Analysis of the global distribution of water isotopes using the NCAR atmospheric general circulation model

Abstract

We have incorporated the cycling of water isotopes into the NCAR atmospheric general circulation model, CAM2. Isotope dynamics mostly follow those of previous isotope GCMs, with fractionation being produced by evaporation at the surface and by cloud processes. A new feature that we have added is the direct estimation of the degree of isotopic equilibration between vapor and raindrops as a function of temperature and rain rate. The model yields a reasonable global pattern of water isotopes in precipitation, but detailed comparison with observations is limited by known inaccuracies in precipitation and temperatures yielded by CAM2. We use the results to evaluate the fundamental controls on water isotopic composition in precipitation. We emphasize that, over much of the surface of the Earth, the concept of Rayleigh distillation is inadequate to understand the large‐scale geographic distribution of water isotopes in precipitation, because the effects of surface fluxes are more important than those of distillation, in particular at low and midlatitudes and over the oceans. In oceanic regions the balance between precipitation and evaporation (P − E), which reflects the large‐scale atmospheric circulation, is the primary determinant of the isotopic composition of precipitation and vapor. Variations of P − E at low latitudes over the oceans produce about 7‰ variation of precipitation δ18O that is independent of temperature variation. Where P > E, the convergence of atmospheric vapor derived from various sources leads to low values and a particularly wide range in δppt. In the tropical and subtropical troposphere the vertical decrease of δ18O in vapor is different from the values expected from Rayleigh distillation because of entrainment, convective mixing, detrainment, evaporation of detrained water, and subsidence of low‐δ18O high‐altitude air. The low δ18O of atmospheric vapor over the oceans at high (ca. 55°) latitudes produces, as a result of kinetic effects associated with evaporation, a zone of heavy (high‐δ18O) evaporation from the oceans. This effect may account for the low δ18O of some high‐latitude ocean surface waters and also helps attenuate the effects of global temperature changes on the isotopic composition of polar precipitation.