Understanding the impact of Anthropogenic Aerosol emissions on North Atlantic Multi-decadal Variability

在过去的150年里，北大西洋的气温在几十年的时间里摇摆不定，要么温暖，要么寒冷。这种现象被称为大西洋多年代际变率（AMV），它与区域气候的一系列重要变化有关。例如，当北大西洋比平均温度低时，大西洋飓风的数量就会减少，但萨赫勒地区或北方欧洲发生严重干旱的风险就会增加。许多研究表明，通过预测大西洋表面温度的年代际变化，我们可以提前几年甚至几十年预测区域气候。计算机气候模式模拟表明，大西洋表面温度的年代际变化是一种自然现象，与大西洋环流强度的变化有关。然而，最近由英国气象局领导的研究对这一范式提出了挑战，认为欧洲和北美重工业排放的颗粒污染物（称为气溶胶）的变化是改变大西洋表面温度的关键因素。这是因为气溶胶可以改变北大西洋云层的反射率。因此，更多的气溶胶通过使云层反射更多的阳光来冷却北大西洋，因此，气溶胶污染物排放的数十年变化可以调节大西洋的温度。到目前为止，对颗粒污染物变化的评估主要集中在简单的措施上（例如平均表面温度）。此外，所提出的机制与海洋和大气环流发生的其他重要变化的证据不一致，而这些变化也会改变大西洋的温度。不幸的是，我们对这种气溶胶污染将如何影响北大西洋的理解受到我们以前气候模拟的弱点的限制。气候模拟在模拟北大西洋和重要的土壤污染过程方面经常有问题。许多不同的过程或外部影响也在同时发生，因此，需要用最先进的模式对简化实验进行详细分析，以取得进展。本项目将使用最先进的气候模拟实验来了解北大西洋和大气层如何对来自北美和欧洲的气溶胶污染排放的简化变化作出反应。这些实验将使用最新版本的英国气象局气候模型，该模型对北大西洋气候的模拟有很大的改进，并已被证明在预测大西洋地区的大气环流，海洋温度和降水变化方面具有前所未有的技能。该模式还大大改进了对气溶胶污染及其与云的相互作用的模拟。学生将重点描述气溶胶污染对北大西洋的影响，并阐明不同过程的相对重要性，这些过程塑造了所造成的时空变化模式。学生还将评估北大西洋的变化如何影响区域气候的其他方面，如飓风或降雨。通过与现有的观察结果进行比较，学生将通过比较实验与观察到的变化来评估模拟反应的真实性。最后，学生将评估在十年时间尺度上对气候预测的影响。提高对气溶胶如何影响这些复杂现象的理解将通过气候模型的发展带来好处，并最终提高北大西洋气候预测的信心。

Over the past 150-years North Atlantic temperatures have swung between periods, many decades long, which were eitheranomalously warm or cold. This phenomenon has become known as Atlantic Multi-decadal Variability, or AMV, and it hasbeen linked to a wide range of important changes in regional climate. For example, when the North Atlantic is cooler thanaverage, there are less Atlantic Hurricanes, but the risk of severe drought in the Sahel region or Northern Europe isincreased. Many studies have shown that by predicting these multi-decadal changes in Atlantic surface temperature wecan improve predictions of regional climate years to decades in advance.Climate simulations with computer models suggest that multi-decadal changes in Atlantic surface temperatures is a naturalphenomenon, and is related to changes in the strength of the Atlantic Ocean circulation. However, recent research led bythe UK Met Office has challenged this paradigm by suggesting that changes in particulate pollution - known as aerosols -that are emitted from heavy industry in Europe and North America was a key factor in changing Atlantic surfacetemperatures. This is because the aerosols can modify the reflectiveness of clouds in the North Atlantic. Thus, moreaerosols act to cool the North Atlantic by making clouds reflect more sunlight, and, thus, multi-decadal changes in theemissions of aerosol pollution could modulate the temperatures of the Atlantic.So far, the evaluation of the changes in particulate pollution has focused on simple measures (e.g. average surfacetemperatures). Furthermore, the proposed mechanisms are at odds with evidence that other important changes in oceanand atmospheric circulation have occurred, which we would also expect to change Atlantic temperatures.Unfortunately, our understanding of how this aerosol pollution will affect the North Atlantic is limited by weaknesses in ourprevious climate simulations. Climate simulations often have problems at simulating the North Atlantic, and the importantaerosol pollution processes. Many different processes or external influences are also occurring at the same time.Therefore, detailed analysis of simplified experiments with state-of-the-art models is needed to make progress.This project will use state-of-the-art climate simulation experiments to understand how the North Atlantic Ocean andatmosphere respond to simplified changes in aerosol pollution emissions from North America and Europe. Theseexperiments will use the latest version of the UK Met Office climate model, which has a much improved simulation of theNorth Atlantic Climate, and has been shown to have unprecedented skill at predicting changes in atmospheric circulation,ocean temperatures and precipitation in the Atlantic Sector. The model also has a significantly improved simulation ofaerosol pollution, and its interaction with clouds.The student will focus on describing the impact of aerosol pollution on the North Atlantic and clarifying the relativeimportance of the different processes that shape the temporal and spatial pattern of changes that result. The student willalso evaluate how the changes in the North Atlantic impact on other aspects of regional climate, such as hurricanenumbers, or rainfall. By comparing with available observations, the student will assess the realism of the simulatedresponse by comparing the experiments with the observed changes. Finally, the student will assess the implications forclimate predictions over a decadal time-scale.Improving the understanding of how aerosols affect these complex phenomena will bring benefits through development ofclimate models, and ultimately, improved confidence in climate predictions of the North Atlantic up to a decade in advance.