Aerosol-Atmosphere-Ocean Interactions: Responses, Mechanisms and Feedbacks in the North Atlantic

北大西洋，即赤道以北的大西洋，在几十年的时间里变暖变冷，整个盆地在20世纪初变冷，在本世纪中叶变暖，从1960年到千年左右变冷，此后略有变暖。变暖和变冷对气候有多种影响，包括每年夏天飓风数量的变化，美国东南部和北非的降雨丰富，以及欧洲冬季气温的变化。一个特别严重的例子是萨赫勒地区在1960年以后的降温期间发生的长期干旱。北大西洋海表温度波动的原因尚不清楚，对于它们是自然气候变率的结果还是外部强迫的结果，存在相当大的争论。在自然变率方面，由于大西洋经向翻转环流（AMOC）的加速和减缓，海温可能发生变化。在AMOC中，冷水在北纬高纬度地区下沉，在全球深处循环，并在上升流后返回赤道以南的表面。在外部强迫方面，人们对人为气溶胶的影响相当感兴趣，这些气溶胶通过将阳光反射回太空来冷却海面。来自北美的气溶胶排放在整个世纪中叶变得更加密集，但从20世纪60年代开始减弱，因此上游气溶胶源对大西洋海温的调制是一个可信的理论。内部变率和外部强迫的支持者之间的争论尚未得到解决，原因包括观测记录较短（只有两个完整的周期），缺乏AMOC观测，以及缺乏对海洋上气溶胶浓度的长期测量。该奖项下的研究利用气候模式模拟和观测数据集解决内部变率与外部强迫的问题。这项工作利用了社区地球系统模式（CESM）进行的大规模综合模拟，其中个别模拟之间的差异纯粹是由于自然气候系统变率。特别是，该项目使用了20世纪气候的模拟集合，其中一种外部强迫被去除，例如，一个集合排除了温室气体的增加，但保留了气溶胶、土地覆盖变化、火山爆发等。除了一个之外的所有模拟套件包括一个集成，该集成仅省略了生物质燃烧产生的气溶胶，同时保留了工业气溶胶，另一个集成仅省略了工业气溶胶。该项目利用这些模拟来观察这两种气溶胶的影响可能存在的差异。此外，该项目使用一种配置生成新的模拟，在这种配置中，通过用一个静止的“板”代替海洋来排除AMOC的影响。该项目还超越了海温变率要么是由于内部变率要么是由于外部强迫的普遍假设，并探讨了AMOC本身受到人为气溶胶影响的可能性。由于北大西洋海温变化的广泛影响，这项工作具有社会价值。特别是，它为预测北大西洋地区气候变化的工作提供了信息，包括一些使用气候模式预测AMOC未来演变的工作。该项目以各种方式开展社区外展活动，包括参与河滨社区活动，与当地组织（包括Osher终身学习研究所）合作，以及通过地球科学教育外展计划（GEOP）与K-12学生合作。此外，该项目还为两名研究生和两名本科生提供支持和培训。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。

The North Atlantic, meaning the Atlantic Ocean north of the equator, warms and cools over multiple decades, with basin-wide cooling in the early 1900s, warming in the mid-century, cooling from about 1960 to the millenium, and slight warming since then. The warming and cooling have a variety of climatic effects, including changes in the number of hurricanes each summer, the abundance of rainfall in the southeast US and North Africa, and winter temperatures in Europe. A particularly severe example is the prolonged Sahel drought that occurred during the cooling period after 1960.The causes of the North Atlantic sea surface temperature (SST) fluctuations are not clear and there is considerable debate as to whether they are the result of natural climate variability or are externally forced. On the natural variability side SST could change due to the speeding up and slowing down of the Atlantic Meridional Overturning Circulation (AMOC), in which cold water sinks in the high northern latitudes, circulates around the world at depth, and returns after upwelling to the surface south of the equator. On the external forcing side there is considerable interest in the effects of anthropogenic aerosols, which cool the sea surface by reflecting sunlight back to space. Aerosol emission from North America became more intensive throughout the midcentury but diminished starting in the 1960s, thus modulation of Atlantic SSTs by upstream aerosol sources is a credible theory. The debate between proponents of internal variability and external forcing has not yet been resolved, for reasons including the short observational record (only two full cycles), the lack of AMOC observations, and the lack of long-term measurement of aerosol concentrations over the oceans.Research under this award addresses the question of internal variability versus external forcing using climate model simulations and observational datasets. The work takes advantage of large ensembles of simulations performed with the Community Earth System Model (CESM), in which the difference between individual simulations is due purely to natural climate system variability. In particular the project uses simulation ensembles of 20th century climate in which one type of external forcing is removed, for instance one ensemble excludes greenhouse gas increases but retains aerosols, land cover change, volcanic eruptions, etc.. The all-but-one simulation suite includes an ensemble which omits only the aerosols from biomass burning while retaining industrial aerosols, and another which omits only industrial aerosols. The project uses these simulations to look at possible differences in the effects of the two aerosol types. In addition, the project generates new simulations using a configuration in which AMOC influence is ruled out by replacing the ocean with a motionless "slab". The project also goes beyond the common assumption that the SST variability is either due to internal variability or external forcing and explores the possibility that the AMOC is itself affected by anthropogenic aerosols.The work has societal value due to the wide-ranging effects of changes in North Atlantic SSTs. In particular it informs efforts to predict climate variations in the North Atlantic sector including some which use climate models to predict the future evolution of the AMOC. The project conducts community outreach in various ways including participation in Riverside community events, partnership with local organizations including the Osher Lifelong Learning Institute, and engagement with K-12 students through the Geoscience Education Outreach Program (GEOP). In addition, the project provides support and training to two graduate students and two undergraduates.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.