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的主要原因是了解其能量收支，关闭倾覆环流，了解深部洋流以及热量、淡水、溶解碳、地球化学示踪和营养物质的输送。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见的解释，但一些观测表明，脊侧峡谷内的混合和上升流是重要的，内波过程并不能完全解释这一现象。深海约有1000个大洋中脊侧翼峡谷，本项目要研究的峡谷就是其中的许多或大部分具有代表性的。因此，如果边界层过程在这个峡谷被证明是重要的，那么它们预计将在世界上大部分海洋中都是重要的。这项研究提出了一个新的假说，解释了一个重要的过程(深海混合)背后的机制，该假说是通过物理模型发展起来的，并且可以用观测程序进行可行和干净的检验。尽管它与潮汐驱动的混合一起发生，但这种机制可能会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正在驱动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，并影响海洋的总体性质。该方法由一系列日益复杂的模型组成，从具有恒定横截面的峡谷到完全逼真的三维地形。一项关键的成果是一套衡量边界层对远离边界的密度场的总影响的指标。这些度量将与地形的特征相关，作为混合参数的第一步，这些参数不会将所有地形效果压缩到单个粗糙度参数中。更广泛的影响：这个项目将提高我们对整个深海中的混合和上升流的理解。这项研究的模拟部分的一个关键目标是开始开发全球模式的混合参数，考虑到峡谷内的边界层和混合过程，并以模式和观测为基础。该项目还将构成博士后研究人员培训的核心部分。

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.