The transfer of gases between atmosphere and ocean is central to the carbon cycle; this transfer relies on many scales of circulation and mixing.

Combined atmosphere-ocean simulations at interannual time scales require more accurate ocean models. The present generation of coupled atmosphere-ocean models exhibits unacceptable drifts (Manabe and Stouffer, 1988). Climate forecast models must be free of even small systemic errors that accumulate over long simulated periods, hiding the signals that are sought. To understand and mimic the paleoceanographic record, a major test of global models, one must be able to carry model integrations over time scales corresponding to thousands or tens of thousands of years. It is not clear that ocean and atmosphere behavior is predictable on scales of decades or longer. The limits of predictability are being explored as a research topic.

The global ocean is so large and its circulation occurs over such a variety of space (tens to thousands of kilometers) and time (days to centuries) scales that ocean circulation modeling has always overwhelmed even the largest supercomputers. This situation will probably remain for some decades to come. Thus it is a major intellectual challenge to design models of ocean circulation with time and space increments small enough to model processes adequately, given foreseeable limitations in computing resources.

Prominent features and processes that must be incorporated more accurately into physical oceanographic models (in a manner consistent with observations) include the effects of complex bottom topography on deep-water masses, deep vertical and horizontal mixing, eddies and fronts in the upper ocean, the interaction of water flow and diffusion of a variety of properties, boundary effects at the seafloor and surface, and the dynamics of shallow and deep boundary currents.

Ocean Mixing

Interior Mixing Large-scale ocean circulation is coupled with, and partially controlled by, small-scale mixing processes. Understanding the places, rates, and mechanisms by which the ocean mixes heat, salt, and momentum is crucial to understanding the circulation of the largest scales and essential to any capability to predict future oceanic states. It is intimidating to realize that to understand the dynamics of large-scale circulation and convective water mass formation, we must also understand the physics acting on the smallest scales (centimeters and millimeters). Heat-



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