Bottom Boundary Layer Turbulence and Abyssal Recipes (BLT Recipes)

自20世纪60年代沃尔特·蒙克的开创性工作以来，海洋学家们一直认为，冰冷的深海上涌调节深海数十年至数千年(从而塑造地球气候)的能力，是由厘米级的湍流混合推动的，与海洋内部的破碎波有关。然而，过去20年对深海湍流的测量结果与这一假设完全相反，相反，它们表明，破碎的海浪混合会导致深海水域“向下涌动”。受这一难题的启发，最近的理论研究发展了一种诱人的另一种观点，即混合在维持深海上升流中的作用。在这一新观点中，上升流是由海底附近薄层(通常是几十米厚)内的高度局域湍流驱动的，统称为底边界层。在这一提议中，我们开始评估这一新范式的有效性，并通过获得第一组同步的、系统的测量结果来弄清楚它是如何工作的：(1)大范围的混合和上升流，(2)它们的内部和底部边界层贡献，以及(3)在具有代表性的深海盆地(东北大西洋的罗卡尔海槽)中支撑这些贡献的过程。为此，我们将与一组美国项目合作伙伴密切合作，进行为期一年的三次研究巡航。为了测量(1)，在第一次巡航中，我们将在盆地东侧的海底附近释放一种可在非常低浓度下检测到的长寿命化学示踪剂，那里的深水从南部流入盆地。我们将在示踪剂释放后6个月和12个月，在随后的两次航行中，通过对示踪剂在盆地中的混合和上升流进行采样，监测示踪剂是如何混合和上升的。就第(2)和第(3)项而言，我们会双管齐下。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，来评估海洋内陆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靠近海底，我们将通过两种方式获得关于跨越底部边界层的混合和上升流的有针对性的测量：(I)在盆地倾斜边界的两个地点，进行为期6个月的两次部署停泊在海底的新型湍流测量仪器；以及(Ii)在每个系泊部署地点进行短期染料的近底部释放，并监测染料在释放后几天内如何混合和上升。我们将把我们对(1)-(3)的测量整合到一个真实的罗卡尔海槽环流数值模型的设计中，并使用该模型对海洋混合的新范例进行严格的定量测试。我们期待这项试验将提供第一个证据，证明海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模型的深刻变革。

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.