| Summary: | Eighteen Degree Water (EDW), the subtropical mode water of the western North Atlantic, is a voluminous, weakly-stratified upper ocean water mass that is formed via a global extreme in ocean-to-atmosphere heat exchange. In this dissertation, data from Argo profiling floats and the CLIVAR Mode Water Dynamics Experiment (CLIMODE) observational campaign are used to quantify EDW formation strength and to identify the relevant mechanisms that drive the interannual variation in EDW renewal. Evaluating the Sargasso Sea buoyancy budget from 2004-2012, we find that interannual changes in winter buoyancy removal, and thus EDW renewal, are more significantly associated with late winter surface forcing than changes in lateral oceanic heat transport.Next, observations from Argo together with acoustically-tracked, isothermally-bound profiling CLIMODE ``bobbers'' are used to investigate the dominant processes that drive the large annual erosion of EDW and quantify EDW destruction rates.We find that EDW destruction is dominated by 1-D vertical diffusion, while mesoscale, along-isopycnal stirring is also significant, explaining approximately 1/3 of the total annual EDW destruction, particularly in the northern Sargasso Sea where larger potential vorticity gradients and enhanced mesoscale activity exist.Finally, we employ CLIMODE profiling floats equipped with oxygen sensors to determine the structure and seasonality of oxygen evolution in the Sargasso Sea and evaluate the physical processes that modify the distribution of oxygen. Applying estimates of vertical mixing estimated from the previous EDW destruction component of the dissertation, we are able to determine the seasonality and depth-dependence of net biological production and consumption and obtain an estimate of the remineralization of organic carbon that occurs in the region.
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