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

这项研究涉及大气风和浮力通量对上层海洋强迫的理论、模拟和观测工作。PI和他的团队利用非线性理论，发现了一种在风力驱动的海洋中产生锋面的新机制。风应力和地面浮力通量的大气强迫通过直接冷却、上层Ekman层水平热量输送增强的冷却和垂直涡度修正的Ekman层体积输送的散度来驱动海洋。后两种强迫效应是非线性Ekman动力学的结果。利用对流参数化，预测了次表层海洋的二维亚惯性响应：它包括锋面的自发发展、这些锋面的下沉、与锋面相关的特殊地点的浮力对流以及由锋面下流高度结构的下风气流的自旋。位涡Q场起着关键的控制作用；Q符号的变化使自旋和不稳定性呈现出不同的形式。当具有恒定密度的表面比绝对动量表面倾斜得更陡峭时，Q会改变符号。非静力数值模拟验证了这些结果。本研究将在此基础上着重讨论海洋环流的三个方面：(I)在全三维海洋中由强风场产生的密度锋和速度锋；(Ii)海洋上层混合层内部和之下的环流的自旋；(Iii)大尺度风对上层海洋中反气旋环流和涡旋的放大和强迫传播。将同时使用三维建模和观测。模式研究涉及面向过程的模拟，目的是了解非线性Ekman动力学、风喷流优先生成反气旋、通过Ekman抽水引起的涡旋伸展传播风力涡旋以及类拉布拉多海盆中风力边界流的演变。另一个目标是使用该理论来指导静力海洋环流模型，以便它们能够捕捉到风驱动的锋生和锋面下流的动力学。将使用的特定观测数据集包括来自拉布拉多海和大西洋亚极锋面的高分辨率卫星散射计风场、来自日本/东海亚极锋面的高分辨率海面飙升和ADCP数据、以及使用埃里克森海滑仪拍摄的高分辨率CTD剖面。SEA-SOAR/ADCP调查现在是锋面/中尺度研究中常用的工具。这项工作开发了一种反方法，利用上述理论来估计非地转翻转环流，并将其驱动力划分为风、冷却和汇流/分流地转流的贡献。除了物理科学之外，这项工作还与生活在海岸线附近、沿海或赤道沿岸上升流地区的生态系统密切相关：这些地区特别受全球变化的影响。通过这种方式，它有助于理解地球的宜居性。这项研究也有助于PI的环境研究本科教学，将科学的方法和成果展示在年轻的学生面前，他们是环境的受益者和保管者。它还激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.