Collaborative Research: Dynamics of the Orkney Passage Outflow

合作研究：奥克尼群岛航道流出的动力学

• 批准号: 1536779
• 负责人: Kurt Polzin
• 金额: $ 78.6万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-10-01 至 2020-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1536779&HistoricalAwards=false
• 关键词: Collaborative; Research; Dynamics; Orkney; Passage

Cold and dense water masses are formed through air-sea interaction near the Antarctic and are funneled through narrow passages as they flow downward into the deep ocean basins. The intense mixing that occurs along the way in these passages sets the properties of the Antarctic Bottom Water, which spreads out to fill the deepest layers over much of the global ocean. One of the most remarkable features of contemporary oceanic climate change is the warming and contraction of Antarctic Bottom Water. This study will make new field measurements and computer simulations to address questions of how long-term variability in the dense waters formed near the Antarctic is translated into downstream variability elsewhere, such as the deep basins of the Atlantic Ocean, and whether the observed warming trends result from diminishing Antarctic contributions to the Meridional Overturning Circulation or from increased mixing. Orkney Passage is a key circulation choke point that governs ocean exchanges between the marginal seas of the Antarctic Continent and the Southern Ocean: approximately 5 Sverdrups (Sv) of newly ventilated Antarctic Bottom Water are funneled through this narrow passage into the Scotia Sea (Naveira Garabato et al., 2002) which represents a significant contribution to the total 15 Sv of Antarctic Bottom Water estimated to pass equatorward of the Antarctic Circumpolar Current's southern boundary (Naveira Garabato et al., 2014). Existing data (LADCP and CTD) reveal the presence of thick bottom boundary layers, 500 meters in vertical extent, with intense thermal wind shear above. Flows within the passage may also exhibit intense horizontal velocity gradients and be hydraulically controlled upstream and/or downstream of these observations: in the highly energetic and variable environment above the bottom boundary layer, overturns exceeding 100 meters extent have been observed. A control volume budget suggests that high levels of mixing must continue downstream of Orkney Passage: the absence of observed mixing in the Scotia Sea interior suggests this enhanced mixing must be located along a boundary. This study will test the hypothesis that enhanced mixing in the downstream boundary current results from overturning generated by tidally-driven cross-slope shear in the Ekman boundary layer. This parameter regime, however, is poorly understood from a theoretical standpoint owing in part to a paucity of direct sampling of flows and diapycnal processes in situations such as this. The dynamics that set turbulent mixing and transports within Orkney Passage and in the boundary current downstream will be investigated using a combination of numerical modeling and field measurements. The fieldwork will complement observations of the diabatic and frictional processes in Orkney Passage being carried out by collaborators in the UK, by employing a mooring downstream of Orkney Passage. Regional numerical simulations using the MITgcm model will be used in planning and interpreting the field measurements, and importantly, in improving methods to accurately represent flows through narrow passages in climate models such as the GFDL MOM6. Ultimately, the observations and simulations will be used to address questions of how long-term variability in the upstream Weddell Sea Deep and Bottom Water properties is translated into downstream variability in the Scotia Sea: whether warming trends of Antarctic Bottom Water in the Scotia Sea and Atlantic Ocean result from diminishing Antarctic contributions to the Atlantic Meridional Overturning Circulation [e.g. Johnson et al. (2008)] or increased diabatic mixing associated with a strengthened Weddell Gyre (Meredith et al., 2011). This study will significantly improve the understanding of continuously stratified, rotating flow dynamics in a sparsely sampled parameter regime with horizontal velocities having strong horizontal and vertical gradients [Rossby number, Froude number ∼ O(1)] and be applicable to many choke points of global deep ocean circulation. The observations will be applied to improve the GFDL ocean general circulation model, among the best of the coupled models contributing to IPCC future climate projections. The project will promote teaching and training, by including 3 undergraduate interns (2 at Princeton, 1 at WHOI), with a particular effort made to recruit under-represented minority students.

寒冷而稠密的水团是通过南极附近的海气相互作用形成的，并通过狭窄的通道向下流入深海盆地。这些通道沿途发生的强烈混合决定了南极底层水的特性，这些底层水扩散到全球大部分海洋的最深层。 当代海洋气候变化最显着的特征之一是南极底层水的变暖和收缩。这项研究将进行新的现场测量和计算机模拟，以解决南极附近形成的稠密水域的长期变化如何转化为其他地方（例如大西洋深盆地）下游变化的问题，以及观察到的变暖趋势是否这是由于南极洲对经向翻转环流的贡献减少或混合增加造成的。 奥克尼海峡是控制南极大陆边缘海和南大洋之间海洋交换的关键环流阻塞点：大约 5 Sverdrups (Sv) 新通风的南极底水通过这条狭窄的通道流入斯科舍海 (Naveira Garabato)等人，2002），这对预计流经赤道方向的南极底水总量 15 Sv 做出了重大贡献。南极绕极流的南部边界（Naveira Garabato 等，2014）。现有数据（LADCP 和 CTD）显示存在厚厚的底部边界层，垂直范围为 500 米，上方有强烈的热风切变。通道内的流动也可能表现出强烈的水平速度梯度，并在这些观测的上游和/或下游受到液压控制：在底部边界层上方的高能和多变环境中，已观测到超过 100 米范围的翻转。控制体积预算表明，奥克尼海峡下游必须继续进行高水平的混合：​​在斯科舍海内部没有观察到混合，表明这种增强的混合必须沿着边界进行。这项研究将检验以下假设：下游边界流的混合增强是由潮汐驱动的埃克曼边界层横坡剪切产生的翻转造成的。然而，从理论角度来看，人们对这种参数体系知之甚少，部分原因是在这种情况下缺乏对流动和二流过程的直接采样。将结合数值模拟和现场测量来研究奥克尼海峡内和下游边界流中的湍流混合和传输的动力学。现场工作将补充英国合作者通过在奥克尼海峡下游使用系泊装置对奥克尼海峡的非绝热和摩擦过程进行的观察。使用 MITgcm 模型的区域数值模拟将用于规划和解释现场测量，更重要的是，用于改进准确表示气候模型（例如 GFDL MOM6）中狭窄通道的流量的方法。最终，观测和模拟将用于解决上游威德尔海深水和底层水特性的长期变化如何转化为斯科舍海下游变化的问题：斯科舍海南极底层水的变暖趋势是否与大西洋是由于南极洲对大西洋经向翻转环流的贡献减少而产生的[例如约翰逊等人。 (2008)]或与加强的威德尔环流相关的非绝热混合的增加(Meredith et al., 2011)。这项研究将显着提高对稀疏采样参数范围内连续分层、旋转流动动力学的理解，其中水平速度具有较强的水平和垂直梯度[罗斯比数、弗劳德数∼ O(1)]，适用于全球深海环流的多个阻塞点。这些观测结果将用于改进 GFDL 海洋环流模型，这是有助于 IPCC 未来气候预测的最佳耦合模型之一。该项目将通过包括 3 名本科生实习生（普林斯顿大学 2 名，WHOI 1 名）来促进教学和培训，并特别努力招收代表性不足的少数族裔学生。