Collaborative research: Quantifying Global and Regional Impacts of the Atlantic Meridional Overturning Circulation (AMOC) Slowdown in the 21st (twenty-first) Century

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A denser liquid will sink below a lighter one just as vinegar sinks below oil in a salad dressing. This sinking effect happens in the North Atlantic around Greenland and Iceland, where warm surface water brought by the Gulf Stream cools in the subarctic climate and becomes denser than the water below it. The cold surface water sinks to great depth where it flows slowly southward, circulating throughout the world's oceans before resurfacing and returning to the North Atlantic. The resulting Atlantic Meridional Overturning Circulation (AMOC) moves surface water northward and deep water southward throughout the entire Atlantic basin. The AMOC transports heat northward over its whole length, not just from the warmer tropics to the colder high latitudes of the Northern Hemisphere but across the equator as well, transferring heat from the Southern Hemisphere into the Northern Hemisphere.Simulations of the response of climate to increases in greenhouse gas concentrations consistently show a slowdown of the AMOC as climate warms, a consequence of warming-induced reductions in the density of surface water in the northern North Atlantic. Density reduction is a direct consequence of warmer water temperature but reductions in saltiness also contribute, as fresh water (which is less dense) is added by melting ice sheets and increased precipitation. Also, the increased moisture content of warmer surface air reduces evaporation, and reduced evaporation means less salty surface water since evaporation removes fresh water leaving salt behind. There is observational evidence for AMOC slowdown but the period of record is somewhat short for a definitive assessment.Several climatic effects of AMOC slowdown have been claimed, including changes in precipitation over Europe, an increase in North Atlantic storms tracking into Europe, changes in the frequency and strength of El Nino events, and a southward shift in the intertropical convergence zone (ITCZ), the narrow rain band that typically sits north of the equator over the tropical oceans. Many authors have argued that the AMOC slowdown is responsible for the North Atlantic Warming Hole (NAWH), a region of minimal warming or even slight cooling found in the North Atlantic south of Greenland in most climate change simulations. A reduction in AMOC heat transport into the northern North Atlantic is a reasonable explanation for the warming hole, but it is possible to produce a NAWH in global warming simulations using a model that does not allow AMOC slowdown.Work supported through this award uses a novel methodology to examine the effects of AMOC slowdown and their connection to the NAWH using simulations from an ensemble of four climate models. The simulations use a novel methodology in which just enough salt is removed from the surface ocean in the northern North Atlantic to prevent the AMOC slowdown that would otherwise occur due to greenhouse gas-induced warming. A preliminary simulation performed with a version of the Community Earth System Model (CESM) produces the NAWH as a consequence of AMOC slowdown, along with a reduction in rainfall over the warming hole and several other climatic impacts. But results from a single model are not definitive given that differences in model formulation can lead to substantial differences in model behavior. The project also includes a detailed examination of the mechanisms through which AMOC slowdown causes local and remote responses.The work has societal relevance due to the human impacts of climate change and the need to help decision makers confronting climate change. The Principal Investigators conduct outreach activities at two museums, the Museum of Riverside and the Peabody Museum of Natural History, including public lectures and symposia as well as teacher development workshops. The project supports a graduate student and a postdoctoral fellow, thereby supporting workforce development in this research area.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.