Atlantic Ocean circulation influences the atmosphere lying above, playing a key role in 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 important metric in assessing the state of ocean circulation and its impacts. It is one of the most important variables evaluated in ocean and climate models, and is thought to be one of the key drivers behind biodiversity distribution and ecosystem connection in the deep Atlantic.
The AMOC is a strong focus of work in iAtlantic: it connects the individual research themes that consider large-scale ocean circulation and its influence on specific regions, guides the setup of ocean observations, and is an important driver in ecosystem connectivity studies. One element of iAtlantic’s work involves setting up, verifying and analysing ocean models, and providing information about historical circulation patterns (hindcasts) in the Atlantic over the past 60 years, in order to underpin analyses and studies of ecosystem connectivity. Ocean scientists at GEOMAR operate two models to fulfil this goal: VIKING20X covering the full Atlantic, and INALT20 (Schwarzkopf et al., 2019) with a specific focus on the South Atlantic and Agulhas region.
In a recently published paper, Biastoch et al. (2021) describe a hierarchy of global 1/4° (ORCA025) and Atlantic Ocean 1/20° nested (VIKING20X) ocean-sea-ice models. It shows that the eddy-rich VIKING20X configuration performed in hindcasts of the past 50-60 years under CORE and JRA55-do atmospheric forcings 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 its long-term temporal evolution, strongly depends on details of atmospheric freshwater fluxes and their application to the models.
The interannual variability of the AMOC shows high correlation between the model experiments and observational data, including the 2010 minimum observed by the RAPID oceanographic mooring array installed in the Atlantic at 26.5°N. This points to a dominant role of wind forcing. The paper explores the ability of the model to represent regional observations in western boundary current (WBC) systems at 53°N, 26.5°N and 11°S, and the question of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC, are also investigated.
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 particularly good indicator of the subpolar AMOC trend during recent decades. 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, which is a major step forward in ocean modelling.
VIKING20X provides the physically consistent spatial and temporal framework of the ocean observations in the North and South Atlantic and for the determination of basin-scale modes – both important components of iAtlantic’s research. The ultra-high resolution models, e.g. of the Walvis Ridge, are embedded into the basin-scale circulation and hydrography through VIKING20X. Output from the VIKING20X hindcasts is also used to transport virtual biological organisms such as mussel or sponge larvae, and these data will be combined with genomic analyses to better understand the role of ocean circulation in ecosystem connectivity.
Read the full paper: Biastoch, A., Schwarzkopf, F. U., Getzlaff, K., Rühs, S., Martin, T., Scheinert, M., 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