Evaluating the Roles of the Ocean, the Atmosphere, and External Forcing in Atlantic Multi-Decadal Variability

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Sea surface temperatures (SSTs) over the Atlantic north of the equator show a form of variability in which the entire basin slowly warms and cools, which is referred to as Atlantic Multidecadal Variability (AMV). AMV can be characterized by the Atlantic Multidecadal Oscillation (AMO), formed as the average detrended SST in the North Atlantic (equator to 55 North) after smoothing. The roughly 100-year record of the AMO shows two cycles of oscillation, with cooler temperatures in the early 1900s, warming from the 1920s to about 1960, cooling again from the 1960s to around the millenium, and warming after that. These temperature variations matter for the surrounding land areas as warmer SSTs are associated with increased rainfall in the southeast US and North Africa, increased hurricane activity, and warmer winters in Europe and North Africa. Generally opposite conditions prevail when the AMO is in its cool phase, for example the Sahel drought occurred during the most recent cold period.The driving mechanisms of AMV are not well understood. Variations in the strength of the Atlantic meridional overturning circulation (AMOC) are the most commonly invoked explanation for the slow fluctuations of the AMO, and simulations in which AMOC strength is deliberately reduced show an AMO-like SST response. But recent work by the PIs and collaborators suggests that AMO-like variability can be produced in models which do not represent the AMOC. They looked at climate model simulations in which the ocean model was replaced by a non-circulating "slab" representing the upper ocean, and these simulations produced the AMO SST pattern and behavior similar to standard climate models. Other work suggests that AMO variability can be generated by external forcing factors such as and anthropogenic aerosols and volcanic aerosols.In this project the PIs perform a series of numerical model experiments designed to address the roles of external forcing and oceanic and atmospheric processes in driving AMO variability, with further effort devoted to understanding AMO/AMV impacts on surrounding landmasses. The work addresses three general questions:1) How much is the ocean contributing to multi-decadal variations in the Atlantic basin? What are the dominant ocean processes responsible for this?2) How much is the atmosphere contributing, including externally forced changes in composition, as well as cloud-radiative feedbacks?3) What are the causes of the impacts associated with the AMV around the Atlantic basin?These questions are addressed using a combination of diagnostic heat budget analysis and numerical experiments with a hierarchy of model configurations designed to isolate or eliminate particular processes. In addition to the standard climate model and "slab" ocean model configurations discussed above, the PIs use an ocean model configuration in which each ocean column simulates vertical mixing but ocean circulation is still not simulated (see AGS-1650209). Another configuration allows ocean circulation (including the AMOC) but does not allow changes in the surface winds to affect the circulation, thereby ruling out explanations in which winds and ocean currents act together to generate the AMO. Research on the role of external forcing uses an aerosol dataset from the Max Planck Institute in which analytically specified plumes represent aerosols from the major emission centers. The work has societal as well as scientific relevance given the connections between AMO variability and surface climate on surrounding landmasses, including hurricane activity that affects the US east coast. The work also promotes the development and use of a hierarchy of models connected to the freely available Community Earth System Model, and these model configurations will be made available to the research community. In addition, the work supports a graduate student and includes outreach to K-12 students and the general public.