Collaborative Research: Dynamics of the Orkney Passage Outflow

南极附近的冷水和稠密水团是通过海气相互作用形成的，向下流入深海盆地时，通过狭窄的通道形成漏斗状。在这些航道沿途发生的强烈混合决定了南极底层水的性质，它向外扩散，填满了全球大部分海洋的最深层。当代海洋气候变化最显著的特征之一是南极底层水变暖和收缩。这项研究将进行新的现场测量和计算机模拟，以解决以下问题：南极附近形成的稠密水域的长期可变性如何转化为其他地方(如大西洋深盆地)的下游可变性，以及所观测到的变暖趋势是由于南极对子午线翻转环流的贡献减少还是由于混合增加而产生的。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一个关键的环流阻塞点：大约5个新通风的南极底层水(Sv)通过这个狭窄的通道流入斯科舍海(Naveira Garabato等人，2002年)，这对估计向南极绕极洋流的南部边界赤道方向流过的南极底层水总量(Naveira Garabato等人，2014年)做出了重大贡献。现有资料(LADCP和CTD)显示，存在垂直范围500米厚的底部边界层，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/或下游受到水力控制：在底部边界层以上的高能量和多变的环境中，观察到了超过100米范围的倾覆。控制量预算表明，奥克尼航道下游必须继续进行高水平的混合：在斯科舍海内陆没有观察到混合，这表明增强的混合必须位于边界上。这项研究将检验这一假设，即下游边界流中的增强混合是由Ekman边界层中潮汐驱动的横坡切变所产生的倾覆造成的。然而，从理论角度对这一参数机制的理解很少，部分原因是在这样的情况下，缺乏对气流和昼夜过程的直接采样。用数值模拟和现场测量相结合的方法来研究在奥克尼航道和下游边界流中设置湍流混合和输送的动力学。现场工作将通过在奥克尼航道下游使用系泊设备，补充英国合作者正在进行的奥克尼航道非绝热和摩擦过程的观测。使用MITgcm模型的区域数值模拟将用于规划和解释现场测量，重要的是，将用于改进方法，以准确地表示通过GFDL MOM6等气候模型中的狭窄通道的流动。最终，观测和模拟将被用来解决以下问题：上游韦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的变化：斯科舍海和大西洋南极底层水变暖的趋势是否源于南极对大西洋经向翻转环流的贡献减少[例如Johnson等人。(2008)]或与加强的威德尔气旋相关的非绝热混合增加(Meredith等人，2011年)。这项研究将极大地提高对水平速度具有较强水平和垂直梯度[Rossby数，Froude数和O(1)]的稀疏采样参数区域中连续分层旋转流动动力学的理解，并适用于全球深海环流的许多阻塞点。这些观测将用于改进GFDL海洋环流模型，这是对气专委未来气候预测做出贡献的最佳耦合模型之一。该项目将促进教学和培训，包括3名本科生实习生(普林斯顿大学2名，世界卫生组织1名)，并特别努力招收代表性不足的少数族裔学生。

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.