Recent studies suggest that Eighteen Degree Water (EDW), the subtropical mode water of the North Atlantic, may play an important role in setting the nutrient reservoir of the subtropical gyre. The findings indicate that advection of EDW injects a wedge of low nutrient water beneath the seasonal thermocline in the North Atlantic subtropical gyre. Furthermore, the presence of the nutrient-depleted water mass may dampen the biological response to vertical processes that flux nutrients upwards throughout the subtropical gyre. This raises important questions regarding how and when the EDW is depleted in nutrients and how the signal is exported to the rest of the subtropical gyre: 1) What are the time scales associated with the biological depletion of inorganic nutrients during water mass formation? 2) What is the nutrient concentration of the EDW upon subduction? and 3) How does the formation of EDW impact the downstream nutrient reservoir and vertical nutrient fluxes?
In this project, researchers at Duke University will address these questions with nutrient measurements in the formation region of the EDW. Fortuitously, an NSF-funded field program, CLIvar MOde water Dynamics Experiment (CLIMODE), is currently ramping up for a comprehensive analysis of the formation, maintenance and destruction of EDW. Though CLIMODE's observational program is comprised of a suite of measurements, it currently lacks any biogeochemical component. This project will provide for the addition of nutrient sampling to the CLIMODE observational program in order to test hypotheses concerning the impact of EDW on the subsurface nutrient reservoir. Specifically, the investigators will add nutrient analyses to the planned hydrographic surveys and will mount a nitrate sensor on each of two CLIMODE moorings. The former will create the first cross-section of nutrients during the winter in the formation region, and the latter will provide an unprecedented time series of nitrate within the subtropical gyre.
The overall goal of this proposed work is to further the understanding of the degree to which the horizontal advection of a subtropical mode water impacts the subsurface nutrient reservoir and the corresponding availability of nutrients to primary producers. Such an understanding is crucial since the subtropical gyre's role as a carbon sink depends on the availability of nutrients to fuel the biological pump, remove organic carbon from the surface ocean, and maintain a favorable air-sea gradient in the partial pressure of carbon dioxide necessary for the solubility pump. Furthermore, since water mass production is known to vary on interannual and decadal time scales, establishing the link between subtropical mode waters and nutrient availability advances our understanding of how primary productivity is impacted by climatic variability.
In terms of broader impacts, it is clear that an improved understanding of how the subsurface nutrient reservoir is replenished and diminished will aid the understanding of how the ocean acts as a carbon sink. The project will also support the research of a pre-doctoral graduate student.
