The Influence of Climate Variability and Change on Stratospheric Intrusions of Ozone over North America

地面臭氧对人类健康有害，有时必须减少产生臭氧的活动，以避免危险的臭氧浓度。由于当地人类活动以外来源的臭氧存在，通过限制生产来维持安全臭氧水平的努力变得复杂，因为这种“背景”臭氧水平越高，就需要更严格的生产上限。背景浓度对空气质量管理提出了挑战，因为背景浓度因地而异，也随时间而变化。因此，管理和减轻地面臭氧的需要促使研究背景的行为和支配其时空变化的基本过程。背景臭氧的很大一部分来自平流层，在那里臭氧是由紫外线自然产生的，可以远距离传播。平流层大致从长途航空旅行的飞行高度向上延伸，因此与地面臭氧的产生和影响分离得很好。但是富含臭氧的空气可以在平流层侵入时下降到对流层，这是罗斯比波破裂的结果。在此过程中，大尺度气流发生强烈剪切，平流层空气形成细丝，延伸并折叠进入对流层，与周围空气混合。平流层的贡献意味着臭氧背景可以受到几种已知会影响平流层的大气和气候变率形式的影响，包括厄尔尼诺/南方涛动（ENSO）事件、北环模态（NAM）和准两年一次涛动（QBO）。这里进行的研究旨在了解控制进入北美对流层的平流层臭氧的位置和数量的过程。考虑了两个因素，第一个是存在于平流层最底层的臭氧量，在那里臭氧可以通过入侵混入对流层。该储存库中的臭氧量受平流层赤道到极翻转环流等过程控制，而平流层翻转环流可能受到ENSO、NAM和QBO变率的潜在影响。第二个因素是北太平洋急流和伴随的风暴路径的南北向移动，它调节了产生入侵北美西部的罗斯比波破裂事件的频率和位置。众所周知，这种急流转移是ENSO和NAM变率的结果。这项研究是通过卫星数据和再分析产品的观测分析，以及利用全大气群落气候模式（WACCM，作为群落地球系统模式的一部分而开发的）的计算机模拟相结合进行的。如上所述，这项工作具有更广泛的影响，因为需要为缓解行动提供信息，以解决地面臭氧污染对健康的影响。除了污染问题外，这项工作还与受平流层侵入影响的几种地表天气形式有关。特别是，深层侵入可以调节雷暴活动并增加地面风，从而增加野火的发生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Ground-level ozone is harmful to human health, and ozone-producing activities must sometimes be curtailed to avoid dangerous ozone concentrations. Efforts to maintain safe ozone levels by limiting production are complicated by the presence of ozone from sources other than local human activity, as higher levels of this "background" ozone necessitate more stringent production caps. The background concentration poses a challenge for air quality management as background levels vary from place to place and also change over time. Thus the need to manage and mitigate ground-level ozone motivates research on the behavior of the background and the fundamental processes that govern its variability in time and space.A significant portion of the background ozone comes from the stratosphere, where ozone is produced naturally by ultraviolet sunlight and can be transported over great distances. The stratosphere extends upward from roughly the flight level of long-distance air travel and is thus well separated from ground-level ozone production and impacts. But ozone-rich air can descend into the troposphere during stratospheric intrusions, which occur as a result of Rossby wave breaking. In this process the large-scale flow becomes strongly sheared and stratospheric air forms filaments which extend and fold into the troposphere and mix with ambient air. The stratospheric contribution implies that the ozone background can be influenced by several forms of atmospheric and climate variability that are known to affect the stratosphere, including El Nino/Southern Oscillation (ENSO) events, the Northern Annular Mode (NAM), and the Quasi-Biennial Oscillation (QBO).Research performed here seeks to understand the processes which control the location and amount of stratospheric ozone entering the troposphere over North America. Two factors are considered, the first of which is the amount of ozone present in the lowest layer of the stratosphere where it can be mixed into the troposphere by intrusions. The amount of ozone in this reservoir is controlled by processes including the equator-to-pole overturning circulation of the stratosphere which can potentially be influenced by ENSO, NAM, and QBO variability. The second factor is north-south shifts of the North Pacific jet stream and accompanying storm track, which modulate the frequency and location of Rossby wave breaking events that produce intrusions over western North America. Such jet shifts are known to occur as a consequence of ENSO and NAM variability. The research is conducted through a combination of observational analysis, working from satellite data and reanalysis products, and computer simulations using the Whole Atmosphere Community Climate Model (WACCM, developed as part of the Community Earth System Model).As noted above, the work has broader impacts due to the need to inform mitigation actions to address the health impacts of ground-level ozone pollution. In addition to pollution concerns the work has relevance to several forms of surface weather that are affected by stratospheric intrusions. In particular, deep intrusions can modulate thunderstorm activity and increase surface winds leading to enhanced wildfire occurrence.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.