Collaborative Research: Examining a New Paradigm for Eighteen Degree Water Formation

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The observations obtained during the CLIVAR MODE Water experiment (CLIMODE) suggest that a significant fraction of Eighteen Degree Water (EDW) formation occurs within the eastward-flowing, separated Gulf Stream (GS). This is because water entering the formation area near 70 degree West under the North Atlantic storm track has warm temperatures, relatively high salinity, high potential vorticity (PV) and low percent oxygen saturation in the EDW source waters, while EDW exiting the region near 50 degree West has lower temperatures, salinity & PV, and higher oxygen saturation. All of the water being discussed is found within the 100 km anticyclonic region just to the south of the maximum downstream Gulf Stream flow. Estimates that 50%-90% of the needed amount of new EDW is formed within this frontal region indicate that a new paradigm of EDW formation may be needed: one that departs significantly from the quasi-one dimensional ideas of purely diabatic formation in the Northern Sargasso Sea and that involves diabatic and wind stress-driven production of new EDW within the Gulf Stream frontal region and vigorous cross-frontal mixing and freshening of the water column associated with sub-mesoscale instabilities and shear dispersion by near inertial waves. This study proposes to examine the robustness of these results through innovative analyses of the observations available from CLIMODE combined with sub-mesoscale resolving numerical simulations nested within the global, eddy-resolving hybrid coordinate ocean model (HYCOM) run with assimilation. In particular this project will investigate the importance of sub-mesocale motions and frontal dynamics on the large-scale budgets of PV and salinity as they relate to EDW formation in the proximity of the GS. Additional case studies will be examined of EDW production during the winters of 2006 and 2007 using extensive shipboard observations, subsurface profiling float measurements, and nested model simulations.In terms of intellectual merit, this project will critically examine the importance to EDW formation of frontal-scale processes with active submesocale instabilities and mixing driven by inertial shear dispersion. While there is emerging evidence that strong episodic heat and buoyancy exchange occurs over the wintertime Gulf Stream and that background oceanic vorticity may be an essential element in the new mode water formation, the role of the stress driven formation process remains an open question which will be evaluated using both existing data and numerical simulations.In terms of broader impacts, if this new paradigm is valid, sub-tropical mode water formation, which is invariably tied to the flanks of strong zonal flows, cannot be adequately understood in coarse, Complex climate models that do not properly resolve or parameterize these frontal dynamics. This research project has a high potential to contribute to fundamental new ways of parameterizing air-sea coupling in regions of strong oceanic fronts. The investigators on this project are all members of academic institutions with strong graduate programs in environmental sciences and one graduate student and a post-doc will be trained and mentored as part of the project.This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.