Collaborative Research: Dynamics of the Orkney Passage Outflow

寒冷而稠密的水团是在南极附近的海气相互作用中形成的，当它们向下流入深海盆地时，会通过狭窄的通道形成漏斗状。在这些通道中发生的强烈混合形成了南极底水的特性，这些底水扩散到全球大部分海洋的最深处。当代海洋气候变化最显著的特征之一是南极底水的变暖和收缩。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：南极附近形成的稠密水域的长期变化如何转化为其他地方（如大西洋深盆地）的下游变化，以及观测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 Sv （Sv）的新通气的南极底水通过这条狭窄的通道进入斯科舍海（Naveira Garabato et al., 2002），这对估计通过南极环极流南边界的南极底水总量15 Sv （Naveira Garabato et al., 2014）的贡献很大。现有资料（LADCP和CTD）显示存在厚底边界层，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，并在这些观测值的上游和/或下游受到水力控制：在底部边界层以上的高能量和可变环境中，已观察到超过100米范围的倾覆。控制体积预算表明，高水平的混合必须在奥克尼通道下游继续进行；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位于边界上。本研究将验证下游边界流混合增强的假设是由潮汐驱动的Ekman边界层跨坡剪切产生的倾覆引起的。然而，从理论的角度来看，这种参数制度的理解很差，部分原因是在这种情况下缺乏对流动和周期过程的直接采样。将采用数值模拟和现场测量相结合的方法，对奥克尼海峡和下游边界流中湍流混合和输送的动力学进行研究。通过在奥克尼通道下游使用系泊装置，实地考察将补充英国合作者对奥克尼通道绝热和摩擦过程的观察。使用MITgcm模式的区域数值模拟将用于规划和解释现场测量，重要的是，用于改进气候模式（如GFDL MOM6）中精确表示狭窄通道流量的方法。最终，观测和模拟将用于解决上游威德尔海深水和底水特性的长期变化如何转化为斯科舍海下游的变化的问题。斯科舍海和大西洋的南极底水变暖趋势是由于南极对大西洋经向翻转环流的贡献减少[例如Johnson等人（2008）]，还是与威德尔环流增强相关的绝热混合增加（Meredith等人，2011）。这项研究将显著提高对稀疏采样参数体系中水平速度具有强水平和垂直梯度的连续分层旋转流动动力学的理解[rosby数，Froude数&amp；#8764;]O(1)]，适用于全球深海环流的许多阻塞点。这些观测结果将用于改进GFDL海洋环流模式，该模式是为IPCC未来气候预测做出贡献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WHOI），并特别努力招收代表性不足的少数民族学生。

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.