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

边界层浮力阻滞对跨大陆架交换的限制

• 批准号: 0849498
• 负责人: Kenneth Brink
• 金额: $ 52.87万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-03-01 至 2012-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0849498&HistoricalAwards=false
• 关键词: Constraints; Cross; shelf; Exchange; Imposed

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.

沿海海洋学的一个经典问题是，大陆架上的水流如何对离岸更远的条件作出反应。考虑这个问题的最常见的动力学框架是这样一种想法，即海底埃克曼输送允许放松对跨等深线流的地转（泰勒-普劳德曼）禁令。由于我们对海底边界层有了更好的了解，因此需要重新审视这一范例，在具有密度分层、海底摩擦和倾斜海底的海洋中，跨等深线浮力输运阻止了海底速度，因此，沿等深线的定常水流不会经历沿等深线的海底摩擦。因此，只要在罗斯比数意义上流动没有大的惯性，陆架上的定常流一旦调整，就必须精确地遵循等深线。跨大陆架尺度由旋转、底坡、分层和流量（但不包括底摩擦）确定。这一结果很大程度上带来了问题（在分层条件下）持久的“扩散”模型，该模型表明，稳定流可以跨越等深线，因为底部摩擦的缓解效果。有人认为，如果流到大陆架从近海，目前将是相对较宽的，因为它流向左边（面向岸上，在北方半球）。然而，如果在陆架边缘，水流被吸引到近海，则当它从面向陆地的左侧流入时，相关的陆架流的宽度将小得多。在这种情况下，底部埃克曼运输是上坡。初步的数值计算结果验证了这些结论与目前的宽度和边界层属性，至少定性。该项目的目标是解决如何与现实的代表性的海底边界层逮捕行为方面的大陆架和公海之间的耦合海洋。在这个更广泛的问题中，问题将被分解，以解决1）岸上流，2）离岸流，3）惯性效应，和4）浮力制动物理学的可能性，结合区域岸上流，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，该项目将对跨大陆架交换是一个重要问题的广泛海洋科学产生影响。例如，该项目的一个具体成果将是量化洋流可以渗透到大陆架上的距离（以及大陆架上的水可以被抽到近海的不同尺度）。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。