Collaborative Research: Rapid Gravitational Adjustment of Horizontal Shear: A Recipe for Energetic Mixing in the Coastal Ocean

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Intellectual Merit The study will extend our knowledge of mixing processes in the coastal ocean through a comprehensive examination of the stability and evolution of horizontally sheared flows in the presence of a horizontal density contrast. This configuration of shear and stratification is common in strongly time-dependent flows in the coastal ocean including estuarine inflows and outflows, separating flows around headlands and islands, and merging flows in morphologically complex regions such as tidal inlets. The resulting buoyancy-driven flow results in tilting and intense stretching of the shear-flow vortices and ultimately a breakdown of the flow leading to intense vertical mixing. Horizontal stratification in the presence of horizontal shear has the potential to strongly enhance mixing, despite the fact that the gravity-driven flow by itself increases the vertical stratification. The research into the dynamics and consequences of this process will involve both high-resolution non-hydrostatic numerical modeling and comprehensive laboratory experiments utilizing particle image velocimetry and laser-induced fluorescence. Scaling arguments for this highly nonlinear evolution have been developed and will guide the investigation. It is expected that the results will inform future parameterizations of mixing processes that are crucial in coastal ocean circulation models. Broader Impacts Coastal mixing processes are important for pollutant transport and dilution, biogeochemical cycles of nutrients and carbon in the productive coastal zone, and the transport of marine larvae and plankton. The results of this study will yield understanding that should lead to improved parameterizations for mixing that can be incorporated into estuarine water quality, coastal ocean circulation, and ecological models. The PIs are both active in their respective academic programs (MIT/WHOI Joint Program and UNC Marine Science) and the results of this research will quickly reach the classroom. The grant will also provide important early career support for undergraduate and graduate students will be incorporated into aspects of the research through the WHOI Summer Student Fellowship Program and the Geophysical Fluid Dynamics (GFD) Summer Program at WHOI. The grant will support the continued operation of the WHOI GFD Laboratory, which has hosted over 100 visiting students and scientists over the last 30 years to undertake a variety of experiments spanning physical oceanography, atmospheric dynamics, fluid dynamics, and geological processes.