Collaborative Research: Investigating the Dynamics in Deep Valleys on the Seafloor With Numerical Experiments and Data Analysis

为了关闭海洋的全球翻转环流，高纬度地区致密水的产生和下沉必须通过浮力的增加和上升流来平衡。这两种过程都与底冲混合密切相关，这意味着混合过程在地球气候系统中是至关重要的。此外，垂直运动是深海环流的主要驱动力，这意味着为了理解和模拟环流，必须知道它的空间分布。过去几十年收集的观测资料表明，深海中最强的混合主要出现在海洋中脊（MORs）的粗糙地形附近，这些地形覆盖了整个海底的50%以上。特别令人感兴趣的是许多山脊侧面峡谷（断裂带），它们每隔50公里左右就会切割缓慢扩张的山脊侧面。这些峡谷不仅是强烈混合的主要场所，而且它们也具有惊人的规则性和自相似性，持续的上翼流动和溢流跨越无处不在的等深山，作为沿轴流的基板。尽管在混合的空间分布和脊侧峡谷环流方面的观测取得了进展，但对海洋中垂直运动的空间分布几乎一无所知。虽然目前的高分辨率海洋模型在许多领域都取得了显著的进展，但上升流场的准确性仍然是一个重要的突出问题。本文将利用数值模拟和现有观测资料的分析，研究作用于MORs附近的动力学，特别是山脊侧峡谷的流动。除了提高我们对粗糙地形附近的横向环流和混合的理解之外，人们可以期望从这项研究中获得的见解有助于在海洋环流模式中开发更准确的上升流和由此产生的深海环流。智力优势：湍流非常依赖于区域几何的细节。即使是最高分辨率的大尺度海洋模式也不能充分解决海底地形的所有精细尺度特征。因此，粗糙地形引起的上升流的参数化和建模是一个根本性的挑战。脊翼峡谷及其相关流动的自相似、规则的几何性质可以极大地促进大尺度模型中相关阻力和混合的表现。这些流动的数值模拟是一项新颖的工作，它将揭示形式阻力，水力控制和时间依赖力在多层流中的重要性。更广泛的影响：该项目的结果将以期刊文章、在国内和国际科学会议上的演讲以及通过项目网站的形式传播。最终，对MORs流动动力学的更好理解将导致对海洋环流的更好理解和建模。由于海洋模型被用于预测各种尺度的环流模式，本研究的结果可能会对气候模型产生更广泛的影响，最终通过影响政策来影响社会。该项目的研究结果也有助于提高我们对热液“产物”（包括热、地球化学示踪剂和动物幼虫）扩散的认识。该研究将为一名物理海洋学博士研究生和一名女性博士获得者提供支持。

In order to close the global overturning circulation of the ocean, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upwelling. Both these processes are intimately linked to diapycnal mixing, which implies that mixing processes are fundamentally important in the Earth's climate system. Additionally, vertical motion is a primary driver for the abyssal circulation, which implies that its spatial distribution must be known in order to understand and model the circulation. Observations collected during the last few decades indicate that the strongest mixing in the deep ocean is primarily found near the rough topography of mid-ocean ridges (MORs), which cover more than 50% of the entire sea-floor. Of particular interest are the many ridge-flank canyons (fracture zones) that incise the flanks of slow-spreading ridges every 50 km or so. Not only are these canyons the main sites of strong mixing, they are also strikingly regular and self similar, with persistent up-flank flows and overflows across the ubiquitous bathymetric hills that act as sills for the along-axial flows. In spite of the observational advances regarding the spatial distribution of mixing and the circulations in ridge-flank canyons, virtually nothing is known about the spatial distribution of vertical motion in the ocean. While current high resolution ocean models have made significant strides towards realism in many areas, the accuracy of the upwelling field remains as one of the important outstanding problems. The dynamics acting near MORs, and in particular the flows in the ridge flank canyons will be investigated by using numerical modeling and analysis of existing observations. In addition to improving our understanding of the lateral circulation and the mixing near rough topography, one could expect insights from this study to contribute towards developing more accurate representations of upwelling and the resulting abyssal circulations in ocean general circulation models.Intellectual Merits: Turbulent flows are strongly dependent on the details of domain geometry. Even the highest resolution large-scale ocean models cannot adequately resolve all the fine-scale features of bottom topography. As such, parameterization and modeling of upwelling induced by rough topography poses a fundamental challenge. The self-similar, regular nature of the geometry of the ridge-flank canyons and the associated flows may greatly facilitate representation of the associated drag and mixing in large-scale models. The numerical modeling of these flows is a novel undertaking, which will reveal the importance of form drag, hydraulic control and time-dependent forcing in stratified flows over multiple sills.Broader Impacts: The results from this project will be disseminated in the form of articles in journals, presentations in national and international scientific meetings, and via a project website. Ultimately, a better understanding of flow dynamics on MORs will lead to an improved understanding and modeling of the ocean general circulation. As ocean models are used to predict circulation patterns at a variety of scales, the results from this study may have the broader impact of aiding climate modeling, which can ultimately impact society by influencing policy. The results from this project may also help improve our understanding on the dispersal of hydrothermal "products", including heat, geochemical tracers and animal larvae. The study will provide support for both a PhD student and a female PhD recipient in physical oceanography.