Upper Ocean Turbulence in Non-Equilibrium Conditions

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Turbulent processes near the surface of the ocean play a key role in weather and climate systems by coupling the ocean with the atmosphere and by distributing nutrients, pollutants, plankton, and bubbles. Wind and waves drive this turbulence, often through complex interactions. Our current conceptual and theoretical framework of these dynamics is based on an equilibrium assumption, in which waves and turbulence are in equilibrium with the wind forcing. However, recent investigations highlight that typical ocean conditions are rarely in equilibrium, but rather are characterized by swell and variable wind waves in terms of frequencies and directions. This study will integrate recent observational, theoretical, and computational progress to systematically assess the influence of non-equilibrium conditions on turbulence near the ocean surface and on the exchange of momentum and heat with the atmosphere. Improving the representation of air-sea interaction and ocean turbulence, will advance coupled ocean-atmosphere models of weather and climate. The ability to predict the distribution of ocean pollutants as well as seasonal fluctuations and secular climate change has important environmental, societal, and economical impacts. The knowledge developed in this study will be incorporated into courses at the University of Delaware. The project will uniquely foster training of students in a collaborative environment with computational and observational experts in oceanography. The PI will participate in public outreach events, such as the annual Coast Day, an open house day for the general public sponsored by the University of Delaware's College of Earth, Ocean and Environment. This occasion provides an opportunity for scientists to inform the general public of the scientific issues that influence the environment and to expose children of all ages to careers in the sciences and engineering.By critically evaluating traditional equilibrium assumptions, the proposed research promises to advance our basic conceptual understanding of ocean surface boundary layer (OSBL) dynamics. Specifically, the study will test the following hypotheses: (1) The evolution of the OSBL depends on complex, non-equilibrium sea states, so that for the same surface fluxes OSBL dynamics vary significantly. (2) A turbulence-resolving model based on the wave-averaged Navier-Stokes equations accurately captures the observed sea state dependent evolution of the turbulent OSBL. (3) The relative importance of breaking wave and Langmuir circulation (LC) effects depends on sea state and OSBL conditions, such as OSBL depth. (4) Extending the existing theoretical and conceptual framework of planetary boundary layers by including wave effects explicitly will provide a more physical description of realistic OSBLs. These hypotheses will be addressed by analyzing observations from recent field experiments in the coastal and open ocean in collaboration with the Woods Hole Oceanographic Institution. Those rare data sets include collocated measurements of waves, surface fluxes, and upper ocean structure, including unique observations of LC characteristics. Observations will be compared to large-eddy simulation (LES) results based on the wave-averaged Navier-Stokes equations. The LES model resolves turbulence and captures both LC and breaking waves. The breaking wave input to the model will be enhanced based on recent progress on wind-wave coupling theory that takes sea state effects into account in collaboration with the National Center for Atmospheric Research. In collaboration with the Leibniz Institute for Baltic Sea Research, the researchers will evaluate common OSBL turbulence models employed in regional and global ocean models. The combined analyses of OSBL observations and process-based LES will provide the needed insights for developing novel, accurate physics-based OSBL models. Thus, the research will contribute to improving the next-generation ocean models and to enhancing our understanding of the coupled ocean-atmosphere system.