Collaborative Research: Bottom Boundary Layer Turbulent and Abyssal Recipes

合作研究：底部边界层湍流和深渊配方

• 批准号: 1756251
• 负责人: Kurt Polzin
• 金额: $ 263.79万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-09-15 至 2023-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1756251&HistoricalAwards=false
• 关键词: Collaborative; Research; Bottom; Boundary; Layer

The classic view of the deep overturning circulation of the ocean is one in which densest waters formed at high latitudes sink and spread along the abyssal basins. Small-scale mixing, such as is caused by breaking internal waves, drives upwelling of these densest waters slowly back toward the surface over the interior of the basins. However, turbulence measurements over the last 20 years have shown that mixing becomes more vigorous toward the ocean bottom, which should result in the sinking of the water masses formed by mixing. Recent work, combining theory, numerical models and turbulence measurements have suggested that the upwelling necessary to bring the water back toward the surface to close the loop happens in thin boundary layers very close to the ocean bottom. This is a region typically avoided in turbulence measurements to prevent the instruments from hitting the bottom. This US-UK joint project will seek the first direct evidence that turbulent mixing drives sinking in the stratified interior and upwelling along thin boundary layers. It has potentially wide impact because it explores the importance of boundary layer upwelling in the overturning circulation, a process that has received little attention to date. Should this experiment succeed in finding evidence for large upwelling confined to deep boundary layers, it will reinvigorate studies of boundary layer turbulence. The field program will compare different approaches to measure turbulent buoyancy fluxes in the ocean, and help settle the ongoing debate on which ones are most accurate. The result of this experiment will have important implications for climate studies, because the ocean uptake of carbon and heat is regulated by the pathways of deep water masses. Finally, the project has a strong educational component through the training of two postdocs at WHOI and SIO, who will lead the analysis of the observations, and one graduate student at MIT, who will run numerical simulations to put the observations in the overall context of the regional circulation and the global overturningStarting with Munk (1966), it is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest waters that sink to the ocean bottom at high latitudes. However, turbulence measurements over the last 20 years have shown that mixing becomes more vigorous toward the ocean bottom, and thus converts light waters into denser ones and not vice versa. Using a combination of theoretical ideas, numerical models, and turbulence measurements, Ferrari et al. (2016), de Lavergne et al. (2016) and McDougall and Ferrari (2017) have argued that abyssal waters are converted from dense to light along weakly stratified bottom boundary layers, where small-scale turbulent buoyancy fluxes decrease to zero to satisfy the no-density flux condition at the ocean bottom. In this view, the lower branch of the meridional overturning circulation is the residual of a large diapycnal sinking, driven by convection at high latitudes and small-scale mixing in the stratified ocean interior, balanced by an even larger diapycnal upwelling along the ocean boundary layers. Callies and Ferrari (2017) illustrate that the confinement of upwelling along boundary layers results in a different abyssal circulation from the classical view pioneered by Stommel (1958) and Munk (1966), with important implications for ocean carbon and heat uptake. Observational support for this emerging view of the overturning circulation is lacking, because tracers are advected rapidly in and out of the boundary layers and thus reflect some average of the diapycnal sinking in the stratified interior and diapycnal upwelling along the boundaries. Vertical profiles of turbulence in the deep ocean generally stop above the boundary layer to avoid hitting the seafloor, and thus miss the crucial decrease of turbulent buoyancy flux through the bottom boundary layer. This US-UK collaborative project will use the Rockall Trough in the Northeast Atlantic as a natural laboratory to study diapycnal upwelling along sloping boundaries. This basin is characterized by rough topography and strong topographic mixing, and is an important conduit of abyssal waters in the North Atlantic. Tracers will be released along the Trough's eastern boundary to see whether their movement is consistent with these new ideas and with inferences in prior work that deep waters enter the Rockall Trough from the south and upwell in the basin. The temporal evolution of the tracers will be compared with diapycnal velocities estimated from buoyancy flux measurements from vertical profilers in the stratified interior and moored sensors across the boundary layer. Diapycnal velocities are expected to be strong and upward in the boundary layer, and downward in the stratified interior. Successful completion of the field program will return the first direct observation of the role played by deep boundary layers in the oceanic overturning circulation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

