Oceanic Fronts, Subduction and Spin-up: Nonlinear Ekman Dynamics

本研究涉及与大气风和浮力通量对上层海洋强迫有关的理论、模拟和观测工作。PI和他的团队发现了一种新的机制，利用非线性理论在风驱动的海洋中产生锋面。大气强迫风应力和表面浮力通量驱动海洋通过直接冷却，冷却增强的水平热输送在上埃克曼层，和发散的埃克曼层体积输送修改垂直涡。后两种强迫效应是非线性Ekman动力学的结果。使用对流参数化，二维次惯性响应的地下海洋的预测：它涉及到自发发展的前线，在这些前线的下沉，浮力对流在特殊的网站相关的前线，和旋转的顺风电流的模式，是高度结构化的锋面下沉。位涡场q起着关键的控制作用; q符号的变化使自旋上升和不稳定性呈现出独特的形式。当等密度曲面比绝对动量曲面倾斜得更陡时，q改变符号。非静力学数值模拟验证了这些结果。本研究将从这一基础出发，并强调海洋环流的三个方面：（一）在全三维海洋中强风场产生的密度和速度锋;（二）海洋上部混合层内和之下环流的旋转;（三）大尺度风对反气旋环流和海洋上部涡旋的放大和强迫传播。将使用三维建模和观察。模型研究涉及面向过程的模拟，旨在了解在上下文中的非线性埃克曼动力学，优先生成的反气旋风射流，传播的风强迫涡流埃克曼泵引起的涡拉伸，和演变的风强迫边界电流在拉布拉多海一样的盆地。另一个目标是使用该理论指导海洋流体静力学环流模型，使它们能够捕捉风驱动的锋生和锋下流的动态。 将使用的具体观测数据集包括拉布拉多海和大西洋副极锋的高分辨率卫星散射计风场、日本/东海副极锋的高分辨率海测和ADCP数据以及Eriksen Seaglider拍摄的高分辨率ctd剖面。Sea-soar/ADCP测量现在是锋面/中尺度研究中常用的工具。建议的工作开发了一种逆方法，使用这种调查与上述理论来估计非地转翻转环流，并将其驱动力划分为风，冷却和汇合/分流地转流的贡献。除了物理科学之外，这项工作与生活在海洋前沿附近、海岸或沿着上升流区域的生态系统关系特别密切：这些地区受到全球变化的压力特别大。这有助于理解地球的可居住性。这项研究也有助于PI的环境研究本科教学，其中科学的方法和结果被放在年轻学生谁是环境的受益者和监护人。它还鼓励公众就环境问题发表演讲。

ABSTRACTOCE-0351191This study involves theory, modeling and observational work related to the forcing of theupper ocean by atmospheric winds and buoyancy flux. The PI and his team have found a new mechanism for the generation of fronts in a wind-driven ocean, using nonlinear theory. Atmospheric forcing by wind-stress and surface buoyancy flux drives the ocean through direct cooling, cooling augmented by horizontal heat transport in the upper Ekman layer, and divergence of the Ekman-layer volume transport modified by vertical vorticity. The latter two forcing effects are a consequence of nonlinear Ekman dynamics. Using a convection parameterization, the two-dimensional sub-inertial response of the subsurface ocean is predicted: it involves spontaneous development of fronts, downwelling at these fronts, buoyant convection at special sites related to the fronts, and the spin-up of downwind currents in a pattern that is highly structured by frontal downwelling. The potential vorticity field, q, acts as a key control; changes in the sign of q enable spin-up and instability to take on distinctive forms. q changes sign when surfaces of constant density slope more steeply than absolute momentum surfaces. Non-hydrostatic numerical simulations verify these results. The present study will start from this basis and emphasize three aspects of the ocean circulation: (i) generation of density- and velocity- fronts by strong wind fields in a fully three dimensional ocean; (ii) spin-up of circulation within and beneath the oceanic upper mixed layer, and (iii), amplification and forced propagation of anticyclonic circulation and eddies in the upper ocean by large-scale winds. Both 3- dimensional modeling and observations will be used. Model studies involve process oriented simulations aimed at understanding in the context of nonlinear Ekman dynamics, the preferential generation of anticyclones by wind-jets, propagation of wind-forced eddies by Ekman-pumping-induced vortex stretching, and evolution of wind-forced boundary currents in a Labrador Sea-like basin. Another goal is to use the theory to guide hydrostatic ocean circulation models so that they are able to capture the dynamics of wind-driven frontogenesis and frontal downwelling. Particular observational data-sets to be used include high-resolution satellite scatterometer wind-fields from Labrador Sea and subpolar fronts of the Atlantic, high-resolution sea-soar and ADCP data from the subpolar front of the Japan/East Sea, and high-resolution ctd sections taken with the Eriksen Seaglider. Sea-soar/ADCP surveys are now a commonplace tool used in frontal/mesoscale studies. The proposed work develops an inverse method to use such surveys with the theory above to both estimate ageostrophic overturning circulation and partition its driving force into contributions from wind, cooling, and confluent/diffluent geostrophic flow.Broader Impacts. Beyond physical science, this work relates particularly closely toecosystems living near ocean fronts, in upwelling regions at coasts or along the Equator: regions which are particularly stressed by global change. In this way it contributes to understanding of Earth's habitability. This research also contributes to the PI's undergraduate teaching in environmental studies, in which the methods and results of science are put before young students who are the beneficiaries and custodians of the environment. It also motivates public lecturing on the environment.