Constraints on Cross-shelf Exchange Imposed by Boundary Layer Buoyancy Arrest

海岸海洋学中的一个经典问题是陆架上的水流如何对离岸更远的情况作出反应。考虑这个问题的最常见的动力学框架是，底部埃克曼输运允许地转（泰勒-普劳德曼）禁止跨等深线流动的想法被放松。由于我们对底部边界层的理解有所提高，这种范式需要重新审视。在具有密度分层、底部摩擦和倾斜底部的海洋中，跨等深线浮力运输阻止了底部速度，因此稳定的沿等深线流动不会经历沿等深线底部摩擦。因此，经过大陆架的稳定气流一旦调整，就必须精确地遵循等深线，前提是气流在罗斯比数意义上没有大的惯性。跨架尺度由旋转、底部坡度、分层和流量（而不是底部摩擦）决定。这一结果在很大程度上对（在分层条件下）持久的“扩散”模型提出了质疑，该模型表明，由于底部摩擦的缓和作用，稳定流动可以穿过等深线。有人认为，如果气流从海上进入大陆架，气流将相对较宽，因为它流向左边（面向北半球的陆地）。然而，如果在陆架边缘，水流被拉向海上，那么相应的陆架电流的宽度就会小得多，因为它从左边流入，面朝陆地。在这种情况下，底部Ekman输运是上坡的。初步的数值结果至少在定性上证实了这些关于电流宽度和边界层性质的结论。这个项目的目标是解决一个具有底部边界层停滞的现实表现的海洋在大陆架和开放海洋之间的耦合方面是如何表现的。在这个更广泛的问题中，这个问题将被分解，以解决大陆架斜坡系统的问题：1)陆上流动，2)海上流动，3)惯性效应，以及4)浮力抑制物理的可能性，结合区域陆上流动，可以解释加利福尼亚暗流等潜流系统。这项工作的广泛影响是双重的。首先，该项目将对广泛的海洋科学产生影响，其中跨大陆架交换是一个重要问题。例如，这个项目的一个具体结果将是量化海洋水流可以穿透到大陆架上的距离（以及大陆架上的水可以被抽到离岸的相当不同的尺度）。第二，该项目将通过促进博士研究生的教育，为海洋科学界的健康做出贡献。

One of the classic problems in coastal oceanography is the question of how flow on the shelf responds to conditions farther offshore. The most common dynamical framework for thinking about this issue has been the idea that bottom Ekman transport allows the geostrophic (Taylor-Proudman) prohibition of cross-isobath flow to be relaxed. This paradigm needs to be revisited because of our improved understanding of bottom boundary layers.In an ocean with density stratification, bottom friction and a sloping bottom, cross-isobath buoyancy transport arrests the bottom velocity, and so steady along-isobath flow experiences no along-isobath bottom friction. Consequently, steady flow over the shelf, once it adjusts, has to follow isobaths exactly, provided the flow does not have large inertia in the Rossby number sense. Cross-shelf scales are set by rotation, bottom slope, stratification and flow volume (but not bottom friction). This result very much brings into question (during stratified conditions) the enduring 'diffusive' model that shows that steady flows can cross isobaths because of the mitigating effects of bottom friction.It is argued that if flow goes onto the shelf from offshore, the current will be relatively wide as it flows away toward the left (facing onshore, in the Northern Hemisphere). However, if, at the shelf edge, flow is drawn offshore, the width of the associated shelf current will be much less as it flows in from the left as one faces onshore. In this case, bottom Ekman transport is up-slope. Preliminary numerical results verify these conclusions with regard to current width and boundary layer properties, at least qualitatively.The goal of this project is to address how an ocean with a realistic representation of bottom boundary layer arrest behaves with regard to coupling between the shelf and the open ocean. Within this broader question, the problem will be broken up to address the shelf-slope system for 1) onshore flows, 2) offshore flows, 3) inertial effects, and 4) the possibility that buoyancy arrest physics, in combination with regional onshore flow, can account for undercurrent systems such as the California Undercurrent.The broader impact of this work is two-fold. First, the project will have impact over the wide range of ocean sciences where cross-shelf exchange is an important issue. For example, one specific result of this project will be to quantify the distance onto the shelf that an oceanic flow can penetrate (and, the rather different scale over which water from over the shelf can be drawn offshore). Second, this project will contribute to the health of the ocean science community by contributing to the education of a PhD graduate student.