Relatively little is known about the role of eddies in controlling subduction in the eastern half of the subtropical gyre. Here, a new tool to study the eastern North Atlantic Ocean is created by combining a regional, eddy-resolving numerical model with observations to produce a state estimate of the ocean circulation. The estimate is a synthesis of a variety of in-situ observations from the Subduction Experiment, TOPEX/POSEIDON satellite altimetry, and the MIT General Circulation Model. A novel aspect of this work is the search for an initial eddy field and eddy-scale open boundary conditions by the use of an adjoint model. In this region of the ocean, no fundamental obstacle exists to constraining the model to both large-scale and eddy-scale motions.

The state estimate is consistent with observations, self-consistent with the equations of motion, and it explicitly resolves eddy-scale motions with a 1/6 degree grid. Therefore, subduction rates, and the processes that control subduction, are readily diagnosed in a physically-interpretable context. Estimates for eddy subduction in the Northeast Atlantic are higher that those that have been inferred by coarse-resolution ocean models. Eddy subduction rates typically have a magnitude of 15 percent of the mean subduction. The findings of this thesis imply that the inability to resolve or accurately parameterize eddy subduction in climate models would lead to an accumulation of error in the structure of the main thermocline even in the eastern subtropical gyre, which is a region of comparatively weak eddy motions.

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