A new mechanism for Mode water formation at a thermohaline ocean front

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In every ocean basin, on the equatorward side of major ocean fronts, layers of weakly stratified waters with nearly homogeneous properties are found. These so-called "mode waters" play an important role in the coupled ocean-atmosphere system by sequestering and releasing heat and carbon dioxide on interannual timescales and by affecting the large-scale circulation through shaping the potential vorticity field. Current climate models have difficulties accurately forming mode waters with the correct temperature and salinity properties, which is a concern given their influence on interannual ocean and climate variability. The mechanisms explaining their formation are not well understood but appear to be shaped by the dynamics of the ocean fronts that mark their poleward extent. A new mode water formation mechanism that has a clear connection to fronts and involves cabbeling, submesoscale lateral mixing, and frontogenesis will be explored. Cabbeling refers to the process by which two water masses of equal density but different temperature and salinity are mixed to create a new, denser water mass, as a result of nonlinearities in the equation of state for seawater. The theory that forms the basis of this project suggests that the properties of mode waters are influenced by cabbeling at fronts. The dynamics necessarily involves submesoscale processes, which have received much attention in recent years, but in the framework of a linear equation of state. The coupling between submesoscale motions and cabbeling through changes in density has the potential for new, rich physics that will be explored in this project. The expected findings should help inform strategies to improve the simulation of mode waters in ocean circulation and climate models and should be important for quantifying the flux of nutrients at the fronts that mark gyre boundaries. The project includes mentoring and support of a promising postdoctoral researcher. A high school science teacher will be given training on laboratory demonstrations that illustrate the physics of the ocean, atmosphere, and climate that can be incorporated in their courses. A simple two-dimension model for the water mass transformation due to cabbeling at a thermohaline front forced by frontogenetic strain and equilibrated by lateral mixing correctly predicts the isopycnal layers where mode waters are observed to reside and suggests that the mechanism could be responsible for persistent, hence significant, mode water formation. This project will test and extend the simple model using a more complete theory and two sets of numerical simulations designed to study the process on scales spanning the width of a basin to the width of a front. The theory will consider partially compensated fronts and use the semi-geostrophic equations to capture the feedback of secondary circulations on frontogenesis and quantify its effect on cabbeling. The objectives of the basin-scale simulations are to test the theoretical prediction for mode water formation in a three-dimensional, double-gyre circulation and to explore the role of lateral mixing and cabbeling in setting the T-S relation of mode waters. The second set of simulations will be configured with a thermohaline front collocated with a sheet of cyclonic vorticity modeled after flow observed in the Gulf Stream. The submesoscale shear instabilities that develop and the isopycnal mixing of temperature and salt they induce will be studied to determine how cabbeling should be parameterized via an eddy diffusivity. In addition, the potential dynamical feedbacks of cabbeling on the submesoscale instabilities will be investigated.