Collaborative Research: Flow, Turbulence and Mixing in Mid-Ocean Ridge Fracture Zone Canyons

概述：为了关闭全球翻转环流，必须降低高纬度地区高密度的深水和底水的密度，并使其上升到较浅的深度。来自副热带南大西洋的现有观测表明，那里深海的大部分混合发生在地形崎岖的中大西洋脊上，特别是在山脊两侧凹凸不平的准规则间隔的“断裂带峡谷”。在那里，密集的水被持续的沿着山谷的水流向脊顶平流（即上升流），这些水流沿着单向密度梯度流下，由强烈的湍流（深流混合）维持。这些推论所依据的大部分数据都是在巴西盆地示踪剂释放实验（BBTRE）期间收集的，该实验沿着南大西洋西部靠近22S的一个单一山脊-侧翼峡谷进行，先前的分析表明，潮汐混合和溢流过程都很重要。因此，很可能必须考虑这两个过程，以便理解和参数化湍流和混合在峡谷中的影响，这些峡谷在所有缓慢扩张的山脊两侧起波纹，这些山脊构成了海底的大部分，特别是在大西洋、印度洋和南大洋。这个后续项目的主要目的是提高我们对缓慢扩展的洋中脊的波纹侧翼上的动力学的理解。由于采样分辨率和站点位置的选择很粗糙，因此不可能从BBTRE数据中回答重要问题，例如潮汐和静止混合的相对重要性。因此，将在BBTRE地区相邻的两个峡谷和中间的地形冲程上进行高分辨率的水文、三维流动、湍流和混合调查，以确定潮汐和与岩石有关的混合的相对贡献。此外，部署在附近两个基泊区域的剖面系泊将用于导出空间综合混合相关浮力通量的时间序列，并研究先前记录的沿峡谷流动的强烈但无法解释的亚惯性变异性。此外，将在两个峡谷之间的鞍座上部署三个小型系泊点，以调查峡谷间的交流。数据分析将包括以前实验的可用数据，包括一组以前未分析过的示踪剂概要。智力优势：缓慢扩张的山脊的波纹状侧翼覆盖了几个主要海洋盆地的大片海底。因此，了解~100 km脊翼峡谷的动力学及其对深海浮力和上升流收支的影响具有全球意义。除了确定两个峡谷中潮汐混合和跨基流的相对重要性外，还将研究潮汐到年时间尺度上湍流和混合的时间变异性，以深入了解沿峡谷流动的强迫、邻近峡谷之间的交换以及峡谷水的最终命运。更广泛的影响：预计在该项目中获得的见解将提高我们对许多具有相似底部地形的不同地区的深海混合的理解，并为更好地参数化大尺度环流和气候模式中湍流和混合的影响提供基础，这些模式无法解决这些小尺度过程。作为该项目的一部分，一名研究生和一名博士后研究员将接受从数据获取到解释的观测物理海洋学各个方面的培训。

Overview: In order to close the global overturning circulation, high-density deep- and bottom waters produced at high latitudes must be made less dense and upwell to shallower depths. Available observations from the subtropical South Atlantic indicate that the bulk of the mixing in the deep ocean there takes place over the topographically rough Mid-Atlantic Ridge, in particular in the quasi-regularly spaced "fracture zone canyons" corrugating the ridge flanks. There, dense water is advected toward the ridge crest (i.e. upwelled) by persistent along-valley currents that flow down the unidirectional density gradients, which are maintained by strong turbulence (diapycnal mixing). Most of the data on which these inferences are based were collected during the Brazil Basin Tracer Release Experiment (BBTRE) along a single ridge-flank canyon in the western South Atlantic near 22S where previous analyses have shown that both tidal mixing and overflow processes are important. Therefore, it is likely that both processes must be considered in order to understand and parameterize the effects of turbulence and mixing in the canyons corrugating the flanks of all slow-spreading ridges, which make up large fractions of the sea floor, in particular in the Atlantic, Indian and Southern Oceans. The primary aim of this follow-on project is to improve our understanding of the dynamics over the corrugated flanks of slow-spreading mid-ocean ridges. Due to the coarse sampling resolution and choice of station locations it is not possible to answer important questions, such as the relative importance of tidal and sill mixing, from the BBTRE data. Therefore, high-resolution surveys of hydrography, three-dimensional flow, turbulence and mixing will be carried out in two neighboring canyons and over the intervening topographic spur in the BBTRE region to determine the relative contributions of tidal and sill-related mixing. Furthermore, profiling moorings deployed on two nearby sill regions will be used to derive time series of spatially integrated mixing related buoyancy fluxes and to investigate the strong but unexplained sub-inertial variability of the along-canyon flow recorded previously. Additionally, three small moorings will be deployed in saddles between the two canyons to investigate inter-canyon exchange. The data analysis will include available data from previous experiments, including a set of tracer profiles that has not been analyzed before.Intellectual Merit: The corrugated flanks of slow-spreading ridges cover large areas of the sea floor of several major ocean basins. Therefore, understanding the dynamics in the ~100 km of ridge-flank canyons and its effects on the buoyancy and upwelling budget of the abyssal ocean is of global significance. In addition to determining the relative importance of tidal mixing and cross-sill flows in two canyons, the temporal variability of turbulence and mixing from tidal to yearly time scales will be investigated to gain insights into the forcing of the along-canyon flows, the exchange between neighboring canyons, and the eventual fate of the canyon waters. Broader Impacts: It is anticipated that insights gained during this project will improve our understanding of abyssal mixing in many different regions with similar bottom topography and provide the basis for better parameterizations of the effects of turbulence and mixing in large-scale circulation and climate models that cannot resolve these small-scale processes. As part of the project, a graduate student and a post-doctoral researcher will be trained in all aspects of observational physical oceanography, from data acquisition to interpretation.