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

摘要：本研究涉及大气风和浮力通量对上层海洋的强迫作用的理论、模拟和观测工作。PI和他的团队利用非线性理论发现了风驱动海洋中锋面形成的新机制。风应力和表面浮力通量的大气强迫通过直接冷却驱动海洋，上层Ekman层的水平热输送增强了冷却，垂直涡度改变了Ekman层体积输送的散度。后两种强迫效应是非线性埃克曼动力学的结果。利用对流参数化，预测了地下海洋的二维亚惯性响应：它包括锋面的自发发展、锋面的下移、锋面相关特殊位置的浮力对流，以及由锋面下移高度结构化的下风向流的自旋上升。位涡场q作为关键控制；q符号的变化使自旋向上和不稳定性呈现出不同的形式。当等密度曲面比绝对动量曲面斜率更陡时，Q的符号发生变化。非流体静力数值模拟验证了这些结果。本研究将从这个基础出发，强调海洋环流的三个方面：(i)在完全三维的海洋中由强风场产生的密度和速度锋；（ii）海洋上层混合层内部和下方环流的自旋上升，以及（iii）大尺度风对上层海洋的反气旋环流和涡流的放大和强迫传播。三维建模和观测都将被使用。模型研究包括以过程为导向的模拟，旨在理解非线性埃克曼动力学的背景，风射流对反气旋的优先产生，埃克曼泵诱导的涡旋拉伸对风强迫涡流的传播，以及拉布拉多海样盆地中风强迫边界流的演变。另一个目标是利用该理论来指导流体静力学海洋环流模型，以便能够捕捉风驱动的锋面形成和锋面下移的动力学。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的高分辨率卫星散射计风场，来自日本/东海亚极锋的高分辨率海跃和ADCP数据，以及由埃里克森滑翔机拍摄的高分辨率ctd部分。海平面上升/ADCP测量现在是锋面/中尺度研究中常用的工具。提出的工作开发了一种逆方法，将这些测量与上述理论结合起来，既可以估计地转翻转环流，又可以将其驱动力划分为风、冷却和合流/分流地转流的贡献。更广泛的影响。除了物理科学之外，这项工作还与生活在海洋前沿、海岸上升流地区或赤道附近的生态系统密切相关，这些地区特别受到全球变化的压力。通过这种方式，它有助于了解地球的可居住性。这项研究也有助于PI在环境研究方面的本科教学，其中科学的方法和结果摆在年轻学生面前，他们是环境的受益者和监护人。它还激发了有关环境的公开讲座。

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.