A RECONSIDERATION OF THE INITIAL CONDITIONS USED FOR STABLE WATER ISOTOPE MODELS

Many of the ''simple models'' used to study and explain the global dis tributions of stable water isotopes assume that the initial isotope co ntent of an air mass can be estimated from local conditions at the oce anic source through the application of a global-scale closure equation . Using a general circulation model fitted with isotope tracer diagnos tics, we show that the closure equation is generally invalid at the lo cal scale, leading to a systematic bias in the modeled isotope content s. The quantitative results obtained in earlier simple model studies m ay need some adjustment in light of this bias, particularly if deuteri um excess was used to characterize conditions prevailing at the evapor ative source.