The Downwelling Limb of the Thermohaline Circulation

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Intellectual Merit: Recent observational, theoretical, and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries. Nonetheless, the dynamics and thermodynamics that control where waters sink from the upper ocean to mid- and deep-depths, and what the consequences of these downwelling regions are for the general circulation, are not well understood. There is a lack of clear understanding on whether the downwelling occurs over the spatial scale of the boundary current or within a narrower, mixing-driven, boundary layer, as suggested by previous theories. The proposed study seeks to explicitly represent these downwelling regions in high resolution, non-hydrostatic numerical model calculations and laboratory experiments. The scientific objectives of the proposed study are to: i) To determine what controls the rate and spatial scales of downwelling near lateral boundaries; ii) To test previous theoretical models of downwelling boundary layers and their influence on the basin-scale circulation; iii) To determine if the details of the near-boundary buoyancy-forced turbulence needs to be explicitly resolved in order to faithfully represent the influence of the downwelling regions on the basin-scale circulation Broader Impacts: The proposed study will have broader impacts spanning both research and educational activities. The science objectives of the proposed study will have direct impact on our understanding of the mean and low frequency variability of the thermohaline circulation, its role in the climate system, and our ability to model it. The processes of mixing and downwelling near boundaries also impacts other branches of oceanography such as biology and chemistry. The results of this study will also be made broadly available to the scientific community through publication in scientific journals, presentation at national and international meetings, and in seminars. Both PIs are staff in a joint educational/research institution within which they have played a role (and plan to continue) in teaching, advising, supervising and training graduate students and summer student fellows (undergraduate). This research will also help maintain the Woods Hole Oceanographic Institution Geophysical Fluid Dynamics laboratory as a teaching and training facility.