Atlantic meridional overturning circulation

Atlantic meridional overturning circulation
R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP. · CC BY 3.0 · source
Name	Atlantic meridional overturning circulation
Othernames	AMOC
Region	Atlantic Ocean
Type	Ocean circulation
Components	Gulf Stream, North Atlantic Current, Labrador Sea, Denmark Strait, Irminger Sea
Significance	Climate regulation, heat transport

Atlantic meridional overturning circulation is a large-scale system of ocean currents in the Atlantic Ocean that transports heat, salt, and nutrients between the tropics and high latitudes. It links surface currents such as the Gulf Stream and North Atlantic Current with deep water formation regions including the Labrador Sea and the Denmark Strait, influencing climate across Europe, North America, and beyond. Research programs by institutions like the National Oceanic and Atmospheric Administration, Woods Hole Oceanographic Institution, and European Space Agency study its variability and potential changes under anthropogenic forcing.

Overview

The circulation functions as part of global thermohaline connectivity involving the Southern Ocean, Arctic Ocean, and basins bordered by Africa and South America. Surface poleward heat transport via the Gulf Stream and North Atlantic Current is balanced by equatorward return flow at depth through pathways near the Mid-Atlantic Ridge and the Charlie-Gibbs Fracture Zone. Basin-scale exchanges connect to features like the Sargasso Sea, the Rockall Trough, and the Irminger Sea, while large-scale atmospheric teleconnections link to the North Atlantic Oscillation, the Arctic Oscillation, and modes such as the El Niño–Southern Oscillation and Pacific Decadal Oscillation that affect variability.

Physical Mechanisms

Density-driven convection in high-latitude regions—especially the Greenland Sea and Labrador Sea—drives deepwater formation through cooling and brine rejection during sea ice formation near Greenland and along the East Greenland Current. Salinity inputs from processes tied to the Amazon River, Mediterranean Sea outflow through the Gibraltar Strait, and exchanges with the Subpolar Gyre modify density gradients. Wind forcing from systems like the Icelandic Low and the Azores High imposes Ekman transport and generates Sverdrup balance across the basin. Boundary currents interact with topography such as the Mid-Atlantic Ridge and seafloor features explored by vessels like RV Knorr and RRS Discovery.

Observations and Measurements

Direct monitoring arrays including the RAPID Climate Change project at 26°N, the OSNAP array across the subpolar Atlantic, and repeat hydrographic sections by GO-SHIP provide time series of transport, temperature, and salinity. Satellite missions from the Jason series and Sentinel-3 measure sea surface height and temperature used with in-situ data from Argo floats, moorings, and gliders deployed by institutions like Scripps Institution of Oceanography. Proxy records from Greenland ice cores, North Atlantic sediment cores, and stalagmites in Iberia inform palaeocirculation reconstructions, while ship-based tracer experiments using chlorofluorocarbons and radiocarbon tie to studies by Lamont–Doherty Earth Observatory and GEOMAR.

Climate Impacts and Feedbacks

The circulation modulates regional climates including the relatively mild winters of Western Europe and the North American eastern seaboard by transporting heat poleward. Interactions with the Atlantic Multidecadal Oscillation affect hurricane activity linked to events like Hurricane Katrina and agricultural productivity in regions such as Sahel. Freshwater inputs from melting in Greenland and ice discharge into the Labrador Sea influence stratification and can alter deep convection, potentially affecting atmospheric patterns tied to European energy infrastructure and fisheries off Iceland and Norway. Feedbacks include changes to carbon uptake in the North Atlantic and interactions with the Atlantic Meridional Mode and the Benguela Current system via teleconnections.

Past and Future Variability

Paleoclimate evidence links abrupt shifts in overturning strength to events such as the Younger Dryas, Heinrich events, and Dansgaard–Oeschger cycles recorded in Greenland ice cores and North Atlantic sediment records. Model ensembles from projects like the Coupled Model Intercomparison Project under scenarios by the Intergovernmental Panel on Climate Change indicate a range of twenty-first-century responses, from gradual weakening to threshold behavior under strong freshwater forcing reminiscent of hypotheses tied to the Meltwater Pulse 1A. Observational trends noted by IPCC assessments and studies from agencies including NOAA and UK Met Office highlight uncertainties in timescales for potential collapse and regional consequences for European agriculture and coastal systems.

Modeling and Predictability

Climate models developed at centers such as National Center for Atmospheric Research, Max Planck Institute for Meteorology, and Met Office Hadley Centre simulate overturning dynamics using coupled ocean–atmosphere frameworks, eddy-resolving configurations, and Earth system models that include biogeochemical cycles studied by Plymouth Marine Laboratory. Predictability depends on initialization from observations (e.g., Argo), representation of mesoscale eddies, and parameterizations of mixing and convection. Forecast systems draw on data assimilation approaches used by ECMWF and ensembles tested in CMIP6 to estimate skill and lead time, while large-scale experiments like those by Project CLIVAR and international collaborations including the World Climate Research Programme coordinate efforts to reduce uncertainty.

Category:Oceanography Category:Climate