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

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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.