Upwelling variability in the California Current: bridging local dynamics and climate variability

沿海上升流的研究在加利福尼亚洋流系统中有着悠久而丰富的历史，然而，对局部、区域和盆地尺度上物理过程之间相互作用的全面描述仍然难以捉摸。季节性上升流由强烈的赤道沿岸风驱动，在塑造美国西海岸的环流和生态系统结构方面起着重要作用。本研究的主要目的是利用高分辨率数值模拟来表征与加利福尼亚洋流系统中风力驱动的沿海上升流相关的全三维、时变环流的变化。更具体地说，模型结果将用于：(i)产生一个区域综合，严格识别和描述主要沿海岬角附近上升流增强的区域，以及（ii）量化区域和盆地尺度过程对局部尺度上升流动力学的影响。这项研究将提供重要的新见解，以了解增强上升流区域如何有助于形成沿岸物理和生物地球化学性质的梯度。特别相关的是与沿海海洋酸化以及美国西海岸大陆架上低pH条件的发生和持续的联系。增强的上升流中心可能在决定近岸pH值的时空变异性方面发挥关键作用，从而在短期和长期生态系统对沿海海洋酸化的响应中发挥关键作用。这项研究将增加对生物地球化学变化在多大程度上归因于与局部上升流动力学相关的物理强迫（由区域和盆地尺度过程调节）的理解，从而提高目前对美国西海岸哪些沿海地区更容易发生频繁和严重酸化事件的认识。此外，该项目将支持加州大学圣克鲁斯分校的一名博士后研究员，他将在最先进的区域海洋环流建模和诊断方法（如伴随计算）方面获得宝贵的培训。项目成果也将整合到UCSC的研究生课程中，并为基于社区的区域海洋模拟系统（ROMS）增强可用的教育工具。最后，pi将在圣克鲁斯的Seymour海洋探索中心进行普通听众讲座，作为正在进行的一系列关于季节性加利福尼亚上升流如何响应不断变化的气候条件以及对生态系统过程和社会经济服务的潜在影响的一部分。沿海上升流趋势和根据全海岸平均（即综合强弱上升流区域）确定的变率可能混合了不同的动力过程，并可能歪曲美国西海岸发生的当地物理和生物地球化学波动的真实性质。因此，这项研究将为主要海岸岬角附近的上升流增强区域如何在局部尺度上影响沿海海洋环流和物理变化提供重要的新见解。数值实验的结果还将有助于确定区域和盆地尺度的变率如何调节上升流中心和周围陆架区域附近的垂直输送和水团性质。该项目最具创新性的方面是，它将描述当地、区域和盆地尺度过程对与沿海上升流相关的全三维、时变海洋环流的影响，而不是依赖传统的上升流代理（例如，Bakun指数），这些代理在强烈上升流或强烈的陆上地转流地区通常是不可靠的。这项研究的另一个重要贡献是，它将证明将区域数据同化解决方案缩小到美国西海岸上升流动态变化的局部尺度的好处。最后，数值模拟的结果将补充和加强另一个研究项目，该项目由美国国家科学基金会的沿海see项目资助，重点是量化加利福尼亚低pH事件的强度、发生频率和可预测性，以及它们对沿海生态系统过程的潜在影响。

Coastal upwelling research has a long and rich history in the California Current System, yet a comprehensive description of the interplay between physical processes at local, regional, and basin scales has remained elusive. Seasonal upwelling, driven by strong equatorward alongshore winds, plays a fundamental role in shaping the circulation and ecosystem structure along the west coast of the United States. The main objective of this study is to use high-resolution numerical simulations to characterize variability in the full three-dimensional, time varying circulation associated with wind-driven coastal upwelling in the California Current System. More specifically, the model results will be used to: (i) produce a regional synthesis that rigorously identifies and describes regions of enhanced upwelling in the vicinity of major coastal headlands, and (ii) quantify the impact of regional- and basin-scale processes on local-scale upwelling dynamics. This research will provide significant new insight into how regions of enhanced upwelling contribute to shaping alongshore gradients of physical and biogeochemical properties. Of particular relevance is the connection to coastal ocean acidification and the occurrence and persistence of low pH conditions over the continental shelf along the U.S. west coast. Enhanced upwelling centers are likely to play a key role in determining the spatiotemporal variability of nearshore pH levels, and, therefore, alongshore heterogeneity in the short- and long-term ecosystem response to coastal ocean acidification. This study will increase understanding of how much biogeochemical variability is attributable to physical forcing associated with local upwelling dynamics (as modulated by regional- and basin-scale processes), thereby advancing current knowledge of which coastal regions along the U.S. west coast are more prone to frequent and severe acidification events. In addition, the project will support a postdoctoral researcher at UC Santa Cruz who will gain valuable training in state-of-the-art regional ocean circulation modeling and diagnostic methods, such as adjoint-based calculations. Project outcomes will also be integrated into the graduate curriculum at UCSC, and enhance educational tools available for the community-based Regional Ocean Modeling System (ROMS). Finally, the PIs will present general audience lectures at the Seymour Marine Discovery Center in Santa Cruz, as part of an ongoing series on how seasonal California upwelling will respond to changing climate conditions, and the potential impacts on ecosystem processes and socio-economic services. Coastal upwelling trends and variability determined on the basis of coast-wide averages (i.e., integrated over stronger and weaker upwelling regions) likely compound different dynamical processes and may misrepresent the true nature of the local physical and biogeochemical fluctuations occurring along the U.S. west coast. Therefore, this research will provide significant new insight into how regions of enhanced upwelling in the vicinity of major coastal headlands contribute to shaping the coastal ocean circulation and physical variability at local scales. Outcomes from the numerical experiments will also help determine how regional- and basin-scale variability modulates vertical transport and water mass properties in the vicinity of enhanced upwelling centers and surrounding shelf regions. The most innovative aspect of the project is that it will describe the impacts of local-, regional-, and basin-scale processes on the full 3-dimensional, time-varying ocean circulation associated with coastal upwelling, as opposed to relying on traditional upwelling proxies (e.g., Bakun index) which are often unreliable in regions of intense upwelling or strong onshore geostrophic flow. Another important contribution from this research is that it will demonstrate the benefits of downscaling regional data-assimilative solutions to the local scales at which alongshore variability in upwelling dynamics occurs along the U.S. west coast. Finally, the results from the numerical simulations will complement and enhance another research project, funded under NSF's Coastal SEES Program, focused on quantifying the magnitude, frequency of occurence, and predictability of low pH events along California, as well as their potential impact on coastal ecosystem processes.