state of the atmosphere is rather complete4 over the Northern Hemisphere, especially over the continents, but it is not suitable for studying decadal climate variability in the Southern Hemisphere. A very uneven spatial and temporal record of the hydrographic and current structure of the world oceans is available starting after World War II, but it is unlikely that the data coverage is sufficient permit deduction of a posterior decadal variations in the ocean climate.

Fundamental to understanding the coupled atmosphere-ocean system is a knowledge of how energy and constituents are exchanged between the two media. On this score, even the recent instrumental record is clearly deficient. For example, there is large uncertainty as to the annual cycle of the turbulent exchange of heat and momentum alone; estimates of the variability in these energy exchanges on the intermediate time scales would be premature.

In summary, it is not possible to discern the past variability of the climate system on the century scale from the instrumental data sources. Recent decadal variations in the atmosphere-ocean system can perhaps be deduced with some confidence from the instrumental data base, although incompletely. The ample spatial voids in the data set (especially in the subsurface oceans) introduce some uncertainty, into determining whether the variability is locally confined or is of global extent. Because of the severe limitations of the existing instrumental record, proxy data sets will play an important role in documenting the intermediate-scale climate variability and, perhaps, in evaluating simulated climate variability (discussed below).

THE TRADITIONAL MODUS OPERANDI AND THE ATLANTIC CLIMATE CHANGE PROGRAM
The Traditional Modus Operandi

The history of atmospheric sciences and oceanography is replete with examples of community-wide intensive research activities that are focused on the precise documentation and analysis of specific, observed phenomena that represent perturbations from a mean (state) that are statistically significant, e.g., the mid-latitude cyclone, the Quasi-biennial Oscillation (QBO), and the Gulf Stream. Major programs have also commonly had as a centerpiece a well-observed phenomenon. Examples include GATE (easterly waves, mesoscale tropical convection), POLYMODE (long-lived coherent eddies in the ocean thermocline), ERICA (rapidly deepening cyclones), CLIMAP (reconstruction of the climate of the last ice age), and the upcoming EPOCS effort (the annual cycle and the boundary layer circulation in the Pacific).5

The Tropical Ocean and Global Atmosphere (TOGA) program provides a good illustration of the traditional research strategies, and the profound effects the limited data base will have on the modus operandi in research on the variability of climate on intermediate time scales. The ENSO phenomenon was the centerpiece for TOGA, although mid-latitude phenomena were also documented and modeled in this program. It is important to recall that prior to TOGA the ENSO was already a reasonably well-documented phenomenon, being a large-scale, large-amplitude perturbation in the atmosphere-ocean system. The emphasis of the TOGA program was on providing an understanding of how and why this climate anomaly was manifested and assessing the predictability of the phenomena. In contrast, for intermediate-scale climate variability the target phenomena are smaller in amplitude, are derived from only a few realizations, and are not completely defined by the historical data.

The methodologies and strategies of the research activities related to the intermediate-scale climate variability will be distinctly different from those related to interannual variability for two additional reasons: (1) the inherent limitations of the data base of directly observed climate state variables (discussed above), and (2) the constraints imposed by limited computational resources coupled with the uncertainty as to the veracity of the simulated phenomena because of the parameterization of the small-scale processes and the (still) poorly understood physics. The science plan, priorities, and ongoing activities of the ACCP and of the nascent Global Ocean-Atmosphere-Land System program (GOALS) duly reflect these constraints.

The Atlantic Climate Change Program

The ACCP was formally initiated after a workshop held at the Lamont-Doherty Earth Observatory of Columbia University in July 1989. The goals of this program are as follows:

  • To determine the seasonal-to-decadal and multidecadal variability in the climate system due to interactions between the Atlantic Ocean, sea ice, and the global atmosphere using observed data, proxy data, and numerical models.

  • To develop and utilize coupled ocean-atmosphere models to examine seasonal-to-decadal climate variability in and around the Atlantic Basin, and to determine the predictability of the Atlantic climate system on seasonal-to-decadal time scales.

  • To observe, describe, and model the space-time variability of the large-scale circulation of the Atlantic Ocean and determine its relation to the variability of sea ice and sea surface temperature and salinity in the Atlantic Ocean on seasonal, decadal, and multidecadal time scales.

  • To provide the necessary scientific background to design an observing system of the large-scale Atlantic Ocean circulation pattern, and develop a suitable Atlantic

4  

Certain state variables are better measured than others. Water vapor, which may be a central component in decadal climate variations, is only crudely measured above the middle troposphere.

5  

Much of the remaining activity can be categorized as process-oriented studies or studies that relate to weather prediction.



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