Climatic controls on interannual variability of precipitation delta O-18: Simulated influence of temperature, precipitation amount, and vapor source region

We use an atmospheric GCM that incorporates stable isotopes and regional va por source tracers in the hydrologic cycle to explore the relationship betw een interannual variability in climate and precipitation delta(18)O globall y. On the basis of a 12-year simulation forced by observed sea surface temp eratures (SSTs), we identify changes in the amount of precipitation and in the contributions of local and nearby vapor sources as the most important d eterminants of simulated interannual isotopic changes. The model simulates weak positive correlation between temperature and isotopic variability only in certain continental regions, mostly in the extratropics. Comparison wit h long observed records of isotopes and climate indicates that the model si mulates realistic patterns of temperature-isotope correlation but may overe stimate the isotopic influence of precipitation amount. perturbations in ci rculation patterns that alter the transport and mixing of air masses at a s ite also change the relative contributions of vapor from different source r egions. Simulated changes in vapor source regions are large, reaching +/- 1 0-15% of the total precipitation, and cause significant isotopic variabilit y in nearly all grid cells. Our results suggest that shifts among vapor sou rces may provide an important control on the interannual isotopic variabili ty observed in modern precipitation and paleoclimatic records. The isotopic variability simulated in this experiment results from the interaction of s everal aspects of climate. Interannual temperature variability generally in volves circulation changes that alter air mass transport, vapor source regi ons, and condensation history; this advective mechanism may explain the rel ative weakness of temperature-isotope correlations in both the model and th e observations.