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.

海洋深部颠覆环流的经典观点是，在高纬度形成的密度最大的水沿着深海盆地下沉和扩散。小范围的混合，例如由破裂的内波引起的，驱使这些密度最高的水在盆地内部缓慢地向上涌回地表。然而，过去20年的湍流测量表明，混合在海底变得更加猛烈，这应该会导致混合形成的水团下沉。最近的研究结合了理论、数值模型和湍流测量，表明将水带回表面以关闭环路所需的上升流发生在非常接近海底的薄边界层中。这是在湍流测量中通常会避免的区域，以防止仪器触底。这个美英联合项目将寻求第一个直接证据，证明湍流混合推动层状内部下沉和沿薄边界层上升。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，这一过程迄今很少受到关注。如果这项实验成功地找到了限制在深层边界层的大上升流的证据，它将重振对边界层湍流的研究。该实地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具有重要意义，因为海洋对碳和热量的吸收受到深水团路径的调节。最后，该项目通过培训WHOI和SIO的两名博士后和麻省理工学院的一名研究生具有强大的教育作用，前者将领导观测分析，后者将进行数值模拟，将观测放在区域环流和全球翻转的整体背景下。然而，过去20年的湍流测量表明，向海底的混合变得更加强烈，从而将轻水转化为密度更高的水，反之亦然。利用理论想法、数值模型和湍流测量的组合，Ferrari等人。(2016)，de Lavergne等人。(2016)和McDougall和Ferrari(2017)认为，深海水域沿着弱分层底部边界层由密变轻，那里的小尺度湍流浮力通量降为零，以满足海底的无密度通量条件。这种观点认为，经向翻转环流的下沉是由高纬度对流和层化海洋内部的小尺度混合驱动的大天顶下沉的残余，并被沿海洋边界层的更大的天顶上升流所平衡。Cally和Ferrari(2017)表明，沿边界层上升流的限制导致了与Stommel(1958)和Munk(1966)开创的经典观点不同的深海环流，对海洋碳和热量吸收具有重要意义。对这一新出现的翻转环流观点缺乏观测支持，因为示踪物在边界层内和边界层之间迅速平流，从而反映出层结内部的日下沉和沿边界的日上涌的某种平均情况。深海中的湍流垂直剖面一般停留在边界层以上，以避免撞击海底，从而错过了通过底部边界层的湍流浮力通量的关键减少。这项美英合作项目将利用东北大西洋的罗卡尔海槽作为天然实验室，研究沿倾斜边界的昼夜上升流。该盆地具有地形粗糙、地形混合强烈的特点，是北大西洋深水的重要通道。示踪剂将沿着海槽的东部边界释放，以查看它们的运动是否与这些新想法以及先前工作中的推断一致，即深水从南部进入罗卡尔海槽，并在盆地中向上流动。示踪剂的时间演变将与从分层内部和跨边界层系泊传感器的垂直剖面仪测量的浮力通量估计的昼夜速度进行比较。在边界层中，昼夜速度预计会很强且向上，而在层结内部则会向下。成功完成现场计划将是对深层边界层在海洋倾覆环流中所起作用的第一次直接观察。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。