Collaborative Research: Deep Circulation over the Flanks of a Mid-Ocean Ridge

大洋中脊顶部以下的深海是全球热量和二氧化碳的主要储存库，因此在控制地球气候方面发挥着重要作用。这些特性在两极附近的海面上形成密集的水柱，进入深海，并通过湍流扩散与上覆层在整个海洋中交换。深层水以比羽流水低的密度流回两极。为了封闭环流，必须通过地热加热的辅助，通过浮力向下的湍流扩散来增加底层水的浮力。湍流扩散已被发现是强烈增强的粗糙地形的区域，但强烈降低与高度以上的底部。矛盾的是，这导致了大多数深海浮力的损失，但最近的理论研究表明，这种损失在全球范围内被底层浮力的增加和沿着盆地两侧的向上流动所补偿。1990年代在巴西盆地进行的一项实验提出了所涉过程的一个例子。在大西洋中脊两侧的断裂带峡谷上方释放的示踪剂的湍流剖面和演变表明，向底部的扩散增加。此外，虽然主要的示踪剂补丁，这是在大多数地形的高处，向极地和向西移动，大部分更深的示踪剂似乎被吸引到东部和进入峡谷，这表明在该地区的激烈混合。我们建议在2.5公里的分辨率模拟示踪剂补丁的演变，以了解和量化的大气环流和横向涡动搅拌，特别是测试基于测量的假设湍流浮力通量的分布。由于测量结果表明，在该地区的浮力通量占主导地位的过程中跨峡谷的窗台，它是必要的，以高分辨率（250米），以测试假设的湍流浮力通量是如何分布在那里详细模拟的流动在该地区的一个测量良好的窗台。本项目所要研究的强迫和水深测量的类型是整个大西洋以及印度洋和太平洋大部分地区洋中脊侧翼的典型。这项研究将与海洋的大部分区域相关，并将提高我们目前对深层翻转环流的返回路径及其在控制地球气候方面的作用的理解。在粗糙地形上产生湍流并传播到内部是一个复杂的过程，有许多潜在的相关机制产生速度剪切和大幅度位移，提供最终的强迫：例如内潮;背风波;底流;甚至风产生的惯性波。最近的研究表明，这种湍流驱动强烈的上升流沿沿着倾斜的海洋地形。作为巴西盆地实地项目的一部分收集的观测结果将为检验这一假设提供关键信息。将运行数值模型来帮助研究人员解释观测结果并测试他们的理论想法。主要目标是将巴西海盆的众多观测结果统一起来，因为其中没有一个观测结果本身能够描绘巴西海盆深海环流的全貌。例如，在巴西盆地示踪剂释放实验（BBTRE）和随后的活动中，示踪剂的采样必然是不完整的。微结构垂直剖面的时空覆盖范围有限。仅在少数几个位置收集了系泊速度。浮标数据提供了一些水包裹轨迹的一瞥。本研究将使用两组数值模拟。一个2.5公里的分辨率模拟集中在区域采样期间BBTRE配置研究示踪剂的分散，和一个250米的分辨率补丁嵌入在较大的模拟，集中在BBTRE峡谷，详细研究的diapycnal浮力通量和上涌，这似乎是最大的沿着峡谷和丘陵测深。

The deep ocean, below the crests of the mid ocean ridges, is a major global reservoir for heat and carbon dioxide, and thus plays an important role in governing the earth's climate. These properties enter the abyssal ocean in plumes of dense water formed at the surface near the poles, and are exchanged throughout the ocean by turbulent diffusion with overlying layers. Deep water flows back toward the poles at lower densities than the plume water. To close the circulation, buoyancy of bottom water must be increased by turbulent diffusion of buoyancy downward, assisted by geothermal heating. Turbulent diffusion has been found to be strongly enhanced over regions of rough topography, but to decrease strongly with height above the bottom. This results, paradoxically, in buoyancy loss in most of the deep ocean, but recent theoretical work suggests that this loss is more than compensated globally by buoyancy gain in a bottom layer and upward flow along the sides of the basins. An example of the processes involved was suggested by an experiment conducted in the Brazil Basin in the 1990's. Turbulence profiles and the evolution of a tracer released above a fracture zone canyon on the flanks of the Mid-Atlantic Ridge showed increased diffusion toward the bottom. Furthermore, though the main tracer patch, which was aloft of most topography, moved poleward and westward, much of the deeper tracer appeared to be drawn eastward and into the canyon, suggesting vigorous mixing in that area. We propose to simulate the evolution of the tracer patch at 2.5-km resolution to understand and quantify the general circulation and lateral eddy stirring, and especially to test measurement-based hypotheses about the distribution of turbulent buoyancy fluxes. Since measurements suggest that buoyancy fluxes in the region are dominated by processes at sills across the canyons, it is necessary to simulate the flow in the region of a well-measured sill at high (250-m) resolution to test hypotheses of how the turbulent buoyancy flux is distributed there in detail. The kind of forcing and bathymetry to be studied in this project typify the flanks of the mid-ocean ridge throughout the Atlantic and much of the Indian and Pacific Oceans. This study will be relevant to a large fraction of the area of the ocean, and will improve our current understanding of the return path of the deep overturning circulation and its role in governing the earth's climate. Generation of turbulence over rough topography and propagation into the interior are complex processes, with many potentially relevant mechanisms of creating velocity shear and large amplitude displacements that provide the ultimate forcing: e.g. internal tides; lee waves; sill flows; and even wind-generated inertial waves. Recent work suggests that this turbulence drives strong upwelling along the sloping ocean topography. The observations collected as part of field programs in the Brazil Basin will provide key information to test the hypothesis. Numerical models will be run to help the investigators interpret the observations and test their theoretical ideas. The main goal is to bring together in a unified picture the numerous Brazil Basin observations, none of which by itself paints a full picture of the abyssal circulation in the Brazil Basin. For example, the sampling of the tracer during Brazil Basin Tracer Release Experiment (BBTRE) and subsequent campaigns was by necessity incomplete. The vertical profiles of microstructure were limited in their spatial and temporal coverage. Mooring velocities were collected at only a few locations. Float data offered a glimpse of the trajectories of a few water parcels. Two sets of numerical simulations will be used in this study. A 2.5-km resolution simulation centered over the region sampled during BBTRE configured to study the dispersion of the tracer, and a 250-m resolution patch embedded in the larger simulation, centered on the BBTRE Canyon to study in detail the diapycnal buoyancy flux and upwelling which appear to be largest along the canyon-and-hills bathymetry.