How does stably-stratified shear-driven turbulence mix our oceans and estuaries?

这项研究的最终目的是减少气候变化对英国和世界社会的有害影响。问题的根源是由化石燃料二氧化碳的温室效应引起的全球变暖。随着我们的大气变暖，我们的海洋也变暖，这直接影响到生物多样性，并导致海平面上升。随着我们的海洋变暖，维持海洋持续运动的力量平衡也会发生变化，扰乱它们在全球范围内的循环。无论是从短期还是从长期来看，这种干扰都令人担忧，因为目前的环流模式至少对我们宜人的气候具有两项至关重要的功能。首先，垂直洋流将多余的热量和碳储存到海洋深处(减缓全球变暖)。其次，南北洋流将热带热量重新分配到更温和的地区(减少极端天气和气候)。因此，这些洋流的减弱可能会加速气候变化，产生长期的社会后果。为了缓解这种影响，科学家们试图通过使用先进的海洋环流数学和计算机模型来预测世界气候将如何演变。然而，这些模型及其预测需要改进，才能更好地造福社会和决策者。这些模型中不确定性的一个严重原因在于具有不同盐度或温度(从而密度)的水流之间的混合。不同密度的水流组织成垂直堆叠的(或“稳定分层”的)层，以不同的速度彼此流动(形成“剪切”流)。这些流动始终是湍流的，这意味着大量微小的混沌“涡流”以复杂和不可预测的方式混合了大得多的层的盐度和温度。需要更好地理解稳定分层剪切流中的这一基本但极具挑战性的湍流混合过程。为此，我将在三个步骤中采用以下科学方法。首先，我将在实验室中使用此类流动的精确、缩小比例的模型。这有两大好处：它使我能够完全控制流动的几何形状、密度差、流速等，使我能够单独测试和了解每个参数的影响；它使我能够使用高科技测量来比以往任何时候都更好地量化混沌涡旋及其混合。第二，我将用湍流混合的数学模型来解释这些新的实验室测量结果，将我的发现推广(或“推断”)到在海洋中发现的真实尺度的流动。这一关键步骤依赖于流体力学中的“量纲分析”的力量，这也是工程师们经常使用的，从较小规模的实验室原型开发新的飞机或船舶设计。第三，我将通过将我的真实规模预测与从船舶上获得的实际测量(通常是稀疏的、昂贵的、不太准确的)进行比较来验证它们的有效性。这一步类似于工程师在生产前进行全面测试，不同之处在于我们无法控制海洋。尽管具有挑战性，但这一“验证”步骤将有助于确保我的整个方法成功地为气候科学家提供更准确的海洋混合模型。除了全球变暖的长期影响外，我还将上述三个步骤应用于较短期的后果：河口咸水入侵。海平面上升、更频繁的干旱、极端风暴潮和更强的潮汐都将增加人口稠密的三角洲(包括英国重要的泰晤士河流域)对海水的逐渐侵蚀。淡水下方密集咸水层的上游入侵及其垂直混合减少了地表淡水的可获得性，对世界各地已经感受到的沿海社区造成了毁灭性的后果。我将开发更准确的咸水入侵混合模型，以帮助缓解这个紧迫的问题。

This research is ultimately motivated by reducing the harmful consequences of climate change on society, in the UK and worldwide.The root of the problem is global warming, caused by the greenhouse effect of carbon dioxide from fossil fuels. As our atmosphere warms, so do our oceans, which directly affects biodiversity and causes sea levels to rise. As our oceans warm, the balance of forces that keep them in constant motion changes too, disrupting their worldwide circulation. This disruption is worrying, both in the short and long term, because the present circulation patterns perform at least two functions vital to our hospitable climate. First, vertical currents store excess heat and carbon deep into the ocean (slowing global warming). Second, North-South currents redistribute tropical heat to more temperate regions (reducing extreme weather and climate). Therefore, a weakening of these currents could accelerate climate change, with long-lasting societal consequences.To mitigate this, scientists try to predict how the world's climate will evolve by using advanced mathematical and computer models of the ocean circulation. However, these models and their predictions need to be improved to be of greater benefit to society and decision-makers. A serious cause of uncertainty in these models lies in the mixing between water currents that have different salinity or temperature (and thus density). Currents of different densities organise into vertically-stacked (or "stably-stratified") layers which flow past one another at different speeds (creating a "shear" flow). These flows are always turbulent, which means that a vast number of tiny chaotic "eddies" mix the salinity and temperature of much larger layers in complex and unpredictable ways.This fundamental but extremely challenging process of turbulent mixing in stably-stratified shear flows needs to be better understood. To do this, I will employ the following scientific approach in three steps.First, I will use an accurate, reduced-scale model of such flows in the laboratory. This has two great benefits: it gives full control over the flow geometry, the density difference, flow speed, etc, allowing me to test and understand the influence of each parameter separately; and it allows me to use high-tech measurements to quantify the chaotic eddies and their mixing better than ever before.Second, I will interpret these new laboratory measurements with mathematical models of turbulent mixing to generalise (or "extrapolate") my findings to real-scale flows found in the ocean. This crucial step relies on the power of "dimensional analysis" in fluid dynamics, which is also routinely used by engineers to develop new aircraft or ship designs from smaller-scale laboratory prototypes.Third, I will verify the validity of my real-scale predictions by comparing them to actual measurements taken from ships (which are usually sparse, expensive, and less accurate). This step is similar to engineers performing a full-scale test before production, except that we have no control over the ocean. Although challenging, this "validation" step will help ensure that my whole approach succeeds in providing climate scientists with more accurate models for ocean mixing.In addition to the long-term effects of global warming, I will also apply the above three steps to a shorter-term consequence: saltwater intrusions in estuaries. Sea level rise, more frequent droughts, extreme storm surges, and stronger tides will all increase the gradual encroachment of seawater in densely-populated deltas (including the important Thames Basin in the UK). The upstream intrusion of a dense saltwater layer beneath the fresh river water, and their vertical mixing reduce the availability of surface freshwater, with devastating consequences for coastal communities already felt around the world. I will develop more accurate models of mixing in saltwater intrusions to help mitigate this urgent problem.