Basin scale forcing of flows on western-boundary shelves

西部陆架流驱动着区域显著的热通量，具有显著的区域气候影响，是连接中高纬度地区的重要生物地球化学输运通道。大陆架和深海之间的水交换是碳和其他重要化学物种在陆地和海洋之间的重要运输途径。最近的工作表明，这些水流的规模和年际变化构成了底栖物种的空间分布及其遗传变异性的结构，并决定了哪些生活史策略在进化上是稳定的。如果没有对推动这些流动的力量的定量了解，就很难理解、预测或模拟这些流动及其影响。从建模的角度来看，如果不理解更大尺度的模型必须准确地模拟什么才能提供足够的边界条件，那么将区域模型嵌入到更大尺度的模型中是有问题的。如果不了解陆架流动变化的时间尺度，以及这种变化的相位和幅度在整个大陆架上可能如何变化，就很难定义观测战略来了解陆架流动的变化。如果不知道沿岸平均流量的来源及其年际变化，就不能对陆架流量进行令人满意的了解。为解决这些问题，将进行一系列面向过程的调查，以澄清导致这些大陆架上的平均和长时间尺度沿岸流的机制，并量化其变化的时间尺度和来源。主要目标是重新建立沿西部边界大陆架的流动与深海动态之间的联系。有了对观测到的沿岸气压梯度来源的清楚了解，就有可能了解海洋/大气系统的大范围变化，如北大西洋涛动和大陆架动力学之间的联系。这将把沿海海洋重新置于全球海洋的更广泛背景中，并允许更好地从数量上了解它们是如何耦合的。该项目将支持培训一名研究生和2至3名本科生，为他们在科学或工业领域的职业生涯做好准备。这名研究生将参加“未来教师培训”项目，该项目旨在培养学生的教学、指导和写作技能。本科生将接受循序渐进的培训，并由PI和研究生共同指导。每个本科生都将在学年做一个简单的开始项目，然后在夏天做一个更独立的项目。这将在毕业所需的顶峰项目中达到高潮。对西部大陆架沿岸流动的研究未能解释沿这些大陆架的平均和长时间尺度流动的动力学。研究得最好的例子是北美的东部大陆架。长期以来，人们一直注意到，为了接近当地的动量平衡，必须调用来源不明的沿等深线的压力梯度。平均风将驱动与观测相反的环流；沿海岸和跨海岸的浮力梯度不足以解释环流。风的年际变化解释了沿岸输送年际变化的不到一半。该项目的主要假设是，这种“平均”流量(以及其年际变化)在很大程度上是由盆地尺度的环流驱动的，特别是沿西部边界的急流。虽然这一想法并不新鲜，但在理解西部边界流动力学方面的最新进展，为这些盆地规模的流动如何驱动陆架流动提供了一个可信的、定量上容易处理的机制。阿根廷货架和中国货架最近的建模工作支持了这一假设。分析工作和理想化数值模拟的结合将解释盆地尺度强迫的变化如何改变沿西部边界陆架的平均和年际变化的水流。这将揭示盆地尺度强迫与陆架与海洋之间的交换之间的联系，西部边界流如何改变陆架水文，以及沿岸地形特征在加强陆架与斜坡之间的相互作用中的作用。

The flow along western boundary shelves drive regionally significant heat fluxes with significant regional climate impacts, and is an important biogeochemical transport pathway linking high and mid-latitude regions. The exchange of water between continental shelves and the deep sea is an important transport pathway between land and sea for carbon and other important chemical species. Recent work has shown that the magnitude and inter-annual variability of these flows structures the spatial distribution of benthic species and their genetic variability, and determines which life-history strategies are evolutionarily stable. It is difficult to understand, predict, or model these flows, and their effects, without a quantitative understanding of the forces that drive them. From a modeling perspective, embedding a regional model in a larger scale model is problematic if one does not understand what the larger-scale model must simulate accurately to provide sufficient boundary conditions. It is also hard to define observational strategies to understand variability in shelf flows without understanding the timescale of their variability, and how the phase and amplitude of this variability may vary across the shelf. A satisfactory understanding of shelf flows cannot be claimed if the source of the mean alongshore flow, and its interannual variation, is not known. To address these questions, a series of process-oriented investigations will be conducted to clarify the mechanisms leading to the mean and long timescale alongshore flows on these shelves and quantify the time-scale and origin of their variability. The main goal is to re-establish a link between the flows along western boundary continental shelves and the dynamics of the deep ocean. With a clear understanding of the source of observed alongshore pressure gradients, it will be possible to understand the links between large-scale variation in the ocean/atmosphere system such as the North Atlantic Oscillation and shelf dynamics. This will place the coastal oceans back into the broader context of the global ocean, and allow for a better quantitative understanding of how they are coupled. The project will support the training of a graduate student and 2 to 3 undergraduates to prepare them for careers in the sciences or industry. The graduate student will participate in the "Preparing Future Faculty" program that develops teaching, mentoring and writing skills in our students. The undergraduates will be progressively trained, and will be mentored by both the PI and graduate student. Each undergraduate will work on a simple starting project for the school year, and then a more independent project over the summer. This will culminate in a capstone project required for graduation. Studies of the alongshore flow along western-boundary continental shelves have failed to explain the dynamics of the mean and long-time scale flows along these shelves. The best studied example is the eastern shelf of North America. It has long been noted there that to close local momentum balances an along-isobath pressure gradient of unclear origin must be invoked. Mean winds would drive a circulation opposite to that observed; along and cross-shore buoyancy gradients are of insufficient magnitude to explain the circulation. Interannual variations in the winds explain less than half of the inter-annual variability in the alongshore transport. The main hypothesis of this project is that this "mean" flow (and perhaps its interannual variation) is largely driven by basin scale circulations, and in particular the swift currents along the Western Boundaries. While this idea is not new, recent advances in the understanding of Western Boundary Current dynamics provide a plausible and quantitatively tractable mechanism for how these basin scale flows can drive shelf flows. Support for this hypothesis is provided by recent modeling efforts for the Argentinean and Chinese shelves. A combination of analytical work and idealized numerical modeling will explain how changes in basin scale forcing alter the mean and interannually varying flow along western-boundary shelves. This will reveal the links between basin scale forcing and exchange between the shelf and ocean, how Western Boundary Currents modify shelf hydrography, and the role of along-shore topographic features in enhancing the interactions between the shelf and slope.