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

深海位于大洋中脊之下，是全球主要的热量和二氧化碳储存库，因此在控制地球气候方面发挥着重要作用。这些特性以在两极附近的表面形成的稠密水羽流的形式进入深海，并通过与上层的湍流扩散在整个海洋中进行交换。深水以比羽流水更低的密度向两极回流。为了关闭循环，必须通过浮力向下的湍流扩散以及地热加热来增加底水的浮力。湍流扩散在地形起伏较大的区域得到了强烈的增强，但随着底部以上高度的增加而明显减弱。自相矛盾的是，这导致了大部分深海浮力的损失，但最近的理论研究表明，这种损失在全球范围内得到了补偿，因为底层浮力的增加和盆地两侧的向上流动。上世纪90年代S在巴西盆地进行的一项实验给出了这一过程的一个例子。在中大西洋海脊两侧的一个断裂带峡谷上方释放的湍流剖面和示踪剂的演变表明，向底部的扩散增加。此外，尽管位于大多数地形高处的主要示踪剂斑块向极地和向西移动，但大部分较深的示踪物似乎向东吸引，进入峡谷，这表明该地区存在强烈的混合。我们建议在2.5公里分辨率下模拟示踪斑块的演化，以了解和量化大气环流和侧向涡动，特别是检验基于测量的关于湍流浮力通量分布的假设。由于测量表明该区域的浮力通量主要由跨越峡谷的基准处的过程控制，因此有必要在高分辨率(250米)下模拟测量良好的基准区的流动，以检验关于湍流浮力通量如何在那里分布的假设。在这个项目中要研究的那种强迫和水深测量在整个大西洋以及印度洋和太平洋的大部分大洋中脊的侧翼都是典型的。这项研究将与很大一部分海洋面积有关，并将改善我们目前对深部颠覆环流的返回路径及其在控制地球气候中的作用的理解。在崎岖的地形上产生湍流并传播到内陆是复杂的过程，有许多潜在的相关机制来产生速度切变和大幅度位移，从而提供最终的强迫：例如，内潮、背风波、基底流，甚至风生的惯性波。最近的研究表明，这种湍流推动了沿倾斜的海洋地形的强烈上升流。作为巴西盆地实地考察项目的一部分，收集的观测数据将为检验这一假说提供关键信息。将运行数值模型来帮助调查人员解释观察结果并测试他们的理论想法。主要目标是将众多的巴西盆地观测结果汇集在一张统一的图片中，其中任何一项本身都不能全面描述巴西盆地的深海环流。例如，在巴西盆地示踪剂释放试验(BBTRE)和随后的活动中，对示踪剂的采样必然是不完整的。微结构的垂直剖面在空间和时间覆盖范围上都是有限的。只在少数几个地点采集到系泊速度。浮标数据让我们得以一窥几个水团的轨迹。本研究将使用两组数值模拟。一个2.5公里分辨率的模拟以BBTRE期间采样的区域为中心，用于研究示踪剂的弥散，而一个250米分辨率的斑块嵌入在以BBTRE峡谷为中心的更大的模拟中，以详细研究沿峡谷和丘陵的水深测量似乎最大的昼夜浮力通量和上升流。

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.