SPEEDY-IER: A fast atmospheric GCM with water isotope physics

The interpretation of variations in the global isotopic composition of precipitation and water vapor can be strengthened using an isotope-enabled atmospheric general circulation model (AGCM). Here we present a fast-physics atmospheric circulation model suitable for long ensemble integrations: the efficient AGCM Simplified Parameterizations, Primitive Equation Dynamics (SPEEDY), with newly added water isotope physics. The model (SPEEDY-isotope-enabled reconstructions (IER)) simulates the hydrological cycle and isotope ratios in atmospheric water at a fraction of the computational cost of Intergovernmental Panel on Climate Change (IPCC)-class GCMs. Despite its simplified physics, SPEEDY-IER captures many key features of the observed range of tropical, subtropical, and midlatitude isotope variability when compared to the Global Network of Isotopes in Precipitation, Stable Water Isotope Intercomparison Group (SWING2) simulations, and satellite observations of isotopes in vapor. The incorporation of water isotopes in SPEEDY required two updates to the model's physics: postcondensational exchange associated with falling rain and soil hydrology. It is evident that these physical processes are essential for a skillful simulation of isotopes in precipitation and vapor. We conduct a suite of sensitivity tests to constrain effective parameters in the rain exchange and land models and assess the impact of the new physics to isotope simulations. The strong sensitivity to parameter choice in these components reaffirms the importance of land-atmosphere interactions and rain-vapor exchange on stable water isotope ratios in the atmosphere and thus on the interpretation of paleoclimate records. The utility of SPEEDY-IER for climate applications is discussed. Key Points