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

大陆架海极为重要，因为它们维持着高水平的初级生产力，并且有能力吸收和隔离大气气体，包括对气候具有重要意义的温室气体。使陆架海能够这样做的关键物理方面是垂直密度分层，这种分层是在整个春季和夏季形成的，当时太阳辐射或淡水的稳定影响克服了湍流混合的不稳定影响。在英国周围的几个地方，如爱尔兰海，在分层和垂直混合良好的水之间形成了成熟的锋面，这是由于众所周知的强大的潮汐流在海床上产生的摩擦的主导作用，它的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，湍流混合和再冰化之间的竞争更加微妙。分层和由此产生的短暂锋面在空间上是短暂的，在时间上是间歇性的。然而，我们目前还无法预测这些边缘分层的陆架海将如何对变化做出反应，因为我们还没有完全了解控制动态平衡的湍流混合和再冰化过程的细节。其中，表面波和内波都对混合产生重要但不明确的影响，而表面和海底附近的横向密度梯度最近被证明可以调节混合和再冰化。从我们无法在用于预测未来变化的数值模式中准确模拟陆架海动力学来看，这种理解的缺乏是显而易见的。这个项目的重要性源于这样一种认识，即由于气候变化，未来地球系统的变化现在是不可避免的，在边缘分层的陆架海中尤其会感觉到，因为湍流混合和再冰化之间的平衡对外部扰动非常敏感。此外，根据欧盟法律的要求，为了使英国40%的电力来自可再生能源，预计英国将大举进军海洋环境，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冷冻的影响方面起着关键作用。我们需要知道能源开采对环境的影响，但我们目前受到限制，因为我们首先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是最重要的，我们计划在凯尔特海南部进行严格的实地研究和监测活动。锋面的存在和强度表现出不受潮汐周期支配的间歇性行为，表明附加过程对局部动态平衡很重要。我们将通过对整个水柱的湍流进行一系列的原位测量和绘制锋面结构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，这些传感器和陆基雷达测量广泛空间区域的表面波和洋流，以测试我们在整个现场测量中确定为重要的过程是否确实是观测到的变化的原因。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型，可以将项目中观察到的过程扩大到大陆架海洋环境。因此，该项目的最终产品将成为理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.