Vertical mixing below the land fast ice on the Arctic shelves: Efficiency and driving mechanisms

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Vertical mixing in the coastal systems on the Arctic shelves is an important environmental factor. It is capable of ventilating the bottom layer that may otherwise become stagnant during ice-covered periods due to the elimination of the wind stress from the water column. Moreover, vertical mixing can favor upward nutrient fluxes from the bottom to the surface layer, preconditioning the formation of coastal zones with high biological productivity. Tides are capable of creating velocity shear across the shelf water column that can induce vertical mixing in the land-fast ice covered Arctic shelves. However, the tidal-driven vertical mixing below the ice has never been assessed due to the lack of long-lasting simultaneous high-resolution velocity, temperature and salinity profiling on the Arctic shelves. An understanding of the tide-driven mixing below the land-fast ice will be essential for predictions of land-fast ice formation and ablation. This is of particular importance for the Canadian Arctic where northerners use the land-fast ice as a transportation system, and extensive exploration of oil and gas is expected. The overarching scientific themes that motivate this research are the (i) stability of the land-fast ice with potential implication for local transportation and (ii) understanding of relative thermodynamic and dynamic contributions to changes in land-fast ice on Arctic shelves, particularly in terms of ongoing climate change. The short term goals are to (i) provide the observationally-based assessment for efficiency of the tidal-driven vertical mixing and (ii) evaluate the tidal-driven vertical fluxes below the land-fast ice during winter. The long-term scientific objectives of this research are to (i) improve our understanding of regional changes in vertical mixing and mechanisms responsible for shear instability below land-fast ice on Arctic shelves, with implications for land-fast ice cover modeling and predictions and (ii) provide a hemispheric-scale interpretation of differences in shear-driven instabilities and vertical mixing along Arctic shelves with a main focus on the Canadian Arctic. The short-term scientific objectives are as follows: to (i) conduct oceanographic observations from the land-fast ice cover at four locations distributed among the different Arctic shelves and utilize data previously obtained using similar observational ideology; (ii) identify the mechanisms responsible for creating velocity shear across the vertically stratified water column; (iii) reveal the role of tidal-driven velocity shear in promoting instability and vertical mixing; (iii) estimate the upward salt, heat and Suspended Particulate Matter (SPM) fluxes associated with shear-driven instability. The observational program consists of ~ 7-month continuous velocity, temperature, salinity and optical under-ice profiling at four land-fast ice covered locations distributed among the different Arctic shelves. These include the northeastern Greenland, Hudson Bay, Canadian Archipelago and Siberian Arctic. These long-term observations are complemented by short-term (~12-36 h) measurements of extremely small scale (order 1 mm) fluctuations of salinity, temperature, and chlorophyll concentration to infer the levels of dissipation of turbulent kinetic energy and in-situ fluxes of heat and salt. Every year, oceanographic equipment is reallocated between these locations. This observational strategy allows a large-scale interpretation of differences in shear-driven instabilities and vertical mixing along Arctic shelves. One Research Associate, three PhD students and two undergraduate summer students are involved in this research.