Assessing the sensitivity of marginally stratified shelf seas within a changing climate

大陆架海极其重要，因为它们维持着高水平的初级生产力，并有能力吸收和封存大气气体，包括具有气候重要性的温室气体。使它们能够做到这一点的陆架海的关键物理方面是垂直密度分层，在整个春季和夏季，当太阳辐射或淡水的稳定影响克服湍流混合的不稳定影响时，垂直密度分层建立起来。在英国周围的几个地方，如爱尔兰海，由于众所周知的强大潮流在海床产生的摩擦主导效应，在分层和垂直混合良好的水之间形成了成熟的锋面，其影响延伸到整个水柱。然而，在英国大部分沿海水域，潮汐混合并不占主导地位，湍流混合和再分层之间的竞争更为微妙。层结和由此产生的短暂锋面在空间上是暂时的，在时间上是间歇性的。然而，我们目前无法预测这些边缘分层的陆架海将如何应对变化，因为我们不完全了解支配动态平衡的湍流混合和再分层过程的细节。其中，表层波和内波对混合都有重要但不清楚的影响，而表层和海床附近的横向密度梯度最近被证明调制混合和再分层。这种认识的缺乏从我们无法在用于预测未来变化的数值模型中准确地模拟陆架海洋动力学中可见一斑。该项目的重要性源于这样一种认识，即由于气候变化，地球系统未来的变化是不可避免的，在边缘分层的陆架海洋中将特别感受到这种变化，因为湍流混合和再分层之间的平衡对外部扰动非常敏感。此外，为了按照欧盟法律的要求，英国40%的电力来自可再生能源，预计英国将大规模扩张到海洋环境中，以开发其能源资源。至关重要的是，从风、波浪和潮流中提取的能量通常在湍流混合和平衡太阳辐射和淡水输入对再分层的影响方面发挥关键作用。我们需要知道能源提取将对环境产生什么影响，但我们目前受到限制，因为我们从一开始就不了解湍流混合的细节。为了更好地了解在边缘分层的陆架海中哪些过程最重要，我们计划在凯尔特海南部进行一次严格的实地研究和监测活动。锋面的存在和强度表现出一种不受潮汐周期支配的间歇性行为，表明额外的过程对局部动态平衡是重要的。我们将通过对整个水柱的湍流进行一系列现场测量和绘制锋面结构图来确定这些过程是什么。然后，我们将使用海底传感器和陆基雷达监测强迫机制和锋面完整性的长期趋势，这些雷达测量广泛空间区域的面波和海流，以测试我们在整个现场测量过程中确定的重要过程是否确实对观察到的变化负责。该项目的最后一个组成部分将把我们的新知识纳入最先进的数值模式中，这些模式可以将在该项目中观察到的过程扩大到大陆架海洋环境。因此，该项目的最终成果将成为了解和预测英国周围海洋环境未来变化的改进工具。

Continental shelf seas are extremely important because of the high levels of primary productivity that they sustain and their ability to absorb and sequester atmospheric gases including climatically important greenhouse gases. The key physical aspect of shelf seas that enables them to do so is the vertical density stratification, established throughout spring and summer when the stabilizing influence of solar radiation or fresh water overcomes the destabilizing influence of turbulent mixing. In several places around the UK, such as the Irish Sea, well-established fronts form between stratified and vertically well-mixed water due to the well-understood dominant effect of friction generated at the sea bed by strong tidal currents whose influence extends throughout the water column. Throughout the majority of UK coastal waters tidal mixing is less dominant, however, and the competition between turbulent mixing and restratification is more delicately poised. Stratification and the resulting ephemeral fronts are transient in space and intermittent in time. We are currently hindered in our ability to predict how these marginally stratified shelf seas will respond to change, however, because we do not fully understand the details of the turbulent mixing and restratification processes that govern the dynamic balance. Amongst these, surface and internal waves both exert an important but unclear influence over mixing, whilst lateral density gradients near the surface and seabed have recently been shown to modulate mixing and restratification. This lack of understanding is apparent from our inability to accurately simulate shelf sea dynamics in numerical models that are used to predict future changes. The importance of this project stems from the realization that future changes to the Earth system are now inevitable due to climate change and will be particularly felt within marginally stratified shelf seas for which the balance between turbulent mixing and restratification is so sensitive to external perturbations. Furthermore, in order to produce 40% of the UK's electricity from renewable sources as required by EU law, a huge expansion into the marine environment to exploit its energy resources is expected. Crucially, the energy to be extracted from the wind, waves and tidal currents normally play a pivotal role in turbulent mixing and balancing the effects of solar radiation and freshwater input on restratification. We need to know what the effects of energy extraction will be on the environment but we are currently limited by not understanding the details of the turbulent mixing in the first place. To better understand which processes are most important within marginally stratified shelf seas, we plan to conduct a rigorous field study and monitoring campaign in the southern Celtic Sea. The existence and intensity of the front displays an intermittent behavior that is not governed by tidal periodicities, indicating that additional processes are important to the local dynamic balance. We will identify what these processes are by conducting a series of in-situ measurements of turbulence throughout the water column and by mapping the frontal structure. We will then monitor the long term trends in the forcing mechanisms and frontal integrity using seabed sensors and land-based radar that measures surface waves and currents over a broad spatial area to test whether the processes that we identified as important throughout the in-situ measurements are indeed responsible for the observed changes. The final component of the project will incorporate our new knowledge into state-of-the-art numerical models that can up-scale the processes observed within this project to the shelf sea environment. The end-product of this project will therefore constitute an improved tool for understanding and predicting future changes in the marine environment that surrounds the UK.