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。由于消除了水柱的风应力，它能够使底层通风，否则在冰雪覆盖期间可能会变得停滞不前。此外，垂直混合有利于从底层向表层向上的养分通量，为高生物生产力海岸带的形成提供了先决条件。潮汐能够在大陆架水柱上产生速度切变，从而在陆地上被冰覆盖的北极大陆架上引起垂直混合。然而，由于在北极大陆架上缺乏长期同时高分辨率的速度、温度和盐度剖面，因此从未对冰下潮汐驱动的垂直混合进行过评估。了解禁陆冰下潮汐驱动的混合对预测禁陆冰的形成和消融至关重要。这对加拿大的北极地区来说尤其重要，因为那里的北方人使用陆地固定冰作为运输系统，预计会有大量的石油和天然气勘探。推动本研究的首要科学主题是：(i)陆固冰的稳定性及其对当地运输的潜在影响；（ii）了解北极陆架上陆固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)对潮汐驱动的垂直混合的效率进行基于观测的评估；（ii）评估冬季陆冰以下潮汐驱动的垂直通量。本研究的长期科学目标是：(i)提高我们对垂直混合的区域变化和北极大陆架上陆速冰下剪切不稳定机制的理解，这对陆速冰的建模和预测具有重要意义；（ii）对沿北极大陆架剪切驱动的不稳定和垂直混合的差异提供半球尺度的解释，主要集中在加拿大北极。短期科学目标如下：(i)在分布于不同北极大陆架之间的四个地点从陆地冰盖进行海洋学观测，并利用以前使用类似观测思想获得的数据；（ii）确定在垂直分层水柱上产生速度切变的机制；（iii）揭示潮汐驱动的速度切变在促进不稳定和垂直混合中的作用；（三）估计与剪切驱动的不稳定性有关的向上的盐、热和悬浮颗粒物（SPM）通量。该观测项目包括在分布在不同的北极大陆架之间的四个陆地冰覆盖地点进行约7个月的连续速度、温度、盐度和冰下光学剖面分析。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地区。这些长期观测由短期（~12-36 h）测量盐度、温度和叶绿素浓度的极小尺度（约1毫米）波动来补充，以推断湍流动能耗散和原位热盐通量的水平。每年，海洋设备在这些地点之间重新分配。这种观测策略可以大规模地解释剪切驱动的不稳定性和沿北极大陆架垂直混合的差异。本研究共涉及1名副研究员、3名博士生和2名本科生暑期生。

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.