Processes drIving Submarine Canyon fluxES

环绕着所有大陆的浅海陆架海通过陡峭的大陆斜坡与深海相连，大陆斜坡从海面下数千米上升到只有几百米。陆架海洋中的浮游植物(微型植物)依赖于沿大陆斜坡向上的深海水流为它们提供生长和繁殖所需的营养物质。浮游植物的光合作用是大陆架海洋食物网的基础，因此所有生物，包括商业上重要的鱼类物种，都受到上坡洋流的影响。顺着大陆坡向下流动也同样关键。通过深海营养物质允许的额外光合作用，将大气中的二氧化碳吸收到陆架水域，只有在碳被运离陆架并被锁在深海中的情况下，才能有效地长期缓冲人为排放。允许洋流在大陆坡上上下流动的机制人们知之甚少，也没有被用于预测未来气候的数值模型准确模拟。在大陆斜坡笔直而平坦的地方，由于水流通常沿着斜坡流动，而不是穿过斜坡，因此横坡运输有限。然而，在大陆架被海底峡谷切割的地方，沿着斜坡的水流被峡谷陡峭的墙壁挡住了。这通过两个关键的物理过程加强了穿过峡谷的横坡输送。第一种是上升和下降--这是由于沿斜坡的水流在陡峭的墙壁上变得不稳定，并被转向或离开陆架造成的。第二个过程是内潮和湍流混合--由地下波冲击陡峭的墙壁而引起。这造成了湍流，并将峡谷边缘上方的深水和浅水混合在一起。由于这两个过程的小尺度和峡谷几何形状的复杂性，观察和破译这两个过程具有挑战性。海底峡谷在世界各地的大陆边缘很常见--由高密度、多泥沙的洋流而不是河流形成--因此它们有可能对全球大陆架上的营养物质总运输量做出重大贡献。该项目旨在观察、理解和预测这两个关键的峡谷过程，以便它们能够被下一代全球模式准确地模拟或现实地近似。为了实现这些目标，我们将深入研究惠塔德峡谷的这两个物理过程，惠塔德峡谷是一个切割凯尔特海大陆架的大型分支系统。在峡谷不同的肢体内，这两个过程预计在跨坡营养盐输送中起到或多或少的作用：一些肢体预计以上升流和下沉流为主；另一些肢体则以内潮和湍流混合为主。我们将使用广泛的技术，包括尖端的自动驾驶车辆、各种船用和系泊仪器，以及最先进的高分辨率海洋模型，来测量关键的海洋属性，帮助我们了解主导过程。具体地说，我们将使用自主海洋滑翔机，配备测量水流速度、溶解营养物质浓度和湍流混合的定制传感器，以确定营养物质在峡谷中的传输。这些滑翔机是由浮力而不是螺旋桨驱动的，所以它们可以用一个电池监测峡谷环境数月。观测和高分辨率模式模拟将是相辅相成的，因此我们能够：(1)调查峡谷几何形状如何控制这两个过程；(2)确定惠塔德峡谷和整个欧洲西北部大陆架断裂的主导过程；(3)评估沿坡流速度、潮汐能和层结变化(海洋分层)如何影响跨坡营养物质输送；(4)改进全球海洋和气候模型对这些过程的模拟。

The shallow shelf seas that surround all the continents are connected to the deep open ocean by a steep continental slope, which rises from thousands of metres beneath the ocean surface to just a few hundred. Phytoplankton (microscopic plants) in shelf seas are reliant on flows of deep ocean water up the continental slope to provide them with the nutrients they require to grow and reproduce. Photosynthesis by phytoplankton is the foundation of the shelf sea food web, so all organisms, including commercially important fish species, are influenced by up-slope currents. Flow down the continental slope is equally critical. Drawdown of atmospheric carbon dioxide into shelf waters by the extra photosynthesis allowed by deep ocean nutrients is only an effective long-term buffer to anthropogenic emissions if the carbon is then transported off the shelf and locked away in the deep ocean.The mechanisms that allow currents to flow up and down the continental slope are poorly understood and are not accurately simulated by the numerical models used to predict future climate. Where the continental slope is straight and smooth, there is limited cross-slope transport because currents generally flow along the slope rather than across it. However, where continental shelves are incised by submarine canyons, along-slope currents are blocked by the canyons' steep walls. This results in enhanced cross-slope transport through the canyons by two key physical processes. The first is upwelling and downwelling - caused by along-slope currents becoming unstable over the steep walls and being turned onto, or off, the shelf. The second process is internal tides and turbulent mixing - caused by subsurface waves breaking against the steep walls. This creates turbulence and mixes deep ocean water with shallow water above the canyon rim. Both processes are challenging to observe and decipher due to their small scales and the complexity of canyon geometry. Submarine canyons are common along continental margins worldwide - created by high-density, sediment-laden currents rather than rivers - so they have the potential to make a major contribution to the total transport of nutrients onto continental shelves globally. This project aims to observe, understand and predict these two crucial canyon processes so that they can be accurately simulated, or realistically approximated, by the next generation of global models. To achieve these aims, we will intensively study both physical processes in Whittard Canyon, a large, branching system that incises the Celtic Sea continental shelf. Within the different limbs of the canyon, the two processes are expected to play greater or lesser roles in cross-slope nutrient transport: some limbs are expected to be dominated by upwelling and downwelling; other limbs by internal tides and turbulent mixing. We will use a broad range of technologies, including cutting-edge autonomous vehicles, a wide variety of ship-board and moored instruments, and state-of-the-art high-resolution ocean models, to measure critical ocean properties and help us understand the dominant processes. Specifically, we will use autonomous ocean gliders equipped with bespoke sensors measuring current velocity, dissolved nutrient concentration, and turbulent mixing, to determine nutrient transport through the canyon. These gliders are driven by buoyancy instead of a propeller, so they can monitor the canyon environment for months on a single battery. The observations and high-resolution model simulations will be complementary so that we can: (1) investigate how canyon geometry controls the two processes; (2) determine which processes dominate in Whittard Canyon and along the whole European northwest shelf break; (3) assess how cross-slope nutrient transport is affected by along-slope current speed, tidal energy, and changes in stratification (layering of the ocean); and (4) improve the simulation of these processes by global ocean and climate models.