Structure of cross-shelf circulation in a buoyancy-influenced, wind-driven Eastern Boundary Current system

受浮力影响、风驱动的东部边界流系统中跨陆架环流的结构

• 批准号: 1332753
• 负责人: Barbara Hickey
• 金额: $ 63.71万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1332753&HistoricalAwards=false
• 关键词: Structure; cross; shelf; circulation; buoyancy

The overarching goal of this project is to understand the patterns and driving mechanisms of subtidal cross-isobath (inshore/offshore) circulation in a wind-forced Eastern Boundary Current (EBC) system such as the California Current System (CSS) where buoyancy sources affect stratification and the mean circulation is seasonally-dependent. Specifically, project objectives are:1) Determine the response of cross-isobath circulation over the mid-shelf to local and remote forcing during the upwelling season as a function of stratification,2) Determine the cross-margin extent of the identified cross-isobath circulation patterns,3) Determine how the cross-isobath response changes seasonally.The objectives will be met through application of a newly developed method of isolating the cross-isobath circulation to 1) analysis of an existing and untapped set of observations and 2) analysis of upcoming NSF-funded Ocean Observatory Initiative (OOI) Endurance Array observations that will span the continental margin year-round at two separate along-shelf locations in the northern CCS.Cross-margin exchange remains one of the least understood processes in EBC systems. Although progress has been made on understanding near-surface dynamics and transports, the origin, structure, and timing of the deeper compensating return flows have proven elusive. Much of our lack of understanding stems from the fact that cross-isobath flows are difficult to accurately isolate: 1) cross-isobath flows are generally much weaker than coincident along-isobath flows, and 2) water column velocity measurements often do not span the crucial surface and bottom boundary layers. Analyses often hinge on removal of the instantaneous depth-averaged cross-isobath velocity in order to relate the residual structure and its temporal variability to external forcing. This approach avoids the otherwise unbalanced depth-averaged along-shelf momentum that results from a slight veering-over of along-isobath currents, as occurs when a meander or eddy passes by a measurement location. However, this methodology can leave significant biases in the vertical structure of both the mean and fluctuating cross-isobath velocity profiles if the along-isobath flow is vertically sheared, as is often the case in EBC systems such as off the western US, Peru and Chile, and parts of Africa. A newly-developed technique that eliminates these biases and has shown promise at a mid-shelf location in the northern CCS will be employed to isolate the cross-isobath circulation in existing and upcoming observational data sets. Except during spring the seasonal mean surface pressure gradient opposes the along-isobath flow in the northern CCS. This offers a dynamical counter-example to other shelf regions such as those off the northeast US where flows are down the pressure gradient.Cross-margin exchange regulates water property distributions (heat, salt, and oxygen), nutrient availability, larval recruitment of invertebrates and fish, and pollutant and sediment dispersal, including export to the open ocean. Because of these wide-ranging and important applications and because understanding of cross-isobath circulation has historically been so elusive, the outcomes of this project will themselves be of broad impact. The project will provide new methodology and insights that may be used and tested in other coastal settings with regional current jets and/or significant eddy fields. The project will provide educational activities for a Postdoctoral Fellow/early career scientist, graduate student teaching, local resource managers, and an undergraduate student. In particular, results will be incorporated into a graduate level class at the UW that attracts biological and fisheries oceanography students with interdisciplinary interests in along- and cross-shelf motion. Regional partners, e.g., from WA Dept. of Health and WA Dept. of Fish & Wildlife, will be trained on the use of OOI data streams and products as tools for coastal resource management. Last, an undergraduate student will gain experience with the use and application of cutting-edge OOI datasets, with the goal of presenting their results at the annual UW Undergraduate Research Symposium.

该项目的首要目标是了解风驱动的东部边界流（EBC）系统（如加州流系统（CSS））中潮下带跨等深线（近岸/离岸）环流的模式和驱动机制，其中浮力源影响分层，平均环流随季节变化。具体而言，项目目标是：1）确定在上升流季节期间作为层化函数的陆架中部跨等深线环流对本地和远程强迫的响应，2）确定已确定的跨等深线环流型的跨边缘范围，3）确定跨等深线响应如何随季节变化，将通过应用新开发的隔离跨等深线环流的方法来实现这些目标：（1）分析现有的和未开发的一组观测数据，（2）分析即将由国家科学基金会资助的海洋观测倡议耐久性阵列观测数据，该阵列将全年在两个独立的沿着跨越大陆边，大陆架位置在北方CCS。跨边际交换仍然是EBC系统中最不了解的过程之一。虽然在了解近地表动力学和输运方面取得了进展，但深层补偿回流的起源、结构和时间却难以确定。我们之所以缺乏了解，主要是因为很难准确地分离出跨等深线流：1）跨等深线流一般比沿等深线流弱得多，2）水柱速度测量往往不跨越关键的表面和底部边界层。分析往往取决于瞬时深度平均横等深线速度的去除，以便将剩余结构及其随时间变化与外部强迫联系起来。这一方法避免了沿等深线流轻微转向造成的沿陆架深度平均动量不平衡，如当曲流或涡流经过测量地点时发生的情况。但是，如果沿等深线的气流受到垂直剪切，这种方法可能会在平均和波动的跨等深线速度剖面的垂直结构中留下重大偏差，美国西部、秘鲁和智利以及非洲部分地区的EBC系统经常出现这种情况。将采用一种新开发的技术，消除这些偏差，并在北方CCS的中间大陆架位置显示出希望，以隔离现有和即将到来的观测数据集中的跨等深线环流。除春季外，在北方CCS，季节平均表面气压梯度与沿等深线流动相反。这提供了一个动态的反例，以其他大陆架地区，如美国东北部的流动是向下的压力gradient.Cross-margin交换调节水的属性分布（热，盐，氧），营养物质的可用性，幼虫招聘的无脊椎动物和鱼类，污染物和沉积物的扩散，包括出口到公海。由于这些广泛和重要的应用，并由于对跨等深线环流的理解历来如此难以捉摸，这个项目的成果本身将产生广泛的影响。该项目将提供新的方法和见解，可用于和测试在其他沿海设置与区域电流射流和/或显着的涡流场。该项目将为博士后研究员/早期职业科学家，研究生教学，当地资源管理人员和本科生提供教育活动。特别是，结果将被纳入一个研究生班在华盛顿大学，吸引生物和渔业海洋学的学生与跨学科的兴趣，在沿着和跨货架运动。区域伙伴，例如，关于WA Dept卫生和西澳大利亚部将对海洋和野生动物管理局的一名官员进行培训，使其了解如何使用OOI数据流和产品作为沿海资源管理的工具。最后，本科生将获得使用和应用尖端OOI数据集的经验，目的是在年度UW本科生研究研讨会上展示他们的成果。