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Natural Climate Variability on Decade-to-Century Time Scales
Temporal evolution of (A) the average tritium, (B) 3He, (C) tritium/3He age, and (D) CFC-11 values of Greenland Sea Deep Water (GSDW; depth > 1500 m). (From Schlosser et al., 1991; reprinted with permission of the American Association for the Advancement of Science.)
too small to be evaluated quantitatively. In contrast, the transient tracer signals are relatively large.
For quantitative evaluation of the transient tracer time series, a model is needed that simulates the basic exchange processes in the study area. In the case of the GSDW, we have to simulate the deep water formation and the coupling of the deep Greenland Sea to the Norwegian Sea and the Eurasian Basin of the Arctic Ocean. For this purpose, we modified a simple box model developed by Heinze et al. (1990) (Figure 21). The model allows deep-water formation
Box model used to simulate the GSDW tracer concentrations as a function of time. (NSDW: Norwegian Sea Deep Water; GSDW: Greenland Sea Deep Water; EBDW: Eurasian Basin Deep Water; EBBW: Eurasian Basin Bottom Water; BS: Barents Sea Water; GSSW: Greenland Sea Surface Water; NSSW: Norwegian Sea Surface Water; Atl: Overflow into Atlantic. (From Schlosser et al., 1991; reprinted with permission of the American Association for the Advancement of Science.)
in the main basins (Greenland Sea, Norwegian Sea, and Eurasian Basin). Exchange of deep water between the basins is implemented on the basis of hydrographic studies (Swift et al., 1983; Aagaard et al., 1985; Swift and Koltermann, 1988; Smethie et al., 1988). The model is then tuned to simulate a variety of steady-state and transient parameters in the deep waters (temperature, salinity, tritium, 3He, CFC-11, 85Kr, and 39Ar). The parameters used to tune the model are the deep water formation rates and the exchange rates of deep water between the individual basins.
The GSDW tracer time series can be reproduced by the model only if the formation rate of GSDW is variable (Figure 22; Schlosser et al., 1991a). The model requires a reduction of the GSDW formation rate by about 80 percent starting in 1980 (±2 years). With such an assumption the model fits the observations perfectly. Simulations of the tracer concentrations expected for the period between 1990 and 2000 for two scenarios with reduced deep-water formation rate (Figure 22, curve 2) and high deep-water formation rate (Figure 22, curve 3) show the sensitivity of the transient tracer approach for this particular region. The currently ongoing monitoring of tritium/3He and CFC-11/CFC-12 in the Greenland Sea should be a very good indicator of changes in the GSDW formation rate.
Existing data indicate that transient tracers are potentially valuable tools for studies of ocean variability on time scales
Comparison of observed and simulated tracer concentrations of GSDW for three scenarios: (1) constant renewal rate of GSDW; (2) GSDW renewal rate reduced by 80 percent starting in 1980; (3) GSDW formation rate reduced by 80 percent starting in 1980, followed by increase to full deep-water formation rate starting in 1990. A-D are as in Figure 20. (From Schlosser et al., 1991; reprinted with permission of the American Association for the Advancement of Science.)