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

该项目的总体目标是了解风驱动的东部边界流（EBC）系统（如加利福尼亚流系统（CSS））中潮下跨等深线（近海/近海）环流的模式和驱动机制，其中浮力源影响分层，平均环流依赖于季节。具体而言，项目目标是：1)确定在上升流季节，作为分层函数的中部大陆架跨等深线环流对局地和远地强迫的响应；2)确定已确定的跨等深线环流模式的跨边缘程度；3)确定跨等深线响应的季节性变化。这些目标将通过应用一种新开发的分离跨等深线环流的方法来实现：1)分析现有的和未开发的一组观测数据；2)分析即将到来的美国国家科学基金会资助的海洋观测倡议（OOI）耐力阵列观测数据，该观测数据将全年跨越大陆边缘，在CCS北部的两个独立的沿大陆架位置。交叉保证金交易仍然是EBC系统中最不容易理解的过程之一。尽管在了解近地表动力学和输送方面取得了进展，但事实证明，深层补偿回流的起源、结构和时间仍是难以捉摸的。我们之所以缺乏理解，很大程度上是因为很难准确地分离出跨等深线流动：1)跨等深线流动通常比沿等深线流动弱得多；2)水柱速度测量通常不会跨越关键的地表和底部边界层。为了将残余结构及其时间变率与外部强迫联系起来，分析常常依赖于去除瞬时深度平均横断等深线速度。这种方法避免了由于沿等深线流的轻微倾斜而导致的沿大陆架平均深度动量的不平衡，当曲流或涡流经过测量位置时就会发生这种情况。然而，如果沿等深线流动垂直剪切，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明显的偏差，这在美国西部、秘鲁和智利以及非洲部分地区的EBC系统中经常出现。一项新开发的技术消除了这些偏差，并在CCS北部的中大陆架位置显示出希望，将用于在现有和即将到来的观测数据集中隔离跨等深线环流。除春季外，季节平均地面气压梯度与北CCS沿等深线流动方向相反。这为其他大陆架区域提供了一个动态的反例，比如美国东北部，那里的气流沿着压力梯度向下。跨边界交换调节着水的性质分布（热、盐和氧）、营养物质的可得性、无脊椎动物和鱼类的幼虫吸收以及污染物和沉积物的扩散，包括向公海的出口。由于这些广泛而重要的应用，以及对交叉等深线环流的理解一直是如此难以捉摸，因此该项目的成果本身将具有广泛的影响。该项目将提供新的方法和见解，可用于其他沿海地区的区域流射流和/或重要的涡流场。该项目将为一名博士后/早期职业科学家、研究生教学、当地资源管理人员和一名本科生提供教育活动。特别是，研究结果将被纳入西澳大学的研究生课程，吸引对沿大陆架和跨大陆架运动有跨学科兴趣的生物和渔业海洋学学生。将对西澳州卫生部和西澳州鱼类和野生动物部等区域伙伴进行培训，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，一名本科生将获得使用和应用前沿OOI数据集的经验，目标是在华盛顿大学年度本科生研究研讨会上展示他们的成果。

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.