SWING: Stable water isotope intercomparison group

Summary

The general purpose of the SWING project is an international intercomparison of current state-of-the-art water isotope general circulation models and related observational isotope data.

Background

For more than four decades the isotopic composition of water stored in various archives (e.g. ice cores, ground water) has been used to study changes in the hydrological cycle on timescales from glacial-interglacial to short term variations. Such changes of the hydrological cycle play a crucial role forcing both past and future variability of the Earth's climate system. However, the interpretation of isotopic variations in terms of climate change is often handicapped by an lack of other relevant observational climate parameters (e.g. temperature, relative humidity, precipitation) both in space and time.

Schematic overview of the hydrological cycle showing approximate depletions in d¹⁸O [based on: GNIP Brochure, IAEA, 1996].

Modelling the isotopic composition of water within the hydrological cycle of general circulation models (GCM) may help to overcome this deficit on available climate data. Isotope GCMs simulate the ¹⁸O/¹⁶O (and/or ²H/¹H) relation as an independent quantity within a closed "model world" where all other relevant climate parameters are known, too. This enables an improved analysis of (simulated) isotope variability in terms of climate change. Modelling of stable water isotopes also offers the potential to improve our understanding of current-day tropospheric and stratospheric water vapour and cloud processes.

To ensure isotope GCM reliability, any isotope GCM simulation must pass a rigorous comparison of simulated versus observed isotope data (model-data-intercomparison). In addition, artefacts in isotope simulations based on a specific GCM model set-up or parameterisation have to be identified. The latter can be achieved by comparing simulation results of several different isotope GCMs run under identical boundary conditions (model-model intercomparison).

Objectives

During the last decade several state-of-the-art GCM models (GISS, ECHAM, LMD, GENESIS and others) have been enhanced by the capability to simulate the atmospheric and/or ocean cycle of H₂¹⁸O and ¹H²H¹⁶O (=HDO). The SWING project will serve as a platform to bring together experts from the various research groups to explore the following topics:

enable an overview about ongoing isotope GCM modelling capabilities
serve as a platform for future common isotope simulation experiments (see Roadmap) of the various research groups
identify the most important need of new observational isotope data in space, time and the various aggregate forms of water
strengthen the linkage between the modelling community and the key contributors of observational water isotope data
serve as an interface to other model intercomparison studies (see Links)

Organizational Form

The SWING project was ignited in 2003 and formally started at a first workshop kindly hosted by the IAEA, Vienna, Feb. 25-27, 2004 (see Workshops). It will continue as a series of workshops plus direct informational exchange among the consortia members. Participation of new project members with a strong interest in stable water isotopes is warmly welcomed.