Collaborative Research: Bottom Boundary Layer Turbulence and Abyssal Recipes

关于海洋深层翻转环流的经典观点是，高纬度地区形成的密度最大的海水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混合，驱使这些密度最大的水的上升流缓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强烈，这应该导致由混合形成的水团下沉。最近的研究结合理论、数值模型和湍流测量表明，将水带回表面以闭合环流所必需的上升流发生在非常接近海底的薄边界层中。在湍流测量中，这是一个典型的避免区域，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，证明湍流混合驱动分层内部下沉和沿薄边界层上涌。它具有潜在的广泛影响，因为它探讨了边界层上升流在翻转环流中的重要性，这一过程迄今为止很少受到关注。如果这个实验能够成功地找到局限于深边界层的大的上升流的证据，它将重新激活边界层湍流的研究。实地项目将比较测量海洋湍流浮力通量的不同方法，并帮助解决关于哪种方法最准确的持续争论。这个实验的结果将对气候研究产生重要的影响，因为海洋对碳和热量的吸收是由深水团的通道调节的。最后，该项目有很强的教育成分，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，他将运行数值模拟，将观测结果置于区域环流和全球翻转的整体背景下。从Munk（1966）开始，人们普遍认为，由破碎的内波引起的小规模混合，驱使密度最大的海水上涌，在高纬度地区沉入海底。然而，过去20年的湍流测量表明，向海底方向混合变得更加强烈，从而将轻水转化为密度更大的水，而不是相反。法拉利等人（2016）、de Lavergne等人（2016）以及麦克杜格尔和法拉利（2017）结合理论思想、数值模型和湍流测量，认为深海水域沿着弱分层的底部边界层从稠密转变为轻，在那里，小尺度湍流浮力通量减少到零，以满足海底的无密度通量条件。在这种观点中，经向翻转环流的下层分支是由高纬度地区的对流和分层海洋内部的小规模混合驱动的大底流下沉的残余，由沿海洋边界层的更大的底流上升流平衡。Callies和Ferrari（2017）表明，边界层上升流的限制导致了与Stommel（1958）和Munk（1966）所倡导的经典观点不同的深海环流，这对海洋碳和热吸收具有重要意义。这种关于翻转环流的新观点缺乏观测支持，因为示踪剂在边界层内外快速平流，因此反映了层状内部的底流下沉和沿边界的底流上升流的一些平均值。深海湍流的垂直剖面通常在边界层以上停止，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。这个美英合作项目将利用东北大西洋的罗克尔海槽作为一个自然实验室，研究沿倾斜边界的潜旋上升流。该盆地地形起伏不平，地形混合性强，是北大西洋深海水的重要通道。示踪剂将沿着海槽的东部边界释放，以观察它们的运动是否与这些新想法一致，并与先前工作的推论相一致，即深水从盆地的南部和上涌进入罗卡尔海槽。示踪剂的时间演化将与从分层内部的垂直剖面仪和跨边界层的系泊传感器测得的浮力通量估计的流速进行比较。底波速度预计会很强，在边界层向上，在层状内部向下。野外项目的成功完成将使我们第一次直接观测到深海边界层在海洋翻转环流中所起的作用。该奖项反映了美国国家科学基金会的法定使命，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。

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.