Collaborative Research: Bottom Boundary Layer Turbulence and Abyssal Recipes

合作研究：底部边界层湍流和深渊配方

• 批准号: 1756264
• 负责人: Matthew Alford
• 金额: $ 196.31万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 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=1756264&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.

海洋深倾循环的经典景观是，在高纬度下沉并沿着深渊盆地散布的最密集的水域。小规模的混合，例如由内波破裂引起的，将这些最密集的水的上升流驱动，缓慢地朝盆地内部的表面缓慢地朝向表面。但是，在过去的20年中，湍流测量表明，混合对海底变得更加剧烈，这应该导致混合形成的水质量下沉。最近的工作结合了理论，数值模型和湍流测量结果表明，将水恢复到表面以关闭环路所需的上升流在非常靠近海洋底部的薄边界层中。这是通常在湍流测量中避免的区域，以防止仪器撞击底部。这个US-UK联合项目将寻求第一个直接的证据，即湍流混合驱动器驱动在分层的内部和沿薄边界层上升。它具有潜在的影响，因为它探讨了倾覆循环中边界层上升的重要性，这一过程几乎没有得到关注。如果该实验成功地找到了局限于深边界层的大型上升流的证据，它将重振对边界层湍流的研究。该现场计划将比较测量海洋中湍流浮力通量的不同方法，并帮助解决有关哪些最准确的辩论。该实验的结果将对气候研究具有重要意义，因为碳和热量的海洋吸收受深水质量的途径调节。最后，该项目通过在Whoi and Sio的两个博士后进行培训，他们将领导观察结果的分析，并在麻省理工学院的一名研究生进行分析，他们将运行数值模拟，以将观察结果置于区域循环的整体环境中，并在全球范围内与Munk的整体宣传（1966年），这是因为它的内部潮流，例如，这是因为这是因为它的潮流而造成的，例如，这是因为这是因为这是因为它的潮流而造成的，因为它是因为这是因为它的潮流而造成的混合，并且是因为这是造成的混合。高纬度的最密集的水沉入海底。但是，在过去20年中的湍流测量表明，混合对海底变得更加有力，从而将轻水转化为密集的水，反之亦然。 Ferrari等人结合了理论思想，数值模型和湍流测量。 （2016），de Lavergne等。 （2016年）以及McDougall和Ferrari（2017）认为，深渊水从致密到光线沿弱分层的底部边界层转化为光，在那里，小规模的湍流浮标降低到零，以满足海底的无密度通量条件。从这种角度来看，子午倾覆循环的下部分支是一个大的Diapynal下沉的残留物，是由高纬度的对流驱动的，在分层海洋内部的小规模混合驱动，由沿海洋边界层沿着海洋层的较大的Diapynal向上平衡。 Callies and Ferrari（2017）表明，沿边界层上升流的限制导致与Stommel（1958）和Munk（1966）开创的经典观点不同的深渊循环，对海洋碳和热吸收具有重要意义。缺乏对倾覆循环的这种新兴观点的观察支持，因为示踪剂迅速进出边界层，因此反映了沿边界沿边界的分层内部和diapynal上下移动中的二比元下沉的平均值。深海中湍流的垂直剖面通常停在边界层上方，以免撞到海底，因此错过了通过底部边界层的湍流浮肿的关键减少。这个US-UK合作项目将使用东北大西洋的Rockall槽作为自然实验室，以研究沿倾斜边界的Diapynal上升流。该盆地的特征是粗糙的地形和强烈的地形混合，是北大西洋中深处水域的重要渠道。示踪剂将沿着槽的东部边界释放，以查看其运动是否与这些新想法相一致，并与先前的工作中的推论相一致，即深水从南部进入岩石槽和盆地上的Upwell。示踪剂的时间演变将与根据分层内部的垂直探测器和边界层的系泊传感器的浮力测量值估计的垂直速度进行比较。预计在边界层中，速度有强度且向上向上，并在分层的内部向下。成功完成现场计划将首先直接观察深边界层在海洋倾斜循环中所扮演的角色。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的知识分子和更广泛影响的评估评估标准来通过评估来支持的。