Bottom Boundary Layer Turbulence and Abyssal Recipes (BLT Recipes)

自从沃尔特·蒙克（Walter Munk）在20世纪60年代的开创性工作以来，海洋学家们一直相信，由厘米尺度的湍流混合与海洋内部破碎的波浪相关，驱动着寒冷的深海水的上涌，这种上涌调节着深海在几十年到几千年里隔绝热量和碳的能力（从而塑造了地球的气候）。然而，过去二十年来对深海湍流的测量结果却与这种说法截然不同，相反，它表明，破碎波的混合驱动了深海水的“下潜”。受到这个难题的启发，最近的理论研究已经发展出一种诱人的替代观点，即混合在维持深海上升流中的作用。在这个新的观点中，上升流是由海底附近的薄层（通常是几十米厚）内高度局部的湍流驱动的，这些薄层统称为底部边界层。在本提案中，我们着手评估这种新范式的有效性，并通过获得第一组同步的系统测量(1)大规模混合和上升流，(2)它们的内部和底边界层贡献，以及(3)支撑这些贡献的过程，在一个具有代表性的深海盆地（东北大西洋的罗克尔海槽）中弄清楚它是如何工作的。为此，我们将与一组美国项目伙伴密切合作，进行为期一年的实地考察计划，包括三次考察巡航。为了测量(1)，在第一次巡航中，我们将在盆地东侧海底附近释放一种寿命很长的化学示踪剂，其浓度非常低，在那里深水从南部流入盆地。我们将在示踪剂释放后的6个月和12个月，在随后的两次巡航中，通过采样来监测示踪剂在填充盆地时的混合和上涌情况。为了衡量(2)和(3)，我们将采用双管齐下的方法。首先，我们将在第一次和第三次巡航中，通过在整个盆地的数百个地点部署湍流测量剖面仪，评估海洋内部的混合和上升流以及向底部边界层的过渡。其次，由于这些船上部署的剖面仪不适合近距离接近海底，我们将通过两种方式获得有针对性的混合和上升流穿过底部边界层的测量结果：(i)在盆地倾斜边界的两个地点进行两次为期6个月的部署，部署一组新型湍流测量仪器，系泊在海底；（ii）在每个系泊部署地点进行近底部释放一种短寿命染料，并在释放后的几天内监测染料的混合和上涌情况。我们将把(1)-(3)的测量结果整合到罗克尔海槽环流的实际数值模型的设计中，并使用该模型对海洋混合的新范式进行严格的定量测试。我们希望这次测试能够提供第一个证据，证明底边界层湍流在驱动深海上升流方面占主导地位，并由此引发我们对深海环流的理解和建模的深刻转变。

Since the seminal work of Walter Munk in the 1960s, oceanographers have believed that the upwelling of cold, abyssal waters that regulates the deep ocean's ability to sequester heat and carbon for decades to millennia (thereby shaping Earth's climate) is driven by centimetre-scale turbulent mixing associated with breaking waves in the ocean interior. Measurements of deep-ocean turbulence over the last two decades, however, starkly contest this scenario, and instead suggest that mixing by breaking waves drives *downwelling* of abyssal waters. Inspired by this conundrum, recent theoretical investigations have developed a tantalising alternative view of the role of mixing in sustaining deep-ocean upwelling. In this new view, upwelling is driven by highly localised turbulence within thin (typically tens of metres thick) layers near the seafloor, known collectively as the bottom boundary layer.In this proposal, we set out to assess the validity of this new paradigm, and figure out how it works, by obtaining the first set of concurrent, systematic measurements of (1) large-scale mixing and upwelling, (2) their interior and bottom boundary layer contributions, and (3) the processes underpinning these contributions, in a representative deep-ocean basin (the Rockall Trough, in the Northeast Atlantic). To this end, we will conduct a year-long field programme involving three research cruises, in close collaboration with a group of U.S. project partners.To measure (1), in the first cruise we will release a long-lived chemical tracer detectable at very low concentrations near the seafloor on the basin's eastern flank, where deep waters flow into the basin from the south. We will monitor how the tracer mixes and upwells as it fills the basin by sampling it at 6 and 12 months after its release, during the two subsequent cruises. To measure (2) and (3), we will adopt a two-pronged approach. First, we will assess mixing and upwelling in the ocean interior and the transition into the bottom boundary layer by deploying turbulence-measuring profilers at hundreds of sites throughout the basin, in the first and third cruises. Second, as these ship-deployed profilers are unsuitable to approach the seafloor closely, we will obtain targeted measurements of mixing and upwelling across the bottom boundary layer in two ways: (i) by conducting two 6-month-long deployments of a cluster of novel, turbulence-measuring instruments moored at the seafloor, at two sites on the basin's sloping boundary; and (ii) by performing a near-bottom release of a short-lived dye at each of the mooring deployment sites, and monitoring how the dye mixes and upwells over several days after its release.We will integrate our measurements of (1)-(3) into the design of a realistic numerical model of the circulation in the Rockall Trough, and use the model to conduct a rigorous, quantitative test of the new paradigm of ocean mixing. We expect this test to provide the first evidence of the dominance of bottom boundary layer turbulence in driving deep-ocean upwelling and, in so doing, trigger a profound transformation in our understanding and modelling of deep-ocean circulation.