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

概述：为了封闭全球翻转环流，必须使高纬度地区产生的高密度深水和底层沃茨密度降低，并向上流至较浅的深度。从亚热带南大西洋获得的观测资料表明，那里深海的大部分混合发生在地形崎岖的大西洋中脊，特别是在使脊侧面起皱的准规则间隔的“断裂带峡谷”。在那里，密集的水平流流向脊顶（即涌）的持续沿谷电流，流下来的单向密度梯度，这是由强湍流（diapycnal混合）。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22 S附近的一个单一的脊翼峡谷，以前的分析表明，潮汐混合和溢流过程都很重要。因此，很可能必须考虑到这两个过程，才能了解构成海底很大一部分，特别是在大西洋、印度洋和南大洋的所有缓慢扩张海脊侧翼的峡谷中湍流和混合的影响，并确定其参数。这一后续项目的主要目的是提高我们对缓慢扩张的大洋中脊波状侧翼的动力学的理解。由于采样分辨率和测站位置的选择较粗，因此无法从BBTRE数据中回答重要问题，例如潮汐和岩床混合的相对重要性。因此，将在两个相邻的峡谷和BBTRE区域的中间地形凸起上进行水文、三维流动、湍流和混合的高分辨率调查，以确定潮汐和与窗台有关的混合的相对贡献。此外，配置停泊在两个附近的窗台区域将被用来获得时间序列的空间集成混合相关的浮力通量，并调查强大的，但无法解释的亚惯性变化的沿峡谷流先前记录。此外，将在两个峡谷之间的马鞍上部署三个小型系泊设备，以调查峡谷间的交换。数据分析将包括从以前的实验中获得的数据，包括一套以前没有分析过的示踪剂剖面图。知识专长：缓慢扩张海脊的波状侧翼覆盖了几个主要洋盆的大面积海底。因此，了解约100公里洋脊-侧翼峡谷的动态及其对深海浮力和上升流收支的影响具有全球意义。除了确定潮汐混合和跨槛流在两个峡谷的相对重要性，湍流和混合从潮汐到每年的时间尺度的时间变化将进行调查，以深入了解强迫沿峡谷流，相邻峡谷之间的交换，和峡谷沃茨的最终命运。更广泛的影响：预计在该项目期间获得的见解将提高我们对具有类似海底地形的许多不同区域的深海混合的认识，并为更好地确定无法解决这些小规模过程的大规模环流和气候模型中湍流和混合影响的参数提供基础。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所有方面的培训。

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.