Effects of rapid Arctic climate change on jet streams and extreme weather (Ref 4606)

在北极，人为排放的二氧化碳造成的低层大气变暖比世界其他地区大几倍（见图1），这一现象被称为北极放大。然而，这种放大的变暖的重要性可能远远超出北极。中纬度急流（图2）及其相关的风暴路径，有效地由较温暖的低纬度和较冷的高纬度之间的温差提供动力。北极放大作用降低了最低层大气的温度梯度，可能会影响急流和风暴。这种变化可能导致极端天气的增加，这对生活在急流路径下的数十亿人的生活和生计尤其令人担忧[例如，Cohen等人，2014年]。北极放大效应是一种强有力的现象，在最先进的气候模型中清晰可见，并且相对较好地理解。然而，急流对北极放大效应的反应更加不确定，并且在不同的气候模型实验之间变化很大（Screen等人，2018; Screen和Blackport，2019）。在我们更好地了解急流受北极放大作用影响的方式之前，我们无法熟练地预测极端天气的未来变化，这是一个重大问题。不同模式之间缺乏强有力的急流响应，这可能反映了它们模拟当今气候的方式存在差异。例如，急流的平均位置在一些模型中比其他模型更北，这可能影响急流在一个模型中与另一个模型相比受到北极放大的影响有多强烈（Smith等人，2017年）。此外，北极放大现象与气候变化的其他强有力特征同时发生，包括热带对流层上层变暖，这也将影响中纬度急流的行为。了解这些过程如何相互作用，以及它们对急流的综合影响是非常重要的。通过了解是什么原因导致模型的不同，我们寻求减少中纬度气候和极端天气的未来预测的不确定性。项目目标和方法我们将使用一个新的，相对简单，但高度可配置的全球气候模型称为ISCA，由汤姆森博士和其他人在埃克塞特（execlim.github.io/IscaWebsite/）开发的实验。与大多数气候模型不同，Isca有能力开启和关闭相关过程，并将大气“轻推”到不同的背景状态。利用这些特征，我们可以研究急流如何以受控的方式响应北极放大，并确定哪些过程是最重要的。我们还将利用已经使用复杂气候模型运行的新实验的输出，作为极地放大模型相互比较项目（PAMIP; Smith等人，2019）由史密斯博士和屏幕教授领导。我们将结合联合收割机的数据从PAMIP和CMIP 6更传统的所有强迫实验的结果，以及从ISCA的结果，以更好地了解当今气候的偏差如何影响急流属性的预测，以及对极端天气的影响。

Project BackgroundThe lower atmospheric warming as a result of manmade CO2 emissions is several times larger in the Arctic than in other parts of the world (see figure 1), a phenomenon known as Arctic Amplification. The importance of this amplified warming could extend well beyond the Arctic, however. The midlatitude jet streams (figure 2), and their associated storm tracks, are effectively powered by the temperature difference between the warmer low latitudes and the colder high latitudes. Arctic amplification decreases this temperature gradient in the lowermost atmosphere, potentially impacting the jet stream and storms. Such changes could lead to an increase in extreme weather, which is a particular concern for the lives and livelihoods of the billions of people who live under the path of the jet stream [e.g., Cohen et al., 2014]. Arctic amplification is a robust phenomenon that appears clearly in state-of-the art climate models, and is relatively well understood. However, the response of the jet stream to Arctic amplification is far more uncertain, and varies substantially between different climate model experiments (Screen et al., 2018; Screen and Blackport, 2019). Until we better understand the ways that the jet stream is affected by Arctic amplification, we cannot skilfully predict future changes in extreme weather, which is a significant problem. This lack of a robust jet stream response across models may reflect differences in how they simulate the present-day climate. For example, the average location of the jet stream is further north in some models than others, which might affect how strongly the jet stream is affected by Arctic amplification in a one model compared to another (Smith et al., 2017). In addition, Arctic amplification occurs alongside other robust features of climate change, including the tropical upper-tropospheric warming that will also influence the behaviour of the midlatitude jet. Understanding how these processes interact, and what their combined effect is on the jet stream is of significant interest. By understanding what causes models to differ, we seek to reduce uncertainty in future projections of midlatitude climate and extreme weather.Project Aims and MethodsWe will conduct experiments using a new, relatively simple but highly configurable global climate model known as Isca, developed by Dr Thomson and others at Exeter (execlim.github.io/IscaWebsite/). Isca has the ability, unlike most climate models, to be able to turn relevant processes on and off, and to 'nudge' the atmosphere to different background states. By using these features, we can investigate how jet stream responds to Arctic amplification in a controlled manner, and ascertain which processes are the most important. We will also make use of output of new experiments already run using sophisticated climate models as part of the Polar Amplification Model Intercomparison Project (PAMIP; Smith et al., 2019) lead by Dr Smith and Prof Screen. We will combine the data from PAMIP and results from more traditional all-forcing experiments from CMIP6, as well as results from Isca to understand better how biases in present day climate can influence predictions of jet stream properties, and the impacts on extreme weather.