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

沿海海洋学中的一个经典问题是，大陆架上的水流如何对更远的近海条件做出反应。考虑这一问题的最常见的动力学框架是底埃克曼输运允许放松地转(泰勒-普罗德曼)对跨等深流的禁止。由于我们对海底边界层有更好的理解，这一范式需要重新审视。在密度分层、海底摩擦和倾斜海底的海洋中，交叉等深浮力输运阻止了海底速度，因此稳定的沿等深流动不会经历沿等深底流的底部摩擦。因此，货架上的稳定流动，一旦调整，就必须严格遵循等深线，前提是流动在罗斯比数意义上没有很大的惯性。跨陆架比例由旋转、底部坡度、层化和流量(而不是底部摩擦)设定。这一结果极大地质疑了(在分层条件下)持久的“扩散”模型，该模型表明，由于底部摩擦的缓解作用，稳定的水流可以穿过等深线。有人认为，如果水流从近海进入大陆架，则流向左侧(面向岸上，在北半球)的水流将相对较宽。然而，如果在陆架边缘，水流被吸引到近海，那么相关的陆架洋流的宽度将会小得多，因为它是从左侧流入的，因为一个人面朝岸边。在这种情况下，底部的埃克曼输运是上坡的。初步的数值结果从海流宽度和边界层性质方面验证了这些结论，至少定性地验证了这些结论。本项目的目标是研究具有真实底部边界层滞留的海洋在大陆架和开阔海洋之间的耦合方面的行为。在这个更广泛的问题中，这个问题将被分解为解决陆架-斜坡系统的1)陆上流动，2)离岸流动，3)惯性效应，以及4)浮力阻挡物理与区域岸上流动相结合的可能性，可以解释暗流系统，如加州暗流。这项工作的更广泛的影响是双重的。首先，该项目将对海洋科学的广泛领域产生影响，在这些领域，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(以及从大陆架上可以将水引向近海的完全不同的规模)。其次，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.