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The Atlantic Ocean circulation influences the above lying atmosphere and plays a key role for climate and climate change. It also shapes the distribution of biogeochemistry in the ocean and determines the connectivity of marine ecosystems. The Atlantic Meridional Overturning Circulation (AMOC) summarises the four-dimensional ocean circulation and provides an integral metric for the state of the ocean circulation and its impacts. It is one of the most important variables evaluated in ocean and climate models.
This report is an introduction and summary of a larger report on the Atlantic circulation variability of the last 50 years as simulated by the state-of-the-art high-resolution model VIKING20X that has been accepted to the international peer-reviewed literature (Biastoch et al., 2021). The last 50 years is the timescale that is of interest for most studies in iAtlantic and the best known in respect to ocean observations.
In a peer-reviewed paper, Biastoch et al. (2021) describe a hierarchy of global 1/4° (ORCA025) and Atlantic Ocean 1/20° nested (VIKING20X) ocean-sea-ice models. They show that the eddy- rich VIKING20X configuration performed in hindcasts of the past 50-60 years under CORE and JRA55-do atmospheric forcing realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the AMOC, and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5° N. This points to a dominant role of the wind forcing. The ability of the model to represent regional observations in western boundary current (WBC) systems at 53° N, 26.5° N and 11° S is explored. The question is whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to the one of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5° N, it is shown that the outflow of North Atlantic Deepwater at 53° N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable of representing realistic forcing-related and ocean-intrinsic trends.
Reference: Biastoch, A. et al. (2021) Regional imprints of changes in the Atlantic Meridional Overturning Circulation in the eddy-rich ocean model VIKING20X. Ocean Science, 17(5), 1177–1211. DOI: 10.5194/os-17-1177-2021