Vol

Vol. 83, No. 37, 10 September 2002

Maps and Animations Offer New Opportunities for Studying the Global Water Cycle

S. J. Birks, J. J. Gibson, L. Gourcy, P. K. Aggarwal, and T. W. D. Edwards; For additional information, contact S. J. Birks, Department of Earth Sciences, University of Waterloo, Waterloo, ON N2L 3G1, Canada

A joint project between the International Atomic Energy Agency (IAEA) and the University of Waterloo was undertaken to re-evaluate and re-configure the IAEA World Meteorological Organization (WMO) Global Network for Isotopes in Precipitation (GNIP) data base to produce global and regional maps of the weighted mean distribution of stable isotopes in annual and monthly precipitation, and to animate the monthly maps to aid in visualization of the dynamic annual fluctuations in the distribution of water isotopes in global precipitation. A Grid Analysis and Display System (GrADS) version of this "climatological" stable isotope data set was developed to produce the maps and animations, and to enhance compatibility with the expanding array of gridded climate-re-analysis data products.

Isotopes in the Water Cycle

The IAEA/WMO GNIP database includes more than 100,000 ¹⁸O, ²H and ³H measurements performed on monthly precipitation samples collected at 550 stations worldwide. Since 1961, the GNIP has served as the primary baseline reference for the distribution of isotopes in modern precipitation, and is used extensively in the fields of isotope hydrology, climatology, oceanography, and paleoclimatology.

The heavy stable isotopes of oxygen and hydrogen, ¹⁸O and ²H, are particularly useful tracers of climatological and hydrological processes because of the systematic mass-dependent partitioning of the three isotopomers ¹H¹H¹⁶O, ¹H²H¹⁶O, and ¹H¹H¹⁸O among solid, liquid, and gaseous phases as water passes through the hydrologic cycle. Variations in the relative abundances of these isotopomers, measured as ¹⁸O/¹⁶O and ²H/¹H ratios and expressed conventionally as ¹⁸O and ²H values¹, as well as the derived deuterium excess parameter d (= ²H – 8 × ¹⁸O), often provide characteristic labeling of tremendous value in studies ranging from tracing of the origin and distillation history of atmospheric moisture parcels to the analysis of runoff generation. Documentation of the spatial and temporal distribution of isotopes in modern precipitation provided by the GNIP data base has also particularly benefited the interpretation of isotopic records of paleoclimate, from natural archives like glaciers, lake sediments, tree rings, speleothems, and ground waters, which provide windows to Earth's past hydrology and climate.

As well, and perhaps even more important, the GNIP data base provides the only comprehensive "training set" for evaluating the ability of general circulation models (GCMs) equipped with water isotope diagnostics to realistically partition the global water budget. This is a particularly rigorous test of a model's skill, and the ongoing development and increasing sophistication of isotopic GCMs is now generating growing demand for more accessible and comprehensive global observational data. As a first step in response to this demand, a joint IAEA-University of Waterloo project was undertaken to re-evaluate and reconfigure existing GNIP station-based data, with the aim of establishing current best-approximation "climatological" long-term weighted annual and monthly mean data, in both station-based and gridded format, and to present these as global and regional colored contour