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

概述：为了关闭全球翻转的环流，在高纬度产生的高密度深水和底层水必须降低密度，向上升至较浅的深度。来自亚热带南大西洋的现有观测表明，那里深海中的大部分混合发生在地形粗糙的中大西洋海脊上，特别是在沿海脊两侧波纹分布的准规则间隔的“断裂带峡谷”。在那里，稠密的水被沿着山谷的持续水流平流到山脊顶端(即上升)，这些水流沿着单向密度梯度流动，而单向密度梯度由强烈的湍流(昼夜混合)维持。这些推断所依据的大部分数据是在巴西盆地示踪器释放实验(BBTRE)期间收集的，该实验沿着西南大西洋近22S的一个单一脊侧峡谷进行，此前的分析表明，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，以便了解和确定峡谷中湍流和混合的影响，峡谷中的波纹覆盖了所有缓慢展开的海脊的侧面，构成了海底的很大一部分，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大洋中脊波纹侧翼上的动力学的理解。由于采样分辨率较低和站点位置的选择，不可能从BBTRE数据中回答重要的问题，例如潮汐和基床混合的相对重要性。因此，将在BBTRE地区的两个相邻峡谷和中间地形刺上进行高分辨率的水文、三维流动、湍流和混合调查，以确定潮汐和与基床相关的混合的相对贡献。此外，部署在附近两个基台区的仿形系泊将被用来得出空间积分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未解释的亚惯性变率。此外，三个小型系泊设施将部署在两个峡谷之间的鞍座上，以调查峡谷之间的交换。数据分析将包括以前实验中可用的数据，包括一组以前从未分析过的示踪剂剖面。智力上的优点：缓慢展开的海脊的波纹侧翼覆盖了几个主要洋盆的大片海底。因此，了解约100公里山脊两侧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，还将调查湍流和混合从潮汐到年的时间尺度的时间变异性，以深入了解沿峡谷流动的强迫、相邻峡谷之间的交换以及峡谷水域的最终命运。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海底地形相似的许多不同区域的深海混合的了解，并为更好地对无法解决这些小规模过程的大尺度环流和气候模型中的湍流和混合的影响进行参数化奠定基础。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.