Understanding Climate Change in the North Atlantic Region

人为温室气体排放对全球平均气候的许多影响现在是众所周知的。然而，区域影响的各个方面仍然存在很大的不确定性，特别是在像北大西洋这样复杂的区域。大西洋西部边缘是世界上中纬度风暴的主要增生区之一。然后，这些气流沿着急流前进，形成北大西洋风暴路径，为欧洲带来降水和较温和的温度。近几十年来，与大西洋急流的强度和位置有关的可变性导致了欧洲冬季气候的巨大变化，从20世纪60年代的严寒到90年代更温和、更潮湿的条件。北大西洋涛动(NAO)描述了这种变化，它本质上是对横跨北大西洋的西风平均强度的衡量。如果西风比往常强，欧洲受大西洋温和海上影响的影响更大，NAO被认为是正相，而如果比正常弱，影响更多的是来自较冷的欧亚大陆(负相)。NAO被广泛认为是影响欧洲的自然气候变化的主导模式。关于NAO变化背后的机制，还有很多需要了解。过去几十年的变化是人为气候变化的结果，还是仅仅是自然变化的结果，目前还没有达成共识。对于北大西洋公约组织对气候变化的反应，现代气候模型也存在分歧。有几个模型预测了未来更积极的情况，但其他模型不能。在风暴路径对气候变化的反应上，气候模型才刚刚开始趋同。在太平洋，大多数目前的模型现在预测风暴路径将向极地移动，但在大西洋，人们对此的看法较少。这里的情况更加复杂，因为不同的模型预测了NAO和大西洋经向翻转环流的不同变化，大西洋经圈翻转环流将热量输送到遥远的北方。模型之间的这些分歧是气候变化对欧洲地区影响的不确定性的一个关键来源。这项工作旨在提高我们对分歧背后原因的理解，以便减少这种不确定性。雷丁大学最近的研究表明，NAO的长期变化在很大程度上是由于大西洋发生的一种特殊天气系统的变化。这些系统是高层大气中大尺度波破裂的结果，与欧洲上空常见的阻塞高压系统非常相似。这些事件频繁发生的时段平均将出现弱西风，因此将被归类为负NAO时段。在这个项目中，将研究来自最新气候模型的数据，这些数据有助于IPCC最新的报告，使用这种新的视角，试图理解他们预测的NAO反应背后的原因。如果我们能够理解模式平均气候的变化将如何影响破浪的可能性或首选位置，那么我们就可以理解它对NAO的反应。气候模型中关于北大西洋风暴路径变化的分歧可能至少部分是因为有几个相互竞争的机制在起作用。例如，大气变暖的模式将推动风暴路径向极地移动，就像在其他海洋盆地中看到的那样，但与子午线翻转环流减弱相关的海表面温度变化可能会导致赤道移动。在这里，我们计划使用英国气象局最近开发的一项新技术来区分这些不同的影响，并量化每一种影响的相对重要性。

Many of the impacts of anthropogenic greenhouse gas emissions on the global average climate are now well-known. However much uncertainty remains over aspects of the regional impact, especially in a region as complex as the North Atlantic. The western edge of the Atlantic is one of the world's premier growth regions for mid-latitude storms. These are then steered along the jet stream, forming the North Atlantic storm track and bringing precipitation and milder temperatures to Europe. Variability associated with the strength and location of the Atlantic jet has caused dramatic variations in European winter climate over recent decades, from the bitterly cold winters of the 1960s to much milder and wetter conditions in the 1990s. This variability is described by the North Atlantic Oscillation, or NAO, which is essentially a measure of the average strength of the westerly winds across the North Atlantic. If the westerly winds are stronger than usual, Europe is affected more by the mild maritime influence of the Atlantic, and the NAO is said to be in a positive phase, whereas if they are weaker than normal the influence is more from the colder Eurasian continent (the negative phase). The NAO is widely recognised as the dominant pattern of natural climate variability affecting Europe. Much remains to be understood of the mechanisms behind changes in the NAO. There is no consensus on whether the changes over the last few decades are a result of anthropogenic climate change or simply due to natural variability. Modern climate models also disagree over the response of the NAO to climate change. Several models predict more positive conditions in the future, but other models do not. Climate models are only just beginning to converge on the response of the storm tracks to climate change. In the Pacific most current models now predict that the storm track will move polewards, but in the Atlantic there is less agreement. Here the situation is more complicated because different models predict different changes in the NAO, and also in the Atlantic Meridional Overturning Circulation in the ocean, which transports heat towards the far north. These disagreements between models are a key source of uncertainty in the regional effects of climate change over Europe. This work aims to improve our understanding of the reasons behind the disagreements, so that this uncertainty can be reduced. Recent work at the University of Reading has suggested that the long-term variations in the NAO are largely due to variations in the occurrence of a particular kind of weather system in the Atlantic. These systems result from the breaking of large scale waves in the upper atmosphere, and are very similar to the blocking high pressure systems often seen over Europe. Periods during which these events occur frequently will exhibit weak westerly winds on average, and so will be classed as negative NAO periods. In this project, data from state of the art climate models contributing to the latest IPCC report will be studied, using this new perspective in an attempt to understand the reasons behind the NAO responses they predict. If we can understand how changes in the mean climate of a model will affect the likelihood, or preferred location, of wave-breaking, we can then understand its NAO response. It is likely that the disagreement over North Atlantic storm track changes in climate models arises at least partly because there are several competing mechanisms at work. For example, the pattern of atmospheric warming would act to push the storm track polewards, as seen in other ocean basins, but sea surface temperature changes associated with a weakening of the Meridional Overturning Circulation are likely to lead to an equatorwards shift. Here we plan to use a new technique recently developed at the Met Office to separate out these different effects, and quantify the relative importance of each one.