Churning the deep: ocean mixing by breaking internal waves

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By investigating the physics behind ocean mixing, this proposed research program will address a key uncertainty in the field of numerical climate modeling and prediction. Ocean mixing occurs at scales that are too small to be explicitly resolved in global climate models, and as a result the influence of mixing must be represented indirectly, by physically based models, or parametrizations. Climate models are extremely sensitive to the distribution of parametrized mixing, and uncertainties in how to represent ocean mixing limit our ability to accurately predict and adapt to future changes in climate. The spatial and temporal distributions of ocean mixing is mostly determined by the dissipation of internal waves, waves that propagate across density interfaces in the ocean's interior and dissipate by breaking, much like waves at the beach. The focus of this proposed research program is on mixing by wind- and tide-driven internal waves, as parametrizations for these processes remain incomplete, only partially accounting for different dynamical regimes, as described below. The program has three objectives. The first is to explore the lifecycles of tidally-driven internal waves in coastal regions using numerical models, with an emphasis on the west coast of North America. This work will emphasize understanding when and where internal tides are dissipated and mix at the coast, with the goal of developing a parametrization for use in climate models. The depths at which wind-driven internal waves dissipate in the open ocean will be studied for the second objective, using multi-year moored observations from the Atlantic, Pacific and Southern Oceans. Theory and numerical models will be used with the observations to understand energy pathways from forcing to dissipation in these dynamically distinct regions. Finally, the third objective of this research program is to understand ocean mixing by the winds and tides on the Scotian Shelf, a conveniently accessible coastal environment with large tides and wind forcing. Historical and ongoing data collection in this region provides a large dataset for exploring these processes. This objective will additionally address the role of ocean mixing on ocean biology, contributing to an understanding of the processes that sustain the commercially important fisheries in the region. Anticipated outcomes of this research are contributions to our understanding of how and where ocean mixing occurs in the global ocean, and the development of new parametrizations that will increase the accuracy of global climate models, enabling us to forecast future changes both in Canada and globally with greater confidence. Over the next five years, this program will train three graduate students and four honours students, developing their skills in applied physics, numerical and computational analysis and oceanography.