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Natural Climate Variability on Decade-to-Century Time Scales
some future directions and problems likely to be addressed over the next few years.
Only a decade after the first numerical general-circulation model (GCM) of the atmosphere had been constructed (Phillips, 1956), the first attempt at a coupled general-circulation model (CGCM) was made in a remarkably prescient series of three papers (Manabe, 1969a,b; Bryan, 1969) published as a single issue of the Monthly Weather Review. The model was geographically simplified (it consisted of a sector of the globe, bounded by meridians, covering only a third of the zonal extent of the globe, a bit more than half the sector was covered by land), and the solar driving was without annual variation, but it contained most of the physics now recognized as important for the climate problem. Water vapor and its changes of phase were computed explicitly; a rudimentary land hydrology model was included (the ''bucket" model) that allowed for land evaporation and runoff; snow and land ice were parameterized; and radiative transfer for visible and infrared radiation was explicitly calculated using the specified clouds. The only major specification was cover from three types of clouds (low, middle, and high), as a function of latitude for use in the radiative transfer calculations. Rainfall and snowfall were explicitly calculated.
The ocean had five levels in the vertical; computed salinity explicitly; used an equation of state for density as a function of the calculated salinity, temperature, and pressure; and included a parameterization for sea ice. Coupling at the surface was accomplished by fluxes of heat and momentum through the surface into the ocean and by sensible and latent heat transfer into the atmosphere from the surface. SST was determined interactively by thermodynamic processes in both the ocean and the atmosphere.
The coupled model could be run for only 100 years of ocean model time, due to computational limitations, but at the end of this time it had reached a quasi-equilibrium in which only the deeper parts of the ocean were still changing. The resulting distribution of surface temperature, while not directly comparable to observations, looked quite reasonable, with the ocean heat transport warming higher latitudes and cooling the tropics. The modeled atmosphere developed eddies and had a wind and thermal structure similar to that observed, while the ocean developed a thermocline and had a density and current structure similar to that observed. Systematic problems were found in the lack of an intertropical convergence zone over the ocean, in a too deep and diffuse thermocline, and in a lack of sufficient meridional heat transport by the ocean circulation. No significant decadal variability was seen in the coupled model.
All succeeding CGCMs followed the basic themes set out in the original Manabe-Bryan papers (Figure 1, from Manabe (1969b), is still the best summary of CGCMs and continues to be widely used). In subsequent years, geography and topography have become more realistic, resolution has improved (but is still severely limited), clouds are now explicitly calculated instead of prescribed, radiation schemes have become more sophisticated and now include aerosols, land-surface parameterizations are more complete (they now describe vegetative types and evapo-transpiration), and ocean models now include more detailed bottom topography and more sophisticated mixing parameterizations. Many organizations other than GFDL are now running longer-term global coupled models, including groups at NCAR, NASA, DOE, and a few universities.
A major spur to a quite different type of coupled modeling came with the investigation of the ENSO phenomenon in the equatorial Pacific. A simplified coupled model, developed by Zebiak and Cane (1987), specified the annual cycle in both the atmosphere and the single-layer ocean (with embedded surface layer) and calculated the anomalies departing from this annual cycle. The model was successful not only in simulating the equatorial aspects of ENSO in and over the tropical Pacific but also at predicting aspects of ENSO a year or so in advance (see Cane, 1991). Only the upper portion of the equatorial ocean was modeled, since only the part above the thermocline is needed to simulate short-term variability (i.e., months to a year or two). Resolution near the equator was enhanced to fully resolve uniquely equatorial processes, especially equatorial waves and upwelling.
More complicated CGCMs without an annual cycle have also been successful in modeling ENSO variability (e.g., Philander et al., 1992). At this point, models with an annual cycle in solar forcing have had some success (e.g., Nagai et al., 1993; Latif et al., 1993) but still have difficulties in simulating the annual response as well as the full range and amplitude of interannual variability. (A recent review is
Box diagram of coupled-model structure. (From Manabe, 1969b; reprinted with permission of the American Meteorological Society.)