Collaborative Research: Vortex dynamics and interannual variability in the Labrador Sea

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Accumulating evidence suggests that deep convective ventilation in the Labrador Sea is partly controlled by mechanisms other than local surface forcing and local density gradients. It now appears that there exists an active eddy-driven restratification which is modulated by variations in boundary current dynamics. Large inter-annual variations in both eddy shedding and buoyancy transport from the boundary current have been observed but not explained. This project will investigate the processes controlling eddy generation and associated buoyancy transport by combining realistic and idealized numerical modeling, data analysis, and theory. Ensembles of numerical experiments with a high-resolution regional model will explore the sensitivity of eddy generation and property transport to variations in local and external forcing parameters. Extended analysis of eddy and boundary current properties in data, centrally the now fifteen-year TOPEX/Poseidon and Jason altimeter records, will allow comparison of modeled and actual vortex characteristics over a wide range of oceanic conditions. Theory, supported by idealized experiments, will provide criteria to test candidate hypotheses as to the nature of the instability, and will suggest possibilities for its parameterization. The net result will be an understanding of the links between local and non-local forcing variability, and the eddy-driven buoyancy fluxes which limit deep convection. This process-oriented study should form an important step toward the larger goal of understanding and accurately modeling variability of the Atlantic Meridional Overturning Circulation in general.Intellectual Merit: This work has a direct benefit to the representation of the Labrador Sea branch of the Atlantic Meridional Overturning Circulation (AMOC) in large-scale climate models, in which details of the narrow boundary current instability are not possible to resolve. In the face of dramatically increasing freshwater discharge from the Arctic, it is critical to understand the transport of buoyancy from boundary current to the convection region, and in particular, to identify the factors underlying its variability. Furthermore, this collaborative project will contribute to the broader effort of realistically representing the effects of mesoscale features on the large-scale circulation in coarse-resolution numerical models.Broader Impacts: The primary societal benefit of this work is its relevance to understanding and possibly predicting variations of the AMOC. Results will be presented in graduate classes by two of the investigators, A. Bracco at Georgia Tech and J. Pedlosky at WHOI-MIT. The two graduate students supported by the project will benefit from exposure to modeling, analytical investigation and data analysis techniques. Analysis algorithms developed in this work will be freely distributed to the greater scientific community, by inclusion in JLAB, J. M. Lilly?s open-source software package for Matlab. The proposed research will also be incorporated into teaching material for high school teachers in the Atlanta area, with the support of the Center for Education Integrating Science, Mathematics and Consulting (CEISMC).