Southern Ocean mixing has the potential to play an important role in the Meridional Overturning Circulation, but considerable uncertainty still exists as to the locations and processes responsible for the most mixing. The DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer release, together with 10 EM-APEX profiling floats and ship microstructure surveys, has quantified the mixing in the Southeast Pacific (west of Drake Passage), revealing modestly elevated mixing due to wind-generated near-inertial waves, but has yet to extend fully into the high-energy environments of Drake Passage and the Scotia Sea. The shear measurements of the EM-APEX (a profiling float with an electromagnetic velocity measurement) have proven a valuable tool for the characterization of the Southern Ocean's internal wave field (the "finestructure" seen in velocity and density profiles with wavelengths between 10m and 1000 m). This inclusion of shear is particularly necessary, given the predominance of near-inertial waves with relatively weak signature in density (i. e., high shear/strain ratios). The combination of high-quality shear and density profiles over the upper 2000 m and sustained duration makes the EM-APEX an economical choice for extending these types of measurements into the less well-sampled regions and times of the year.

In this project, the investigator and his student will take full advantage of the existing DIMES dataset by (a) conducting an in-depth analysis of both mesoscale and fine-scale shear and stratification data from the initial EM-APEX, along with surface fields from satellite altimetry, to understand the connections between the internal wave field and mesoscale currents, winds, and bottom topography and (b) deploying 8 additional EM-APEX to extend the observational array's coverage within Drake Passage and the Scotia Sea, fill gaps in the seasonal cycle, and increase the robustness of the results.

Broader Impacts: The project has important implications for the development of model parameterizations, such as being developed by the Climate Process Team on Internal Waves and Mixing. The incorporation of internal wave mixing processes into large-scale circulation models is essential for accurate predictions of circulation and future response to climate change. Without parameterizations based on comprehensive observations and solid physical understanding of mechanisms, there is little hope that the mixing in large-scale models that do not directly simulate internal waves will be correct. Much of the project will be conducted by a graduate student, Byron Kilbourne, and will form the basis of his Ph.D. dissertation. This is a valuable educational opportunity, considering the novel nature of the measurements and the need for full-time focus in understanding this complex region. It also provides the benefit of training a talented new member of the ocean science workforce. Educational outreach will be conducted through the Seattle Aquarium and Pacific Science Center, with an emphasis on bringing basic information on the physical oceanography of the Southern Ocean to a public forum.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Eric C. Itsweire
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University of Washington
United States
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