Collaborative Research: Critical Layers and Isopycnal Mixing in the Southern Ocean

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Climate-scale ocean models unanimously stress the key regulatory function played by the oceanic overturning circulation in the Earth's climate and biogeochemical cycles over decadal and longer time scales. Yet in their quest to resolve many topical climate problems, the models credibility is challenged by their extreme sensitivity to the representation of mixing processes in the Southern Ocean. This peculiarity of model behavior reflects the unique role of mixing in mediating the vertical and horizontal transports of water masses in the Antarctic Circumpolar Current (ACC), which shape the overturning circulation through their respective impacts on the overturning rate and inter-ocean exchange. The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was recently funded by NSF to measure directly eddy mixing along density surfaces in the ACC. Eddy mixing will be measured by releasing a chemical tracer and 150 floats at one vertical level. New theoretical results, not available at the time, suggest that eddy mixing rates are strongly enhanced at critical levels in the vertical. The goal of this project is to extend the DIMES project and release 50 additional floats at a shallower level to test the hypothesis that critical levels control the rate of upwelling and downwelling of water masses in the Southern Ocean.Intellectual Merit. Conceptual models of global meridional overturning and numerical predictions for future climate are strongly sensitive to the methods used to represent mixing along and across the ACC. Theory suggest that mixing rates vary greatly in the horizontal and in the vertical. Climate ocean models unanimously stress that model skill is strongly sensitive to these variations. The DIMES project will provide the first direct observations of mixing in the Southern Ocean and will likely deliver a wealth of new information about eddy transport in this part of the ocean. However tracer and float deployments are planned only at one level and will not provide information about the vertical variability of eddy mixing. The additional 50 floats to be deployed at ashallower level will capitalize on the opportunity to learn about a key aspect of eddy mixing in the Southern Ocean.Broader Impacts. This proposal has potentially wide impact because it is designed to further our understanding of a central component of the climate system. The proposed work will also contribute to the improvement ofmixing and stirring in large-scale ocean models such as the MITgcm. Finally, there is strong educational component through the training of a graduate student and a post-doc, and the development of new curricula to introduce students in the MIT/WHOI Joint Program to the role of the Southern Ocean in the climate system.