The Equatorial Mixed Layer and Upper Ocean Mixing

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Diurnal cycling of the tropical mixed layer is central in the dynamics governing air-sea exchanges of heat and moisture which, in turn, are central to a variety of equatorial phenomena including Tropical Instability Waves, the Madden-Julian Oscillation, El Nino/Southern Oscillation, tropical storm cyclogenesis, and the overarching cycle of monsoon seasons. The enhanced stratification below the seasonal mixed layer is also important, as it regulates vertical fluxes into the surface layer from the cooler, nutrient rich upwelling waters below.Understanding the processes that control turbulent mixing of momentum and tracers in this transition layer is an outstanding problem in upper-ocean physics. Based on both modeling results and prior observations, this project's investigators will pursue the hypothesis that turbulent structures in an active mixed layer, combined with shear across the upper pycnocline, generate energetic Near-N Internal Waves (NNIW). These NNIW are confined to the region of elevated buoyancy frequency in the uppermost pycnocline. When they break, they contribute to turbulent dissipation and mixing across the transition zone. In the tropics, the most likely candidate for this kind of NNIW generation is nighttime buoyancy-driven convection combined with the Equatorial Undercurrent shear. The tropical generation of NNIW will compared with previous temperate-latitude observations of the analogous generation by Langmuir circulation and inertial currents, using new observations taken with a combination of a High-resolution Phased Array Doppler sonar, the fast-profiling CTD, and the High-resolution Acoustic Doppler systems aboard the R/V Revelle.Intellectual merit. The project focuses on a link in the air/ocean system that is vitally important yet incompletely understood: net mixing through the uppermost layers of the sea. The mixed layer and uppermost pycnocline together form a crucial barrier between the atmosphere and deep ocean, controlling the net fluxes of heat, momentum, nutrients, and greenhouse gases. This uppermost layer is also vital to marine life, setting the scene where light and nutrients meet, and where pollutants and contaminants are dispersed. Appropriate scaling of these fluxes has been shown to be important to global air-sea coupled simulations, which can be sensitive to both the degree of surface mixing and to the mixed layer depth. Broader impacts. The results will help improve air-sea exchange estimates, and hence both environmental and climate effects, thereby helping improve the information by which long-term policy decisions are made. The results and interpretations will be disseminated broadly to enhance scientific understanding and facilitate both decision making and further scientific work.The proposed activity will provide training and scientific opportunities for a graduate student, and data to support ongoing scientific studies involving students, post-docs, and summer interns. Partnerships and collaborations across institutions will be actively developed.