ABSTRACT

Efforts to address climate change impacts are complicated by the difficulty of distinguishing between the effects of external forcing, predominantly caused by human activities, and the natural internal variability of the climate system. Decade-to-decade variations are typically characterized by a set of climate modes such as the Atlantic Multidecadal Oscillation (AMO), in which the Atlantic north of the equator has alternating episodes of warming and cooling that last 20 to 40 years. The climate modes are generally regarded as natural variability because they can be found in observations from the 1800s and earlier, before the rise of industrial emissions. For this reason regional climate change is often viewed as the superposition of natural variability in the form of climate modes and externally forced change which is distinct from the modes, with a different spatial pattern and involving different physical mechanisms.

Here the Principal Investigators (PIs) question this assumption and pose a more challenging hypothesis: prior to 1950 climate modes are largely natural variability arising from atmosphere-ocean coupling, but after 1950 the modes are largely externally forced, and the relationship between the ocean and atmosphere is altered by the external forcing. The hypothesis is motivated by the PIs' recent work with simulations of 20th century climate which include forcing by the radiative effects of aerosol pollution. The dominant sources of aerosol emissions have shifted from western Europe and the eastern US to China and southern Asia, thus the response to shifting aerosol radiative forcing could cause regional climate change on the decadal timescale. The hypothesis is also predicated on the idea that the climate system responds to external forcing through the dynamics that generate the internal variability modes, so that the response to external forcing amounts to an excitation of the natural internal modes.

The research is conducted with a variety of climate model simulations including large ensembles (LEs) of simulations performed with the Community Earth System Model (CESM). These include LEs of simulations with no external forcing and single-forcing simulations which isolate the response to aerosol forcing. The work also takes advantage of the hierarchy of simpler configurations developed for CESM, including the "pencil ocean" configuration developed under AGS-2040073, which represents turbulent vertical heat transfer in the ocean but does not simulate the lateral movement of water in ocean currents.

The work has societal relevance given the need to address climate change impacts at the regional level. If the PIs' hypothesis is correct it will require some revision of uncertainty estimates for regional climate projections, which do not currently take into account uncertainty in the future evolution of aerosol emissions. The work also has implications for climate prediction efforts, in which it is commonly assumed that climate modes can be predicted from the current state of the atmosphere and oceans and a sufficiently accurate representation of the coupling between the two (see for example AGS-2231237). The work is also directly relevant to climate impacts on coastal communities, particularly given that the AMO has a substantial effect on Atlantic hurricane activity. The researchers work with the Miami Climate Resilience Committee and the Resilient305 Research Collaborative provides a direct application of research results to community-based decision making. The project also provides support and training to a postdoctoral associate, a graduate student and an undergraduate, and the PIs perform a number of outreach activities including the University of Miami Women in Science day, an annual event for girls in the 6th and 7th grade.

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.

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