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Ocean circulation is driven by wind stress and fluxes of heat and fresh water at the sea surface. One of the primary questions we wish to answer with a numerical general circulation model is how heat and salt are transported vertically and horizontally through the ocean. Because mixing in the ocean occurs primarily along isopycnal (or, more correctly, isentropic) surfaces, an isopycnal-layer circulation model, coupled with a parameterization of mixed-layer physics, is a natural vehicle for representing the pathways of buoyancy through the ocean. The Parallel Oregon State University Model (POSUM) is an independent implementation of an isopycnal coordinate, primitive equation model designed specifically to improve on the representation of the thermal and haline circulation in presently available models.
Some of the important features needed to achieve the objective of improved representation of thermohaline circulation include: topographic features which intersect isopycnal surfaces, isopycnals which outcrop at the sea surface, a discrete-density mixed layer model, and exact conservation of mass withing a density class. A variety of experiments involving idealized domains and forcings confirm the successful implementation of these features in POSUM.
A secondary goal in the design of the model is efficient execution on the current generation of massively parallel computers. Computational efficiency is improved by splitting the barotropic and baroclinic modes and integrating the slow baroclinic motions over a much longer time step than the fast barotropic gravity waves. Using this method, it is not necessary to assume a rigid lid. Implementation on massively parallel computers is simplified by adhering strictly to a data parallel programming style.
Application of this model to realistic ocean circulation is now underway. We are focusing our efforts on the North Pacific basin because of the interest in that region associated with WOCE. Particular attention is being dedicated to the diagnosis and analysis of the subtropical water masses, and their dependence on the specification of the surface buoyancy fluxes and diapycnal fluxes.
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