Ocean Circulation, Lateral Transfers of Nutrients, and the Air-Sea Flux of CO2

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ABSTRACTOCE-0525974Biological productivity is regulated by the availability of essential nutrients. In the oceans, export of sinking organic particles inexorably transports nutrients towards the deep, denser ocean waters. Ultimately, physical transport must return those nutrients to the lighter, surface waters where photosynthesis can occur. However, observed diapycnal mixing rates in the thermocline are too small to balance the downward flux of organic matter. Therefore, a significant (perhaps dominant) contribution to the diapycnal transfer of nutrients must occur as lateral transport in the surface ocean. This is facilitated by air-sea buoyancy fluxes and governed by the residual mean flow, which in the surface ocean is the net effect of lateral Ekman and eddy transfers. In this research, researchers at the Massachusetts Institute of Technology will use a hierarchy of models, ranging from simplified scaling arguments to numerical models of ocean circulation and biogeochemical cycles, to address the following hypotheses: 1. The surface residual mean flow supports lateral, diapycnal transfers of nutrients across the Antarctic Circumpolar Current and sustains the productivity of the Atlantic Basin on long timescales. An analogous process at the inter-gyre boundary of the North Atlantic helps to sustain the nutrient budget and productivity of the subtropical gyre. 2. Patterns of air-sea carbon flux in the vicinity of the Antarctic Circumpolar Current and inter-gyre boundary can be understood and predicted in terms of the transport of carbon and nutrients by the residual mean flow and isopycnal eddy stirring. 3. The transport of nutrients and carbon by the residual circulation of the Southern Ocean has a significant control on the global ocean carbon pumps and atmospheric carbon dioxide levels on century and longer timescales, regulated by climatic changes in air-sea momentum and buoyancy fluxes. Among the broader impacts of the research, the work will lead to continued development of the MIT ocean circulation and biogeochemical model. The model is a freely available tool and there is a growing international user base for the tracer and biogeochemical packages. The work also has a strong training component that includes support for a graduate student. In addition, the investigators are engaged in writing an interdisciplinary book of ocean biogeochemical cycles for graduate students.