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

北极大陆架沿岸系统的垂直混合是一个重要的环境因素。它能够使底层通风，否则在结冰期间由于消除了水柱的风应力而可能变得停滞。此外，垂直混合可以促进从底层到表层的营养盐通量向上，预处理具有高生物生产力的海岸带的形成。潮汐能够在大陆架水柱上产生速度切变，从而在陆地坚冰覆盖的北极大陆架上引起垂直混合。然而，由于北极大陆架缺乏长期同时高分辨率的速度、温度和盐度剖面，冰下潮汐驱动的垂直混合从未被评估过。了解潮汐驱动的混合下面的陆地坚冰将是必不可少的陆地坚冰的形成和消融的预测。这对加拿大北极地区尤为重要，因为那里的北方人使用陆地坚冰作为运输系统，预计将进行广泛的石油和天然气勘探。推动这项研究的首要科学主题是：（一）陆地坚冰的稳定性，对当地交通的潜在影响;（二）了解北极大陆架陆地坚冰变化的相对热力学和动力学贡献，特别是在持续的气候变化方面。短期目标是（i）提供基于观测的评估潮汐驱动的垂直混合的效率和（ii）评估潮汐驱动的垂直通量以下的陆地固定冰在冬季。这项研究的长期科学目标是：（一）提高我们对垂直混合区域变化和北极大陆架固陆冰下剪切不稳定性机制的认识，与陆地快速冰盖建模和预测的影响，（ii）提供了剪切差异的半球尺度解释，驱动不稳定性和垂直混合沿着北极大陆架，主要集中在加拿大北极。短期科学目标如下：㈠在分布于北极不同大陆架的四个地点对陆地固定冰盖进行海洋学观测，并利用以前用类似观测思想获得的数据; ㈡查明造成垂直分层水柱速度切变的机制;（iii）揭示潮汐驱动的速度剪切在促进不稳定和垂直混合中的作用;（iii）估计与剪切驱动的不稳定相关的向上的盐、热和悬浮颗粒物（SPM）通量。观测计划包括~ 7个月的连续速度，温度，盐度和光学冰下剖面分布在不同的北极大陆架之间的四个陆地固定冰覆盖的位置。这些地区包括格陵兰岛东北部、哈德逊湾、加拿大群岛和西伯利亚北极。这些长期观测的补充，短期（~12-36小时）测量的极小尺度（1毫米）的盐度，温度和叶绿素浓度的波动，推断湍流动能的耗散水平和原位通量的热量和盐。每年，海洋学设备在这些地点之间重新分配。这种观测策略允许大规模的解释，在剪切驱动的不稳定性和垂直混合沿着北极大陆架的差异。一名研究助理，三名博士生和两名本科暑期学生参与了这项研究。

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.