Stable water isotopes in the MITgcm (checkpoint 64w)

Abstract

Abstract. We present the first results of the implementation of stable water isotopes in the ocean general circulation model MITgcm. The model is forced with the isotopic content of precipitation and water vapor from an atmospheric general circulation model (NCAR IsoCAM), while the fractionation during evaporation is treated explicitly in the MITgcm. Results of the equilibrium simulation under pre-industrial conditions are compared to observational data and paleoclimate records (the oxygen isotopic composition of planktic foraminiferal calcite). The broad patterns and magnitude of the stable water isotopes in annual mean seawater are well captured in the model, both at the sea surface as well as in the deep ocean. However, the surface water in the Arctic Ocean is not depleted enough, due to the absence of highly depleted precipitation and snow fall and slightly enriched river runoff. This shortcoming is also recognizable in the isotopic composition of the seawater-salinity relationship in mid-latitudes. Deep ocean characteristics of the vertical water mass distribution in the Atlantic Ocean closely resemble observational data. Apart from the systematic offset of the modeled oxygen isotopic composition of planktic foraminiferal calcite towards lower values, the comparison with proxy data shows a good agreement. We summarize that the offset is mainly caused by gametogenic calcification and a matter of choice of the applied paleotemperature equation. Overall, the newly developed stable water isotope package opens wide prospects for long-term simulations in a paleoclimatic context.