Modeling Effects of Greenland Ice Sheet Melting on AMOC Variability and Predictability.

Recent observations reveal accelerated melting of the Greenland Ice Sheet (GrIS). Projections of future effects suggest continuing ice loss at increasing rates for business-as-usual anthropogenic greenhouse gas emissions scenarios. Additional meltwater fluxes into the surrounding North Atlantic ocean will increase the buoyancy of surface waters, which may reduce their rates of convection, subduction and sinking to the deep ocean and hence slow down the AMOC. Detailed estimates of the GrIS mass balance show that it is influenced by North Atlantic climate variability, suggesting a possible feedback between the GrIS and the AMOC. However, most comprehensive climate models currently do not include interactive ice sheets. Thus projections of future climate change performed with these models (including CMIP5) do not consider impacts of GrIS melting on AMOC variability although it is well known that the AMOC is sensitive to freshwater fluxes to the North Atlantic. The probabilities of AMOC reduction and shutdown for a given greenhouse gas emission scenario are therefore poorly known. Moreover, previous studies of AMOC internal variability and predictability did not consider feedbacks between the GrIS and the AMOC. Here we propose to organize a model intercomparison project, involving the major climate modeling centers around the world, aimed at quantifying the effects of GrIS mass balance changes on current and future AMOC variability and predictability including uncertainty estimates. Realistic meltwater scenarios will be developed based on a new approximation of GrIS surface mass balance changes. The meltwater will be distributed to the ocean along the Greenland coast using a realistic runoff scheme. The range of meltwater scenarios will consider uncertainties associated with estimating future mass balance changes. Different state-of-the-science climate models will be forced with these scenarios in addition to standard radiative forcing in order to quantify the AMOC response to warming and meltwater input as well as the uncertainty of model AMOC sensitivities to the imposed forcings. Probabilistic AMOC projections will be computed based on the multi-model ensemble. Simulations with an interactive scheme of GrIS mass balance changes will be used to quantify the effect of ice sheet – ocean interactions on AMOC variability and predictability on decadal to centennial time scales. The model experiments will be carefully analyzed in order to understand responses and model differences. The probability of an AMOC shutdown in the coming two centuries will be quantified. The project will lead to international collaboration between scientists at different modeling centers and a new collaboration between global climate modelers and an expert on observations and detailed mass balance modeling of the GrIS.

Relevance to NOAA’s Long Term Goal of Climate Adaptation and Mitigation: Scientific understanding of the interactions between the cryosphere and the ocean will be advanced through realistic modeling of feedbacks between the GrIS and the AMOC. The combined potential impacts of global warming and melting of the GrIS on the AMOC will be assessed on decadal to centennial time scales. The project will lead to useful predictions of likely climate impacts including associated uncertainties, which can support mitigation and adaptation choices by decision makers. The PIs are actively engaged in education and outreach activities that will continue to improve public climate literacy. Results of this project will be published in the peer reviewed literature and disseminated as broadly as possible, e.g. through press releases via Oregon State University’s News and Research Communications office and interviews with reporters.