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.
The Gulf Stream in the North Atlantic is the most important area in the world ocean for the exchange of heat between the ocean and the atmosphere. Curiously, associated with the Gulf Stream, and in fact all of its sister currents, such as the Kuroshio, are unusual anomalies in the thermocline known as mode waters. They are characterized by their excessive thickness, and thus are important reservoirs of heat. This grant aimed to explain what creates the North Atlantic form of mode water. Numerical experiments were conducted and models analyzed to address this question. We found the mode waters largely owe their existence to the excessive heat loss in the region of the Gulf Stream, and that the ocean weather-like variability played a critical role in distributing the heat anomalies throughout the subsurface North Atlantic. This picture supports the theoretical underpinnings of one of two competing theories for the existence of mode water. In addition, we were able to examine the importance of the Gulf Stream interactions with the east coast of the United States. Whereas these interactions have been traditionally viewed as dominantly frictional, we found they were almost frictionless; quite a surprise indeed. This work will influence the next generation of coupled climate models. We now have strong modeling evidence for the role of ocean weather in maintaining the structure of the mid-latitude ocean heat anomalies. Coupled models historically have not bothered with properly modeling ocean weather, and this work argues they therefore miss essential dynamics which are of considerable significance to the mid-latitude climate. Without ocean weather, projections for climate change from coupled models are questionable. Our results about the relatively weak frictional character of the Gulf Stream also calls for a revisitation of the existing theories of the Gulf Stream.
