What controls the meridional distribution of water isotopes?

Meteoric δ18O is often used as a proxy for temperature (T) or precipitation amount (P), but these empirical relationships differ greatly by location, for reasons that are not well understood. Here we propose a simple framework for representing the zonal-mean distribution of meteoric δ18O in terms of three variables: 1) evaporation, 2) condensation temperature, and 3) the length scale of moisture transport. Energetic and thermodynamic constraints dictate how each of these variables responds to climate change, providing insight into the mechanisms controlling δ18O variability globally. At high latitudes, we find that the positive δ18O -T regression slope in the ice core record mostly reflects temperature-driven variability in the spatial pattern of ocean evaporation, challenging the conventional interpretation that is based on a simple understanding of Rayleigh distillation. In the tropics, the spatial pattern of evaporation is bound to be more uniform, and variability in δ18O is thus driven by predictable changes in the length scale of moisture transport, resulting in negative δ18O -T and δ18O -P regression slopes in the intertropical convergence zone. This framework thus provides a simple, unified explanation for much of the observed meridional variability in temporal correlations between temperature, precipitation, and δ18O in the paleo record.