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

海岸上升流的研究在加州海流系统中有着悠久而丰富的历史，但对局部、区域和盆地尺度上的物理过程之间的相互作用的全面描述仍然难以捉摸。在赤道沿岸强风的推动下，季节性上升流对美国西海岸的环流和生态系统结构的形成起着至关重要的作用。这项研究的主要目的是使用高分辨率数值模拟来表征与加州海流系统中由风驱动的沿海上升流相关的全三维、时变环流的可变性。更具体地说，模式结果将用于：(I)制作区域综合，严格识别和描述主要沿海海角附近的上升流增强区域，以及(Ii)量化区域和流域范围的过程对当地尺度上升流动力的影响。这项研究将对上升流增强的区域如何有助于形成沿岸物理和生物地球化学特性的梯度提供重要的新见解。特别重要的是与沿海海洋酸化的联系，以及美国西海岸大陆架上低pH值条件的出现和持续。增强的上升流中心可能在决定近岸pH值水平的时空变异性方面发挥关键作用，因此，在沿海海洋酸化的短期和长期生态系统反应中，近岸异质性可能发挥关键作用。这项研究将增加对生物地球化学变异性在多大程度上可归因于与局部上升流动力学相关的物理强迫(受区域和盆地规模的过程调制)的理解，从而增进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现有认识。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方面获得宝贵的培训，例如基于伴随的计算。项目成果还将纳入加州大学圣迭戈分校的研究生课程，并加强以社区为基础的区域海洋模式系统(ROMS)的教育工具。最后，PI将在圣克鲁斯的西摩海洋探索中心举办普通听众讲座，这是正在进行的一系列活动的一部分，主题是加州季节性上升流将如何应对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。根据全海岸平均(即，在较强和较弱的上升流地区进行综合)确定的海岸上升流趋势和变异性可能会合成不同的动力过程，并可能歪曲美国西海岸发生的局部物理和生物地球化学波动的真实性质。因此，这项研究将对主要沿海海角附近上升流增强的区域如何有助于塑造沿海海洋环流和地方尺度上的物理变异性提供重要的新见解。数值试验的结果还将有助于确定区域和流域尺度的可变性如何调节增强的上升流中心和周围陆架地区附近的垂直输送和水团特性。该项目最具创新性的方面是，它将描述地方、区域和盆地规模的过程对与沿海上升流有关的全三维、时变海洋环流的影响，而不是依赖于传统的上升流替代指标(例如，巴昆指数)，后者在强烈上升流或强烈的陆上地转流区域往往是不可靠的。这项研究的另一个重要贡献是，它将展示缩小区域数据同化解决方案的好处，这些解决方案适用于美国西海岸上升流动力的近岸变异性发生的局部尺度。最后，数值模拟的结果将补充和加强由NSF海岸SEES计划资助的另一个研究项目，该项目侧重于量化加州沿线低pH事件的规模、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.