfactorily explained by the model. In an idealized GCM with physics that closely approximates the assumptions in the advection-condensation model, the reconstructed delta and q distribution are quite close to that in the model. When compared to an isotopic simulation from a full-physics GCM nudged by reanalysis data, the advection-condensation model reproduces the first-order aspects of the delta and q distribution but underestimates the variability. In both cases, the errors can potentially be attributed to insufficient variability in the representation of domains contributing water vapor to the subtropical RH minimum and, in the case of the isotopic reanalysis, to the omission of cloud microphysics. This study suggests that long-term monitoring of water vapor isotopic ratios may provide a way to distinguish between different mechanisms for the projected moistening of the subtropics.