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: 1536453
• 负责人: Sonya Legg
• 金额: $ 8.42万
• 依托单位: Princeton University
• 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 2018-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1536453&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 strengthenedWeddell 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，当它们向下流入深海盆地时，通过狭窄的通道形成漏斗。这些通道中沿着发生的强烈混合决定了南极底层水的性质，这些底层水扩散到全球大部分海洋的最深层。 当代海洋气候变化的最显著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实地测量和计算机模拟，以解决以下问题：南极附近密集沃茨的长期变化如何转化为其他地方的下游变化，如大西洋的深盆地，以及观察到的变暖趋势是由于南极对经向翻转环流的贡献减少还是由于混合增加。 奥克尼通道是控制南极大陆边缘海和南大洋之间的海洋交换的关键环流瓶颈：大约5个Sverdrups（Sv）新通风的南极底层水通过这个狭窄的通道进入斯科舍海（Naveira Garabato等人，2002年），这是一个显着的贡献，总15西弗南极底层水估计通过赤道的南极绕极流？s南部边界（Naveira Garabato等人，2014年）。现有的资料（LADCP和CTD）显示，存在厚的底部边界层，垂直范围为500米，上面有强烈的热风切变。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/或下游受到水力控制：在底部边界层上方的高能量和可变环境中，已经观察到超过100米的翻转。控制量预算表明，高水平的混合必须继续下游的奥克尼通道：观察到的混合在斯科舍海内部的情况表明，这种增强的混合必须位于沿着边界。这项研究将测试的假设，即增强混合的下游边界流的结果，由潮汐驱动的跨坡剪切在埃克曼边界层中产生的翻转。然而，这个参数制度，从理论的角度来看，部分原因是缺乏直接采样的流量和diapycnal过程的情况下，如这是知之甚少。将采用数值模拟和现场测量相结合的方法研究奥克尼通道内和边界流下游的湍流混合和输送动力学。实地考察将补充观测的非绝热和摩擦过程中奥克尼通道正在进行的合作者在英国，采用系泊下游的奥克尼通道。使用MITgcm模式的区域数值模拟将用于规划和解释实地测量，重要的是，改进方法，以准确地表示通过GFDL MOM 6等气候模型中狭窄通道的流量。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：斯科舍海和大西洋南极底层水的变暖趋势是否是由于南极对大西洋经向翻转环流的贡献减少所致[例如，约翰逊等人（2008年）]或与强化威德尔环流相关的非绝热混合增加（Meredith等人，2011年）。这项研究将大大提高连续分层，旋转流动动力学在一个稀疏采样的参数制度，水平速度具有强大的水平和垂直梯度[罗斯比数，弗劳德数#8764; O（1）]的理解，并适用于全球深海环流的许多瓶颈点。这些观测结果将用于改进GFDL海洋环流模式，该模式是有助于IPCC未来气候预测的最佳耦合模式之一。该项目将促进教学和培训，包括3名本科生实习生（2名在普林斯顿大学，1名在世界卫生组织），并特别努力招聘代表性不足的少数民族学生。