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Natural Climate Variability on Decade-to-Century Time Scales
surface salinity and hydrographic data). Nonetheless, the state of the (Atlantic) climate system, as realized from direct measurements of the climate state variables, will remain insufficient to permit assessment of the natural climate variability on decadal and longer time scales, especially away from the earth's surface. Thus, within the ACCP there are also ongoing studies to identify sources of proxy data that will define and constrain the variability in the Atlantic climate system. These studies include an analysis of dendroclimatic and ice-core data surrounding the North Atlantic as well as an analysis of deep-sea, high-sedimentation-rate cores. Encouraging preliminary results from these studies were presented by D'Arrigo et al. (1992), Bond et al. (1992), and Keigwin and Boyle (1992) at the 1992 ACCP Principal Investigators' meeting.
The usefulness of a single proxy data source for inferring the intermediate-time-scale variability in climate or a climate state variable is often limited for two reasons. First, common proxy data are not usually available for the entire globe (e.g., coral is found only in the tropics). Thus, a proxy source, in isolation, cannot be used to differentiate local and global climate variations. Second, climatic implications from a single proxy source can be ambiguous. For example, the cambial activity of trees depends on both seasonal temperatures and precipitation; for many species of trees it is difficult to determine uniquely the relationship between these two climatic variables and the cambial activity. Many proxy climate indicators must be used to ensure consistency and remove any ambiguity in the historical climate record assembled from the proxy data, and to better assess whether the climate variability is regionally confined or global in extent. The utility of this effort is illustrated by the remarkable progress made on the climate of the last glacial maximum (CLIMAP, 1981). In this regard, the IGBP Past Global Change project (PAGES) is timely (IGBP, 1992; see also Bradley, 1991).
IDENTIFYING POTENTIAL CLIMATE VARIABILITY ON DECADE-TO-CENTURY TIME SCALES FROM NUMERICAL MODELS: STRATEGIES AND LIMITATIONS
Numerical models will be the primary tools by which the mechanisms responsible for decade-to-century-scale climate variability are identified. More important, these same models will frequently be the instruments that scientists use to identify target intermediate-time-scale climate phenomena, because instrumental data exist only for the last century and, as mentioned above, for only a limited domain of the climate system. For the same reasons it will often be difficult to determine whether simulated phenomena are ever realized in nature. Thus, our confidence that a simulated phenomenon could occur will result only from an a priori assessment of how accurately the model simulates many well-documented phenomena.
A model to be used for studying natural variability in the climate system on the intermediate time scales must include complete and interactive modules for the four central media: the atmosphere, global oceans, the global terrestrial marine biosphere, and sea ice. An important aspect of this system for the intermediate-time-scale climate studies is an accurate and complete representation of the hydrologic cycle (OCP, 1989). Climate variability models are the result of marrying the individual models for the four media, each of which has first been tested in isolation by prescribing the appropriate boundary conditions for the state of the adjacent media. (When appropriate, the flux of energy is prescribed at the boundaries.) The tests of the uncoupled models do not guarantee a good coupled climate model, but are practical first steps.
Finally, full general-circulation models (GCMs) for the atmosphere and global oceans are required to study the intermediate-time-scale climate variability. There is no evidence from the observational data that the intermediate-time-scale climate variability is regionally confined—for example, to within a hemisphere, an ocean basin, or to the near-surface ocean—as the interannual climate variations seem to be.
Prerequisite Constraints for the Uncoupled Modules
The foremost test of the uncoupled component modules is the ability of the model to reproduce the seasonal cycle, and thus the annual mean state, when forced by the imposed boundary conditions. The annual cycle is an test case for validating uncoupled component modules, because in most cases the annual cycle is well known for many of the climate state variables; in some cases the product moments are also reasonably well known (e.g., the meridional heat transport in the atmosphere). For the coupled atmosphere and land-surface modules, the diurnal cycle provides a second excellent test. The diurnal cycle of near-surface fields of air temperature, moisture, clouds, and wind are well-observed quantities over land. An accurate simulation of these cycles throughout the year provides a rigorous test of the impact of the combined boundary-layer physics and surface-flux parameterizations that act to maintain the simulated climate.
The general-circulation models for the atmosphere (AGCMs) are routinely validated by comparing the simulated climatology with that observed for variables or features that include the jet structure, variation of the height of selected geopotential surfaces in the troposphere, the zonal mean distribution of temperature, zonal wind, and SLP. It is of crucial importance for climate variability, however, that the models accurately simulate the observed annual cycle of all the boundary-layer fluxes: momentum, sensible, convective, latent, and radiative fluxes at the surface, and outgoing long-wave flux at the top of the atmosphere.