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Natural Climate Variability on Decade-to-Century Time Scales
The difference in density (in kg m-3) at 500 m depth between the two pentads, 1970-74 and 1955-59. Regions with negative values are stippled and denote higher density during the 1955-59 pentad. (From Levitus, 1989a; reprinted with permission of the American Geophysical Union.)
long-term change inferred from differences in 5-year means is extremely unreliable. Nonetheless, Bjerknes's hypothesis that ocean dynamics play an important role in interdecadal variability is supported by recent studies of the decadal variability of the North Atlantic atmosphere-ocean system (e.g., Kushnir, 1992; Pan and Oort, 1983).
Nearly all the numerical and theoretical research activities on intermediate-scale climate variability have taken place during the last half-decade, for a variety of reasons. First, there is a growing demand that scientists assess and predict the anthropogenic impact on climate. Essential (but insufficient) to accomplish this goal are an accurate statement of the present climate from observations of the state variables, a rigorous program that results in multiple independent forecasts of the anthropogenically forced change in climate, and a comprehensive inventory of the intermediate variability in the present natural climate system so one can plan an efficient monitoring strategy to confidently assess the accuracy of the forecast climate change from the future observed climate.
There is another reason why the interest in the intermediate time scale variability of the climate system has sharply peaked during the past 5 to 10 years that is external to the greenhouse warming problem. The TOGA and EPOCS programs of the 1980s resulted in the documentation, simulation, and skillful model prediction of the El Niño/Southern Oscillation (ENSO) phenomenon. Through these extraordinarily successful programs scientists have explicitly demonstrated for the first time that rich variability in the climate system can result solely from the interaction between the oceans and the atmosphere. More important, this period of research marked the advent of a new era. With a few notable exceptions, for the first time the atmospheric scientists began to focus on the sub-monthly circulation anomalies in the troposphere, and the oceanographers began to abandon the default assumption of a world ocean in a quasi-steady state.3
The research activities of the last decade also created a modest population of scientists that are actively performing basic research on both oceanic and atmospheric circulation, and on the response of a climate system composed of atmosphere coupled with the global oceans. As a result, there are now numerous studies that document coordinated interannual variability in the atmosphere-ocean system, and many studies wherein isolated phenomena have been simulated and the essential physics documented. Thus, the extraordinary interest of the scientific community in identifying and analyzing the variability in the full climate system on decade-to-century time scales through modeling and observational studies can be attributed to both the research focus on the interannual variability of the coupled atmosphere-ocean climate system and the practical problems that have arisen in the detection of an anthropogenically forced climate change.
In this paper, I discuss the constraints inherent in assessing both the actual variability of the climate system on the intermediate time scales and the physical and dynamical processes that are likely to be responsible for this variability. The methods for validating the "modes" of intermediate-scale climate variability that are produced by numerical and analytical models necessarily represent a change from the traditional modus operandi. A good example of the unique blend of modeling and observational research and monitoring efforts that is required to assess intermediate-scale variability is found in the charter for the Atlantic Climate Change Program (ACCP) of the National Oceanic and Atmospheric Administration (NOAA). I describe the ACCP and briefly review the evidence for a directly observed variability in the atmosphere/ocean/sea-ice system that has recently become a focus of the ACCP. The implications for modeling and modeling strategies are discussed, and a summary is presented at the end of the paper.
THE LIMITATIONS OF THE HISTORICAL INSTRUMENTAL RECORD
The primary limitation on the study of climate variations on the intermediate time scale is that the instrumental record
In The Evolution of Physical Oceanography: Scientific Surveys in Honor of Henry Stommel, Wunsch writes: "Until very recently, the ocean was treated as though it had unchanging climate with no large-scale temporal variability." There are no references to Stommel's 1961 paper in the entire volume, which was published in 1981!