CLIMODE II Analysis Phase

1. Evolution and Fate of EDW in the North Atlantic Subtropical Gyre

We propose to examine the physical processes associated with the evolution (formation, circulation, and destruction) of Eighteen Degree Water (EDW) within the subtropical gyre of the North Atlantic Ocean. EDW is the archetype for the anomalously thick and vertically homogenous mode waters that are typical of all subtropical western boundary current systems. EDW is associated with a shallow overturning circulation that carries heat northward and is an interannual reservoir of anomalous heat, nutrients and CO2. Understanding the annual cycle of EDW evolution, and in particular its associated circulation pathways and destruction mechanisms, is important because though EDW is isolated beneath the stratified upper-ocean at the end of each winter, it may reemerge in subsequent years to influence mixed layer properties and consequently air-sea interaction and primary productivity.
We will investigate the pathways of EDW circulation, the processes of EDW destruction, and the role of EDW in modulating ocean-atmosphere heat exchange and nutrient supply to the euphotic zone. We plan to synthesize a broad spectrum of observations including a substantial in-situ dataset collected during the recently completed CLIVAR Mode Water Dynamics Experiment (CLIMODE). This coordinated field effort included a large array of profiling floats, a moored array, and several hydrographic surveys. To supplement this unique dataset we will also examine contemporary and historical hydrography, Argo profiling floats, surface drifters, satellite altimetry and ocean color fields, and output from several eddy-resolving OGCM simulations.
The proposed work will test the long-hypothesized climatic importance of EDW as an element of the North Atlantic's shallow overturning circulation and as a short-term reservoir of heat, nutrients, and carbon. Significantly, this study will advance understanding of the various processes that destroy EDW. In addition, this study will, for the first time, compare and contrast the roles of the large-scale, low-frequency circulation and the mesoscale eddy field in EDW dispersal and destruction. Finally, the proposed work will provide a foundation for comparative studies since mode waters are found in every ocean basin.

2. Dynamics of EDW from CLIMODE Observations and its Climate Implications

Subtropical Mode Water (STMW), an isothermal layer that forms on the equatorward side of western boundary current (WBC) in response to wintertime cooling, is central to understanding climate variability in mid-latitude regions because it integrates anomalies in both the ocean and atmospheric to contribute to climate system memory. The STMW region has a large capacity to store heat, and its heat storage rate has been shown to depend both on air-sea uxes and on ocean circulation. The volume of STMW is so large that several years of air-sea interaction alone cannot dissipate it; after formation, it is partially re-entrained in subsequent winters to again interact with the atmosphere. Many processes have been identified that could affect STMW formation or its subsequent destruction. A primary goal of the NSF-funded CLIMODE (CLIVAR Mode Water Experiment) is to quantify the processes contributing to the evolution of the STMW of the western North Atlantic, commonly referred to as 18-degree-water (EDW) because of its nearly constant temperature. An extensive set of measurements has been obtained over the 2-yr field program; analyses and modeling of the observational period can help evaluate the relative importance of the processes contributing to EDW evolution. A primary motivation is that CLIMODE analyses will lead to improvements in climate modeling; we propose here to provide a link between the CLIMODE-specific analyses, longer period variability, and the need for metrics to evaluate and verify climate models. The EDW region with its large heat storage and air-sea fluxes, variable poleward heat transport, and energetic ocean circulation is a prime candidate for memory in the atmosphere-ocean system. The proposed research is an examination of the interannual-to-decadal variations in EDW volume, of the processes that contribute to it, and its impact on air-sea interaction. Some competing processes in EDW evolution (warm water advection by the Gulf Stream, mixing, and oceanic heat loss through air-sea fluxes) have variability linked to climate indices such as the North Atlantic Oscillation. We will examine whether important processes can be monitored using proxy variables and thus link the field program results to the longer climate record to evaluate the importance of each process, the predictability of EDW evolution, and the ability of EDW to contribute to climate memory.

3. Examining a New Paradigm for Eighteen Degree Water Formation

CLIMODE observations 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 70W 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 50W has lower temperatures, salinity & PV, and higher oxygen saturation. All of the water being discussed is found within the 100km anticyclonic region just to the south of the maximum downstream GS 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 submesoscale 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 submesoscale resolving numerical simulations nested within the global, eddy-resolving HYCOM model run with assimilation In particular we propose to investigate the importance of submesocale 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 our nested simulations. We 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 we propose to evaluate using both existing data and numerical simulations.

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