A Study to Enravel the Abyssal Circulation of the Northeast Pacific Ocean and its Double Silica Maximum

在东北太平洋盆地发现的世界上最古老的水向上运动是全球海洋环流中最不为人所知的方面之一，取决于复杂的局部效应。在全球范围内，过去几十年来越来越清楚的是，底层水在“传送带”中返回到表层海洋的流动远远不是最初设想的准均匀向上运动和循环。将利用所有现有的历史数据以及模式、统计和数值方法，对整个东北太平洋盆地的深部和深海环流进行高分辨率分析。该项目还将回答一个长期存在的关于北太平洋双二氧化硅极大值起源的问题。该项目的研究人员在华盛顿大学海洋学本科教育中发挥了领导作用，并将利用这项研究的结果向学生说明支撑简单示意图的真实世界过程的复杂性，这些简单示意图通常在讨论深海环流在长期气候变化中的作用时找到。调查人员将在华盛顿大学海洋学网站上发布并存档海洋属性的三维领域。这些领域可能有助于其他研究人员从实验设计到更好地了解北太平洋深处的示踪剂分布。最后，这项工作将为一名本科生和一名研究生提供支持和培训。该项目的第一个目标是检验一个关于环流场与北太平洋双硅极大值之间关系的假说。北太平洋拥有惊人的中深度溶解二氧化硅(Si)的最大值，这是一种对海洋生物地球化学很重要的再生营养物质。先前的工作指出，胡安德·富卡海脊西侧存在第二个密度更高的硅含量最高值，其间存在硅含量最低值，这排除了该地区底水和中层之间简单的上涌连接。当时的假设是，中深部的硅柱来自位于海脊以东的卡斯卡迪亚盆地底部。最近的研究表明，卡斯卡迪亚盆地的物源不足以维持这一特征。该项目的工作假设是，额外的来源位于海脊以西，需要一条令人惊讶的复杂连接，通过一条曲折的向上螺旋路线沿西翼向南到达门多西诺断裂带以北的一个深缺口，然后沿着海脊系统的东侧向北进入卡斯卡迪亚盆地。沿着这条流动路径，底部加热和昼夜混合降低了流体的密度，直到它溢出到边界山脊，为中深羽流提供食物。通过卡斯卡迪亚盆地的流动路径的下游部分已由调查者在最近的工作中记录下来，他提出了对上游西侧流动的详细分析，采用了类似的地转逆方法。第二个目标是以前所未有的分辨率将反计算扩展到日期线以东和北纬25度以北的整个区域。利用1990年以前的数据进行的一项分析显示，太平洋的深部环流令人惊讶地复杂，表现为东西两个不同的经向输送区域，由多个带状拉长的深涡连接起来，但最近的研究不同意这一点。对东北太平洋较高分辨率的倾覆进行详细分析，并使用基于位涡预算的根本不同的反演技术，将用来对这一问题以及其他悬而未决的问题产生新的见解。

The upward movement of the world's "oldest" water found in the Northeast Pacific Basin is one of the least understood aspects of the global ocean circulation and depends on complex local effects. Globally, it has become increasingly clear over the last few decades that the return flow of bottom water to the surface ocean in the "conveyor belt" is far from the quasi-uniform upward motion and circulation that was originally envisioned. A high resolution analysis of the deep and abyssal circulation for the entire Northeast Pacific basin will be conducted using all available historical data and model statistical and numerical methods. The project will also answer a long-standing question about the origin of the North Pacific double silica maximum. The investigator for this project has a leading role in undergraduate education in Oceanography at the University of Washington and will use the results from this study to illustrate to students the complexities of the real-world processes underpinning the simple schematics that are commonly found in discussions of the role of the deep ocean circulation in long-term climate variation. The investigator will publish and archive the three-dimensional fields of ocean properties on the University of Washington Oceanography website. These fields may be helpful to other researchers for uses ranging from experiment design to better understanding of tracer distributions in the deep North Pacific. Finally, the work will provide support and training for an undergraduate and a graduate student.The first objective of the project is to test a hypothesis about how circulation fields relate to the North Pacific double silica maximum. The North Pacific hosts a striking mid-depth maximum of dissolved silica (Si), a regenerated nutrient important for ocean biogeochemistry. Previous work noted a second much denser maximum of Si over the western flank of the Juan de Fuca Ridge, with an intervening Si minimum that precludes a simple upwelling connection between bottom water and mid-depth in this region. The hypothesis then was that the mid-depth Si plume is fed from the bottom of Cascadia Basin, located east of the ridge. Recent work revealed that Cascadia Basin sources are not sufficient to maintain this feature. The working hypothesis for this project is that the additional source lies west of the ridge, requiring a connection - one that is surprisingly complex - via a convoluted upward-spiraling route south along the western flank to a deep gap just north of the Mendocino Fracture Zone, and then north into Cascadia Basin along the eastern flank of the ridge system. Along this flow path, bottom heating and diapycnal mixing decrease the density of the fluid until it spills over the bounding ridges to feed the mid-depth plume. The downstream part of the flow path through Cascadia Basin has been documented in recent work by the investigator who proposes a detailed analysis of the upstream western flank flow, applying a similar geostrophic inverse methodology. The second objective is to extend the inverse calculation to the entire region east of the dateline and north of 25 N at unprecedented resolution. An analysis using data available before 1990 revealed surprising complexity to the deep circulation in the Pacific, in the form of distinct regions of meridional transport in the west and east, connected by multiple zonally-elongated deep gyres, but more recent studies disagree. A detailed analysis of the northeast Pacific overturning at higher resolution, and using a fundamentally different inverse technique based on the potential vorticity budget, will be used to yield new insight on this issue, as well as other unanswered questions.