A Lagrangian approach to quantifying isopycnal dispersion and the role of mixing in the Eastern North Atlantic Oxygen Minimum Zone

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In the ocean interior, microbial respiration creates a layer of low oxygen waters that is inhospitable to many marine organisms. This layer is most severely oxygen-depleted along the eastern margins of every tropical and subtropical basin, in the so-called Oxygen Minimum Zone (OMZs). OMZs which are dynamically isolated from the oxygenated surface layers play a role in ocean biogeochemistry and ecosystem structure that is disproportionate to their size. For instance, OMZs control the global nitrogen cycle, as they are the principal locations where bio-available nitrogen is removed from the ocean. Such nitrogen removal is accompanied by the production of carbon dioxide, providing a source of the greenhouse gas to the atmosphere. The OMZs also influence the distribution of large pelagic fish Moreover, OMZs are predicted to expand under global warming, an effect that is already evident in observational records and strongest in the Atlantic Ocean. This project focuses on understanding of these critical regions by describing the dynamics and quantify the rates that govern isopycnal dispersion and mixing in the tropics using a high resolution Lagrangian set data collected under previous NSF funding. This research is expected to advance conceptual models of the processes controlling oxygen concentrations in the OMZs and their representation in ocean circulation models. A graduate student will be trained in modern oceanographic analysis techniques and the results from the project will be incorporated in the classroom. Visits to middle school classrooms through the Rhode Island Teacher At Sea program will discuss oceanographic topics each spring by incorporating discussions on ocean hypoxia and how acoustic methods are used to observe circulation.Ocean circulation models do not accurately simulate the intensity of the ocean's OMZs, a persistent problem that is hypothesized to be linked to their representation of isopycnal mixing. A quantification and process-level understanding of such mixing has been historically challenged by the scarcity of high-quality data collected in these regions. From 2003-2005, NSF supported the Lagrangian Isopycnal Dispersion Experiment (LIDEX) to overcome these challenges. The LIDEX data, which included Lagrangian trajectories from the deployment of 92 isopycnal RAFOS floats at the edge of the Atlantic OMZ, are poised to address the problem of isopycnal mixing in the tropics. The LIDEX floats were successfully ballasted to drift on two different density surfaces and released in clusters at five locations near the western edge of the Atlantic OMZ. The resulting trajectories represent among the most accurate oceanic observations of Lagrangian particles to date. All floats carried temperature sensors and half were equipped with oxygen sensors. 62 floats were tracked continuously four times a day for 5 months, and 49 floats completed a 15-month mission with only one short gap. Synthesis of these data can provide valuable insight into mixing in this region, and ultimately help improve predictions of the fate of OMZs. The following three critical questions will be addressed:1. What are the processes and rates of dispersion on isopycnal surfaces from several kilometers to hundreds of kilometers?2. How does the diffusivity calculated from the float dispersion compare with other attempts to quantify mixing, and what does this comparison tell us about the flux of oxygen into the OMZ?3. Do the representations of subgridscale turbulence in coarse resolution ocean general circulation models prevent them from successfully simulating OMZ intensity?