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Natural Climate Variability on Decade-to-Century Time Scales
salinity, these trace substances carry time information. Bomb tests of the early 1960s and the ever-growing use of halocarbons in industry since the 1930s are among the sources for these tracers. It is fairly well known at what rate over time they have been injected into the atmosphere, and/or at what rate they decay or are destroyed. Schlosser and Smethie (1995) describe the nature and measurement of these "transient tracers." (Because of the particularly sparse nature of the observations in space and time, they emphasize the need to apply a model for interpreting the data most of the time.) They demonstrate the utility of tracers for studying decadal-scale variability by presenting two specific examples, and suggest that transient tracers, with their unique time-history information, be employed as part of an ongoing climate monitoring system.
TOWARD THE FUTURE
We noted earlier that deep convection occurring in the marginal seas surrounding the North Atlantic provides the sources for waters found in the North Atlantic deep western boundary current. Besides the tracer-based studies presented by Schlosser and Smethie, evidence for interdecadal variability has been documented in temperature and salinity records for other areas of the North Atlantic. Examples are presented by Swift, Lazier, and Dickson in this section. Swift (1995) discusses the freshening in recent decades of both deep and upper waters of the northern North Atlantic, and speculates that it is related to long-term shifts in the wind-driven ocean transport. Lazier (1995) argues that although in a broad sense LSW is characterized by a relative salinity minimum coincident with a relative stratification minimum at depths of 1000 to 2000 m in the ocean, its properties cannot be tracked properly over time by plotting temperature or salinity in the traditional way, on a constant-density surface, since LSW is not necessarily formed at a constant density year after year.
The work by Dickson (1995), who describes interdecadal variability of physical exchanges and transfers in the Irminger Sea and through the Denmark Strait, is a good example of the impact of geographically isolated variability on local, regional, and global scales. Locally, the impact is socioeconomic, affecting nearby cod fisheries. Regionally, the variability is tied in with the Great Salinity Anomaly (Dickson et al., 1988) through anomalous ice and freshwater production and export from the Arctic. Finally, the deep water formed in the Irminger Sea contributes to the total North Atlantic Deep Water production, which in turn is part of the global thermohaline circulation.
It should be pointed out that interdecadal variability is not limited to marginal seas and boundary currents, although such examples are prominent. Levitus et al. (1995) document interdecadal changes in the temperature and salinity fields in the interior of both the subpolar and subtropical gyres of the North Atlantic, by applying appropriate averaging techniques to the vast but irregular (in space, time, and quality) historical data base of the North Atlantic.
In addition to developing our ability to observe interdecadal changes in pivotal areas likely to be associated with more widespread climate change, we would of course like to be able to predict future changes. This will require continued effort in coupled ocean-atmosphere model development, using historical data for testing and validation purposes, as well as acquisition of real-time data as input for prediction of future climate states. The oceanic community recognizes these issues and has begun to address them in recent years with several programs of broad scope. Among these is the Tropical Ocean and Global Atmosphere (TOGA) Program which was initiated to increase understanding of ENSO events, but has contributed as well to our data base in the Pacific, especially the equatorial Pacific. Recently, TOGA has successfully made the transition from a scientific investigation to an operational monitoring system, maintaining an extensive upper-ocean network in all three oceans, with greatest concentration in the tropical Pacific. Though geographically limited, it might be regarded as a prototypical model for more extensive future systems. At high latitudes, increased effort is now being applied to understanding the complex interactions of the atmosphere/ocean/sea-ice system, as we have come to realize its significant role in determining the thermohaline circulation. This effort includes both more observational work than historically has been possible, and intensive modeling studies by a variety of individuals.
Two programs under way that are more attuned to longer periods of variability are the Atlantic Climate Change Program (ACCP) and the World Ocean Circulation Experiment (WOCE). The first of these seeks to determine the nature of interactions between the meridional circulation of the Atlantic Ocean, sea surface temperature and salinity, and the global atmosphere. Attaining this goal, it is noted, will require documentation of the general characteristics of decadal/century modes of Atlantic variability for model validation. WOCE, which is internationally coordinated and funded, has as its primary scientific objective "to understand the general circulation of the global ocean well enough to be able to model its present state and predict its evolution in relation to long-term changes in the atmosphere" (U.S. WOCE Office, 1989). WOCE includes both observational and modeling components, and also addresses data management issues. The Global Ocean Observing System (GOOS) comprises the operational extensions of programs such as GOOS and ACCP; its design will rely to a large extent on the scientific background provided by the research experiments. This international project, with its large amount of support, will be the vehicle for collecting much long-term data useful