Boundary Layer Effects on Flow and Mixing in Deep Ocean Canyons

概述：确定深海混合和上升流的时空分布主要是为了了解其能量收支，关闭翻转环流，了解深海洋流和热量、淡水、溶解碳、地球化学示踪剂和营养物质的运输。观察表明，大多数深海混合发生在崎岖的地形上，比如世界大洋中脊的侧翼。虽然潮汐驱动的内波是对这种混合最常讨论的解释，但许多观察表明，山脊两侧峡谷内的混合和上升流是重要的，并且不能完全用内波过程来解释。在深海中大约有1000个洋中脊侧峡谷，本项目研究的峡谷是其中许多或大部分峡谷的代表。因此，如果边界层过程在这个峡谷中被证明是重要的，那么它们预计在世界上大部分海洋中都是重要的。本研究提出了一种新的假设，用于解释一个重要过程（深海混合）背后的机制，该假设是用物理模型发展起来的，并且可以用观测程序进行可行和清晰的验证。智力优势：提出了一种新的深海混合和上升流机制：扩散驱动的边界层与复杂地形的相互作用。虽然它与潮汐驱动混合同时发生，但这一机制可能显著改变深海混合的预期分布。该项目通过一系列模拟实验来研究边界层是否驱动了南大西洋峡谷内的环流，以阐明这些边界层如何与复杂的地形相互作用并影响海洋的整体特性。该方法由越来越复杂的模型层次结构组成，从具有恒定横截面的峡谷到完全逼真的三维地形。一个关键的可交付成果是一组衡量边界层对远离边界的密度场的总影响的指标。这些指标将与地形特征相关，作为混合参数化的第一步，这些参数化不会将所有地形效应分解为单个粗糙度参数。更广泛的影响：这个项目将提高我们对整个深海混合和上升流的理解。本研究建模部分的一个关键目标是开始为考虑峡谷内边界层和混合过程的全球模式开发混合参数化，并以模式和观测为基础。本项目也将成为培养博士后研究人员的核心部分。

Overview: The main reasons to identify the spatial and temporal distribution of mixing and upwelling in the deep ocean are to understand its energy budget, to close the overturning circulation and to understand deep currents and the transport of heat, fresh water, dissolved carbon, geochemical tracers, and nutrients. Observations suggest that the majority of deep-ocean mixing happens over rough topography, like the flanks of the world's mid-ocean ridges. Though tidally-driven internal waves are the most commonly discussed explanation for this mixing, a number of observations suggest that mixing and upwelling within the ridge flank canyons is significant, and that it is not fully explained by internal wave processes. There are about 1,000 mid-ocean ridge flank canyons in the deep ocean, and the canyon to be studied in this project is representative of many or most of them. Therefore, if boundary layer processes are shown to be significant in this canyon, they are expected to be significant throughout much of the world's oceans. This study presents a novel hypothesis for the mechanism behind an important process (abyssal mixing), developed with physical models, and which can be feasibly and cleanly tested with an observational program.Intellectual Merit: A novel mechanism for abyssal mixing and upwelling is proposed: diffusion-driven boundary layers interacting with complicated topography. Though it occurs in conjunction with tidally-driven mixing, this mechanism may significantly change the expected distribution of deep-ocean mixing. This project investigates whether boundary layers are driving the circulation within a canyon in the South Atlantic using a series of modeling experiments to elucidate how these boundary layers interact with complicated topography and affect bulk properties of the ocean. The approach consists of a hierarchy of models with increasing complexity, from canyons with constant cross-sections to fully-realistic three-dimensional topography. A key deliverable is a set of metrics for the total impact of the boundary layers on the density field far from the boundary. These metrics will be related to the characteristics of the topography as a first step toward mixing parameterizations that do not collapse all topographic effects into a single roughness parameter. Broader Impacts: This project will improve our understanding of mixing and upwelling throughout the deep ocean. A key goal of the modeling portion of this study is beginning to develop mixing parameterizations for global models that take into account boundary layer and mixing processes within canyons and are based on both models and observations. This project will also form a central portion of the training of a postdoctoral researcher.