海洋深层翻转环流的经典观点是，高纬度地区形成的沉积沃茨下沉，并沿着深海盆地扩散。小规模的混合，例如由破碎的内波引起的，驱动这些密集沃茨的上涌慢慢地回到盆地内部的表面。然而，过去20年的湍流测量表明，混合在海底变得更加剧烈，这应该会导致混合形成的水团下沉。最近的工作，结合理论，数值模型和湍流测量表明，将水带回表面以闭合回路所需的上升流发生在非常接近海底的薄边界层中。这是湍流测量中通常会避免的区域，以防止仪器撞击底部。这个美英联合项目将寻找第一个直接证据，证明湍流混合驱动分层内部的下沉和沿着薄边界层的上涌。它具有潜在的广泛影响，因为它探讨了边界层上涌在翻转环流中的重要性，这一过程迄今为止很少受到关注。如果这项实验成功地找到了局限于深层边界层的大的上升流的证据，它将重振边界层湍流的研究。该现场项目将比较不同的方法来测量海洋中的湍流浮力通量，并帮助解决正在进行的关于哪种方法最准确的争论。这项实验的结果将对气候研究产生重要影响，因为海洋对碳和热的吸收是由深水物质的路径调节的。最后，该项目有一个强大的教育组成部分，通过培训WHOI和SIO的两名博士后，他们将领导对观测结果的分析，以及麻省理工学院的一名研究生，他们将进行数值模拟，将观测结果置于区域环流和全球翻转的整体背景下。例如由破碎的内波引起的，驱动下沉到高纬度洋底的沉积沃茨的上涌。然而，过去20年的湍流测量表明，混合在海底变得更加剧烈，从而将轻沃茨转化为密度更大的海水，而不是相反。Ferrari等人（2016年）、de Lavergne等人（2016年）以及McDougall和Ferrari（2017年）综合使用理论思想、数值模型和湍流测量，认为深海沃茨沿着沿着弱分层的底部边界层从稠密转变为稀薄，在那里小尺度湍流浮力通量降至零，以满足海底无密度通量条件。在这种观点中，赤道翻转环流的下分支是大的底辟下沉的残余，由高纬度的对流和分层海洋内部的小尺度混合驱动，由沿着沿着海洋边界层的更大的底辟上涌平衡。Callies和Ferrari（2017年）说明，上升流沿着边界层的限制导致了与Stommel（1958年）和Munk（1966年）开创的经典观点不同的深海环流，对海洋碳和热吸收具有重要意义。这种新出现的翻转环流观点缺乏观测支持，因为示踪剂被迅速平流进出边界层，因此反映了分层内部的底辟下沉和沿着边界的底辟上涌的某种平均值。深海湍流的垂直剖面一般在边界层上方停止，以避免触及海底，从而错过了通过底部边界层的湍流浮力通量的关键性减少。这项美英合作计划将利用东北大西洋的罗卡尔海槽作为天然实验室，研究沿着倾斜边界的底辟上升流。该海盆的特点是地形粗糙，地形混合强烈，是北大西洋深海沃茨的重要通道。将沿着海槽的东部边界沿着释放示踪剂，以观察其运动是否与这些新观点以及先前工作中的推论一致，即深水沃茨从盆地的南部和上游进入罗科尔海槽。示踪剂的时间演变将进行比较，从垂直剖面仪在分层的内部和整个边界层的系泊传感器的浮力通量测量估计diapycnal速度。在边界层中，横旋速度很强且向上，而在分层的内部，则向下。成功完成现场项目将返回第一次直接观测到的海洋翻转环流中深层边界层所起的作用。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。