Assessing the Impacts of Arctic Amplification on the Midlatitude Circulation with a Hierarchy of Atmospheric General Circulation Models

北极放大指的是北半球高纬度地区比全球其他地区变暖更快的趋势。这种效应在近几十年的变暖趋势和温室气体引起的气候变化的气候模式模拟中都有记录，并且与北极海冰的减少和欧亚大陆北部秋季积雪的减少密切相关。本项目探讨了北极放大对北半球中纬度地区天气和大气环流的潜在影响。这项工作的动机来自最近的模拟，该模拟表明，在夏末和初秋，北极海冰的减少促进了北环模的负相位，在这个阶段，北极上空的海平面压力很高，北半球的急流和风暴路径向南移动。进一步的动机来自最近的说法，即北极放大导致大气阻塞事件的频率增加，这与极端天气事件有关，包括2013/2014年冬季美国的冷空气爆发以及最近欧洲的夏季热浪。该研究使用不同复杂程度的模型层次，考察了北极放大对大气环流的影响，包括急流、风暴路径和阻塞事件。在这种建模方法中，使用最简单的模型来分离和研究在更复杂和现实的模拟中发现的行为的基本动力学机制。这项研究的一个关键问题是，北极放大将与全球变暖的其他后果一起起作用，包括热带对流层上层的变暖，以前的工作表明，热带对流层上层的变暖导致喷流和风暴路径向极地移动，这将在一定程度上抵消北极放大的影响。第二个问题是平流层在北极和中纬度环流之间的联系中可能发挥的作用。先前的研究表明，极地帽层平流层的环流变化可以影响中纬度的天气，而且这些影响可以缓慢向下传播，导致高纬度强迫和中纬度响应之间的时间滞后长达几个月。如上所述，由于北极放大可能导致北半球中纬度地区，特别是美国和欧洲人口稠密地区的天气模式发生变化，因此本研究的主题具有更广泛的社会影响。即使是急流和风暴路径的细微变化也会导致降雨量和可用水资源的显著变化，阻塞事件是冬季极端寒冷和夏季极端热浪的重要来源。这里进行的研究可能对长期气候预测和近期天气预报都有价值。

Arctic amplification refers to the tendency of the high latitudes of the Northern Hemisphere to warm faster than the rest of the globe. This effect has been documented both in the warming trends of recent decades and in climate model simulations of greenhouse gas-induced climate change, and is closely related to the decline of Arctic sea ice and reductions in Fall snow cover over northern Eurasia. This project addresses the potential for Arctic amplification to influence the weather and atmospheric circulation of the middle latitudes of the Northern Hemisphere. Motivation for the work comes from recent simulations which suggest that reductions in Arctic sea ice in late Summer and early Fall promotes the negative phase of the Northern Annular Mode, in which sea level pressure is high over the Arctic and the Northern Hemisphere jet streams and storm tracks are shifted southward. Further motivation comes from recent claims that Arctic amplification has resulted in an increase in the frequency of atmospheric blocking events, which are associated with extreme weather events including the cold air outbreaks over the US during the winter of 2013/2014 as well as recent summer heat waves over Europe.The research examines the effect of Arctic amplification on atmospheric circulation, including jet streams, storm tracks, and blocking events, using a hierarchy of models of varying levels of complexity. In this modeling approach, the simplest models are used to isolate and study the fundamental dynamical mechanisms responsible for the behaviors found in more complex and realistic simulations. A key concern of the research is that Arctic amplification will act in concert with other consequences of global warming, including the warming of tropical upper troposphere, and previous work suggests that the warming of the tropical upper troposphere leads to a poleward shift of jets and storm tracks which will somewhat offset the impact of Arctic amplification. A second concern is the possible role of the stratosphere in mediating connections between the Arctic and the midlatitude circulation. Previous work shows that circulation changes at stratospheric levels over the polar cap can influence midlatitude weather, and that these influences can propagate slowly downward, resulting in time lags up to a few months between high-latitude forcing and midlatitude response.As noted above, the topic of this research has societal broader impacts due to the potential for Arctic amplification to produce changes in the weather patterns of the Northern Hemisphere midlatitudes, in particular over the populous regions of the US and Europe. Even subtle shifts of jet streams and storm tracks can yield significant changes in rainfall and available water resources, and blocking events are an important source of extreme winter cold and summer heat waves. The research conducted here may be of value for both long-term climate projections and near-term weather forecasting.