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.
Eighteen Degree Water (EDW) is a type of water mass known as mode water that resides to the south of the Gulf Stream. Mode waters are voluminous and ubiquitous, and are thought to play an important role in interannual climate variability through the temporary removal of heat and carbon dioxide from the atmosphere. How mode waters form is not well understood. They are most prevalent during the winter and tend to be found on the equatorward side of major ocean currents like the Gulf Stream, suggesting that the properties of these currents and strong wintertime winds and cooling shape mode water formation. These currents are characterized by fronts, i.e. sharp boundaries between water masses of distinct temperature, salinity, and density. This project was framed around the hypothesis that mode water formation is intimately linked to the dynamics of these fronts. Analysis of observations collected near the Gulf Stream in the winter of 2007 as part of the CLIvar MOde water Dynamic Experiment (CLIMODE) combined with numerical and theoretical calculations were used to test this hypothesis, as well as investigate questions related to how heat, salt, and other tracers are mixed across fronts and how the ocean circulation dissipates its kinetic energy. The defining characteristic of mode water is its anomalously low value of potential vorticity (PV), a tracer, which, like a dye, can be used to track the movement of water, but in addition, contains information about the dynamics. Thinking in terms of the PV has primarily been reserved for inviscid, adiabatic conditions where PV is conserved. It can also yield fundamental insights into mode water formation in the upper ocean where winds, heating, and cooling break PV conservation. Winds blowing along fronts are particularly effective at changing the PV. When winds are directed down-front, i.e. in the direction of a frontal current, the PV is reduced. Analysis of the CLIMODE observations revealed that this process of PV reduction by winds occurred along the length of the Gulf Stream and generated negative PV at the front (see Fig. 1). The winds combined with cooling, lead to the formation of a varietal of EDW in the vicinity of the current. This varietal of EDW was fresher than its counterpart found further south in the Sargasso Sea. The freshening of this water mass was attributed to the entrainment of relatively fresh water of subpolar origin into the saltier waters of the Gulf Stream. This finding opened the question as to what drove the mixing that was responsible for the entrainment. We posited that internal waves could play a role in the process, and set out to test this hypothesis. Strong, variable wintertime winds generate a type of internal wave known as a near-inertial wave (NIW). Theory and numerical simulations show that these waves interact strongly with fronts and tend to be trapped in frontal regions. Dramatic examples of trapped NIWs have been observed in the Gulf Stream during CLIMODE, where waves with crests extending 600 m below the surface were recorded (see Fig. 2). We developed a theory to understand the underlying physics behind this phenomenon. The theory suggests that fronts should act as what could be described as "NIW surf zones", where NIWs amplify and break as their propagation is slowed approaching a front. The combined action of the wave’s sheared velocity field and the small-scale turbulence induced during breaking leads to lateral mixing of tracers, as demonstrated in numerical simulations (see Fig. 3). At the Gulf Stream, this mixing could contribute to the exchange of water between the subpolar and subtropical gyres, a process that has important implications for not only EDW formation but also the entrainment of nutrients into the subtropics, the nutrient depleted "deserts of the sea." The research conducted as part of this project has also shed light on the energetics of the circulation. One of the great conundrums in physical oceanography is closing the energy budget of the large-scale circulation. While the main sources of kinetic energy (KE), the winds and tides, are well known and quantified, the sinks of KE remain uncertain. Although the circulation does lose energy to mesoscale eddies, because the dynamics of these eddies is strongly constrained by the Earth’s rotation, the KE in the mesoscale is primarily transferred back to larger scales and away from the small-scales where viscous dissipation can act. Submesoscale currents (i.e. flows with length scales between 100 m and 10 km) that form at ocean fronts are thought to bridge the gap and transfer KE from the mesoscale to small-scale turbulence. The research conducted as part of this project has shown that certain submesoscale phenomena, namely symmetric instability, which forms when winds blow down-front (see Fig. 4), and the interaction of fronts, eddies, and NIWs, are particularly effective sinks of KE.
