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

Copyright 2002 American Geophysical Union

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 maps. Depiction of the annual cycle by animating the monthly maps has the added benefit of graphically demonstrating the inherently dynamic nature of global isotope climate, and sets the stage for the ultimate goal of depicting time-series of actual maps, rather than a stationary average "climatological" cycle.

Data Selection and Calculations

Of the 550 GNIP stations in the data base, 389 were judged at present to have sufficient monthly-composite 18O, 2H and precipitation data to calculate amount-weighted annual and monthly means (Figure 1). The GNIP data base used in these maps spans 1961-1999, although data for many stations is only available for portions of this period. To improve global coverage, data from all time periods were included when calculating the long-term means (see Uses and Limitations). The 18O, 2H taken from the GNIP data base were amount weighted according to:

where Pi and i are monthly precipitation amount (mm) and the 2H or 18O value (‰ VSMOW), respectively.

Fig. 1. (a) Mean annual amount-weighted 18O (‰) in global precipitation and the locations of the 389 GNIP stations are shown. (b) Station data were interpolated onto a 2.5º x 2.5º grid.

As noted above, the deuterium excess parameter d was calculated using: d = 2H - 8 × 18O
Only consecutive 12-month periods were used in calculating the weighted annual means, omitting additional months that were not part of a complete year. Weighted monthly means were calculated using all months available, including data from partial years of monitoring.
Interpolation Scheme

GNIP station data were interpolated onto a global 2.5° x 2.5° grid (c. 280 km x 280 km at the equator) using a Cressman objective analysis in GrADS (see Figure 1b). In this type of analysis multiple passes are made through the grid at subsequently lower radii of influence. New values for each grid point are determined using a correction factor for that point with each successive pass. Stations within the radius of influence for that grid point are used to determine the correction factor used. A distance-weighted formula is then applied to all such errors within the radius of influence of the grid point to arrive at a correction value for that grid point. Before the next pass is made, the correction factors are applied to all grid points. The sequence of radii used for the successive passes in this interpolation were r =10, 7, 4, 2, and 1 grid cells. Grid cells lacking a station within four grid cells, about 1113 km, were left blank.

Discussion

As expected, the new global mean annual amount-weighted 18O distribution (Figure 1) interpolated from all of the GNIP data contains most of the major features of previous interpolations [Yurtsever and Gat, 1981; Jouzel et al., 1998], including clear expression of the classical "effects"--for example, latitude, continental, altitude, amount--as well as some regional features not resolved in previous versions, such as low 18O values in the Tibetan Plateau. It is also apparent with this interpolation scheme that large areas of the world oceans and continents, such as northeastern Eurasia and Saharan Africa, have insufficient data to support contour mapping without excessive interpolation. Comparisons of time-series of GNIP station data from different climatic regions [Rozanski et al., 1993] have illustrated the relationship between the amplitude of the seasonal isotopic signal and the seasonality of climate parameters in that region; however, animations of amount-weighted monthly 18O (Figure 2) add to our understanding by demonstrating the dynamic spatial and temporal variability of the isotope fields.
Fig. 2. An animation of monthly amount-weighted 18O (‰) is shown.

One goal in producing the maps, animations, and data base was simply to stimulate interest in incorporating isotopes in climate studies. Another goal was to produce a gridded data set that is compatible with the expanding array of gridded climate-re-analysis data products, which are often more practical to use and more readily available than station-based observational data for regional or global-scale investigations. These data sets include the U.S. NCAR/NCEP (1948-1998), the European ECMWF, and the U.S. NASA/DAO re-analyses, all of which incorporate model data for several atmospheric layers and a wide variety of parameters such as precipitation amount, vapor pressure, temperature, wind, soil moisture, surface roughness, and potential evaporation. For example, precipitable water, a parameter available from the NCAR/NCEP re-analysis data set, may be useful for identifying source areas and determining the degree of rainout along precipitation trajectories, an established control on continental isotope variations. The south-north precipitable water and amount-weighted monthly 18O gradients clearly show the loss of moisture as air masses rainout across eastern North America (Figure 3). Other global and regional data sets are being produced continually from satellite and radar observations and process and modeling studies connected to multidisciplinary global hydroclimatology programs such as the Global Energy and Water Cycle Experiment (GEWEX). Conversely, enhanced use of the available re-analysis data sets will also foster development of improved understanding of the isotope processes.

Fig. 3. This animation of monthly amount-weighted 18O (‰) overlain with precipitable water (kg/m2) contours was obtained from the NCAR/NCEP reanalysis data set.

Uses and Limitations

The maps and animations were created to improve our understanding of the spatial distribution and the dynamic seasonal nature of the isotopic composition of precipitation. They have not yet been ground-truthed for interpolation between GNIP stations. Further statistical analysis is being undertaken to evaluate the interpolation scheme and the spatial error distribution. Note that the maps depict climatological means based on available observations. No effort has been made to normalize the data set to account for fluctuations or unidirectional changes in isotopic composition at any of the stations over the 1961-1999 period. A monthly data set is also being developed for this type of analysis. The maps, animations, and long-term mean database can be accessed free of charge at: http://isohis.iaea.org./

Note: The notation expresses the relative abundances of 18O and 2H as deviations in per mil (‰) from a given standard, normally Vienna Standard Mean Ocean Water, such that SAMPLE = 1000((RSAMPLE/RVSMOW)-1), where R is 18O/16O or 2H/1H.

Acknowledgments

All of the interpolations and all maps were created using the GrADS software (Copyright 1988-1999) developed by Brian Doty, Center for Ocean-Land-Atmosphere Studies (COLA), Institute of Global Environment and Society, Calverton, Maryland, USA (http://grads.iges.org/grads/).

The global climatological fields are based on a re-analysis data set obtained from the Climate Data Assimilation System (CDAS) re-analysis data set, using climatological means for 1979-1995 (http://wesley.wwb.noaa.gov/cdas_data.html).

References

Jouzel, J., R. D. Koster, G. Hoffmann, and A. Armengaud, Model evaluations of the water isotope-climate relationships used in reconstructing palaeotemperatures, Proc., Symp. on Isotope Techniques in the Study of Environmental Change, International Atomic Energy Agency, Vienna, 14-18 April 1997, 485-502, 1998.

Rozanski, K., L. Araguas-Araguas, and R. Gonfiantini, Isotopic patterns in modern global precipitation, in Climate Change in Continental Isotopic Records, edited by P. K. Swart, K. L. Lohmann, J. McKenzie, and S. Savin, AGU Geophys. Monogr. 78, 1-37, 1993.

Yurtsever, Y., and J. Gat, Chapter 6. Atmospheric waters, in Stable Isotope Hydrology--Deuterium and Oxygen-18 in the Water Cycle, edited by J. R. Gat and R. Gonfiantini, Tech. Rep. Ser. 210, International Atomic Energy Agency, Vienna, 103-142, 1981.