Seasonal Sensitivity of the Midlatitude Circulation to Future Climate Warming

温室气体浓度增加引起的气候变化一般表现为全球平均温度的上升，但当地所经历的气候影响将部分取决于全球变暖引起的大气环流变化。 特别是，气候模式模拟表明，中纬度高层的西风急流将向两极移动，因此，平均北方半球急流将向北迁移，南半球急流将相应地向南移动。 这些变化虽然很小，但却很重要，因为中纬度地区的降雨和风暴模式在很大程度上取决于急流的位置。 温室效应导致的急流向极地移动是全球变暖模拟的一个强有力的特征，至少从2005年起就一直是一个研究课题，但尚未充分探讨这种响应的季节性和地理特征。 该项目的初步工作表明，在南半球，最大的变化是在秋季（3月至5月），其次是夏季（12月至2月），冬季（6月至8月）和春季（9月至11月）。在北大西洋，从大到小的顺序是秋季（9月至11月），夏季（6月至8月），春季（3月至5月）和冬季（12月至2月）。北太平洋的排序也是不同的，秋季领先，其次是冬季，春季和夏季。到目前为止，已经提出了几种理论来解释为什么急流在全年纵向平均的意义上向极地移动，但这些理论都没有提供季节性（即为什么秋季的变化最大）或区域变化的明确直觉。该奖项下的研究包括对再分析产品的观测数据的分析，以及对耦合模型相互比较项目和使用社区地球系统模型的大型模拟档案库的未来气候的模拟。 这种分析之后的数值实验，使用不同程度的复杂性模型，旨在隔离和研究可能导致季节性和地理上不同的喷气机转变的物理机制。 需要考虑的一个问题是，温室变暖的模式通常包括热带地区深层对流层变暖最大值以及两极附近的表面捕获变暖（称为极地放大）。急流的转变可以被看作是对热带和极地影响的综合反应，这可能具有相反的影响和非常不同的季节性。由于急流转变对当地气候的强烈影响，这项工作具有社会和科学意义，例如，急流向北移动可能会导致目前顺风地区的干燥，白天急流位置伴有偏北雨雪。 此外，该项目将支持和培训两名学生，从而为该研究领域的未来劳动力做出贡献。PI和她的学生还将在年度地球科学会议上组织一次特别会议，以宣传研究结果，并鼓励对该主题进行进一步调查。

The climate change induced by increases in greenhouse gas concentrations is generally characterized as an increase in globally averaged temperature, but the climate impacts experienced locally will be determined in part by the changes in atmospheric circulation that are induced by global warming. In particular, climate model simulations suggest that the westerly jet streams found at upper levels in the middle latitudes will shift towards the poles, so that the mean Northern Hemisphere jet stream will migrate northward, with a corresponding southward shift for its the Southern Hemisphere counterpart. These shifts are small but consequential, as patterns of rainfall and storminess in the middle latitudes are determined in large part by the positions of the jet streams. The poleward shift of jet streams as a consequence of greenhouse warming is a robust feature of global warming simulations and has been a research topic since at least 2005, but the seasonality and geography of the response has not yet been fully explored. Preliminary work for this project shows that in the Southern Hemisphere the largest shift is in the Fall (March to May), followed by Summer (December to February), Winter (June to August), and Spring (September to November). In the North Atlantic the order is, from largest to smallest, Fall (September to November), Summer (June to August), Spring (March to May), and Winter (December to February). The ordering is again different in the North Pacific, with Fall leading by a large margin followed by Winter, Spring, and Summer. By now several theories have been proposed to explain why the jets shift poleward in an year-round, longitudinally averaged sense, but none of these theories provides a clear intuition for the seasonality (i.e. why the shift is largest in Fall) or the regional variations. Research under this award consists of analysis of observational data from reanalysis products and simulations of future climate from the Coupled Model Intercomparison Project and the Large Ensemble archive of simulations using the Community Earth System Model. This analysis is followed by numerical experiments using models of varying levels of complexity, intended to isolate and study possibly physical mechanisms leading seasonally and geographically varying jet shifts. One issue to be considered is that the pattern of greenhouse warming generally includes a deep tropospheric warming maximum in the tropics together with surface trapped warming near the poles (referred to as polar amplification). The jet shifts can be regarded as a combined response to the tropical and polar influences, which are likely to have opposing effects and very different seasonalities.The work is of societal as well as scientific interest due to the strong impacts of jet shifts on local climate, for instance a northward shift of jet streams could cause drying in the region downwind of the present-day jet location accompanied by excessive rain and snow to the north. In addition, the project will support and train two students, thereby contributing to the future workforce in this research area. The PI and her students will also organize a special session at an annual geosciences meeting to publicize results of the study and encourage further investigation of the topic.