AMOC

AMOC
R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP. · CC BY 3.0 · source
Name	Atlantic Meridional Overturning Circulation
Acronym	AMOC
Region	North Atlantic Ocean, South Atlantic Ocean
Type	Thermohaline circulation, ocean current
Significance	Climate regulation, heat transport, carbon cycle

AMOC

The Atlantic Meridional Overturning Circulation is a large-scale system of ocean currents that redistributes heat, salt, and carbon across the Atlantic Ocean and influences climate across Europe, North America, and the Tropics. It links surface currents such as the Gulf Stream and North Atlantic Current with deep currents that flow toward the southern hemisphere, interacting with atmospheric systems like the North Atlantic Oscillation and the Intertropical Convergence Zone. Research spans observational programs led by institutions like the Atlantic Meridional Overturning Circulation (RAPID) array? and modeling centers such as the National Oceanic and Atmospheric Administration, the Met Office, and the Max Planck Institute for Meteorology.

Overview

The circulation transports warm, saline surface waters northward via pathways including the Gulf Stream and North Atlantic Drift and returns cold, dense waters southward as deep western boundary currents and abyssal flows that feed the Southern Ocean. It is driven by a combination of wind forcing associated with systems like the Azores High and buoyancy forcing related to surface cooling and brine rejection during North Atlantic Deep Water formation. Major components include water mass formation in regions near the Labrador Sea and the Greenland Sea, connections with the Antarctic Circumpolar Current, and exchanges across basins via passages such as the Danish Strait.

Physical Mechanism

Buoyancy-driven processes convert warm, light surface waters into cold, dense deep waters through convective events tied to winter cooling and sea-ice interactions in areas adjacent to Greenland, Iceland, and Scotland. The thermohaline aspect couples with wind-driven gyres like the Subpolar Gyre and Subtropical Gyre, producing a meridional overturning cell that can be described by streamfunction diagnostics used in studies at the Scripps Institution of Oceanography and Woods Hole Oceanographic Institution. Salt advection from regions influenced by the Mediterranean Sea and the Caribbean Sea modifies density gradients, while freshwater inputs from Greenland ice sheet melt and river discharge such as the Amazon River affect stratification.

Variability and Observed Changes

Instrumental and proxy records reveal variability on seasonal to millennial timescales. Observational arrays and paleoarchives studied by groups at the British Antarctic Survey, the Lamont–Doherty Earth Observatory, and the Alfred Wegener Institute show changes associated with events like the Younger Dryas and the Little Ice Age. Recent instrumental programs report trends in transport and density that have prompted debate about a possible long-term weakening, with analyses published by teams at the European Centre for Medium-Range Weather Forecasts and the National Aeronautics and Space Administration. Variability is linked to atmospheric modes including the Arctic Oscillation and to teleconnections with the El Niño–Southern Oscillation.

Climate Impacts

Through poleward heat transport it modulates temperature gradients that influence weather regimes across Western Europe, the United States Northeast, and parts of North Africa. Changes in overturning strength affect sea level along coastlines such as the US East Coast and influence storm tracks related to the North Atlantic Oscillation and the European storm track. Impacts extend to biogeochemical cycles: ventilation of the deep ocean controls oxygenation relevant to regions studied by the International Geosphere-Biosphere Programme, and carbon sequestration linked to overturning affects atmospheric carbon dioxide concentrations monitored by programs at the Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration.

Modeling and Projections

Climate models developed at centers like the Met Office Hadley Centre, the Geophysical Fluid Dynamics Laboratory, and the Max Planck Institute for Meteorology simulate AMOC responses to greenhouse gas forcing, freshwater input scenarios, and coupled atmosphere–ocean feedbacks. Projections vary: multimodel assessments in reports by the Intergovernmental Panel on Climate Change indicate a range from modest decline to more pronounced weakening by the end of the 21st century, contingent on emissions pathways such as those defined by the Representative Concentration Pathways. High-resolution regional models and Earth system models incorporating ice-sheet dynamics from groups at Columbia University and ETH Zurich explore thresholds and potential rapid transitions reminiscent of paleo-events like those inferred from Heinrich events.

Measurement and Monitoring

Sustained observations combine moorings, ship-of-opportunity hydrography, autonomous floats like Argo, and remote sensing by satellites operated by agencies such as European Space Agency and NASA. The RAPID–MOC program at University of Southampton and partner institutions uses arrays across the Strait of Gibraltar and at 26.5°N to estimate transport, supplemented by programs in the Irminger Sea and the Iceland Basin. Paleo-proxies including foraminiferal assemblages studied at the British Geological Survey and sediment cores curated at the National Oceanography Centre provide historical context. International collaborations such as the Global Ocean Observing System coordinate data synthesis and accessibility.

History of Research and Discovery

Foundational concepts trace to early work by oceanographers like Henry Stommel and Walter Munk, who formulated theoretical frameworks for thermohaline circulation and western boundary currents. Empirical advances came with mid-20th-century expeditions led by institutions including the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution, and later technological milestones such as satellite altimetry from missions like TOPEX/Poseidon and in situ observing systems developed by the National Oceanic and Atmospheric Administration. Paleoceanographic reconstructions by researchers at Lamont–Doherty Earth Observatory and Alfred Wegener Institute linked abrupt climate shifts to changes in overturning, shaping modern concerns addressed by interdisciplinary consortia including the World Climate Research Programme.

Category:Oceanography