among moisture fields (clouds and water vapor) and motion fields. Particular issues are cloud-water vaporization feedback, cloud formation (including vertical structure, radiative, circulation, and other feedbacks), and the model representation of those processes, which often occur as subgrid-scale processes. Also important are the relationships of cloud formation and evolution and surface boundary conditions.

The relationship between atmospheric circulation variability and external radiative forcing has not been clearly resolved. Numerous studies have tried to identify periodic behavior in the atmospheric spectrum due to periodic changes in solar forcing. Because the lower atmosphere absorbs only a small part of incoming solar radiation, which varies by only 1 W/m2 over a solar cycle, it is hard to see how such a weak signal could affect climate, unless a positive feedback existed in the atmosphere. Nonetheless, the evidence suggesting such a relationship is often compelling, justifying a concerted effort to understand the potential mechanisms.

Because of the likelihood that anthropogenic change is already imprinted in records of climate variability over the past century, there is a strong need to obtain paleorecords of past atmospheric conditions for instrumental and proxy data and to increase the volume of the archives through data “archeology” (reconstruction of past climate data) and additions of new data. These efforts should proceed in parallel with establishing clear guidelines for future atmospheric observations and careful planning of observational networks, so that adequacy, continuity, and homogeneity of the records are assured. The future observations should describe both state variables (winds, pressure, temperature, humidity, and rainfall) and forcing and other related variables (solar radiation, clouds, aerosols, and chemical composition).

Key Scientific Questions About Atmospheric Circulation

How much of the dec-cen variability is unforced? For example, are dec-cen variations of the PNA, NAO, and other climate patterns driven by inherent natural climate system variations, reflecting nonlinear internal interactions, or coupled interactions (in all cases, interactions that would effectively extend the intrinsic atmospheric timescales)? Or are the variations driven predominantly by changes in radiative forcing, due to anthropogenic increases of greenhouse trace gases, natural or anthropogenic aerosols, and/or variations in solar irradiance?

How does large-scale circulation change on dec-cen timescales, and how does it interact on these scales with regional and higher-frequency changes? Better documentation is needed of large-scale circulation changes to identify how they covary with regional climate states, storm tracks, and weather systems that typically vary over shorter timescales. In other words, how do variations in the mean climate state influence the spatial and temporal distributions of the higher-



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