Atlantic Meridional Overturning Circulation

Atlantic Meridional Overturning Circulation
Name	Atlantic Meridional Overturning Circulation
Caption	A simplified schematic of the circulation
Type	Thermohaline circulation
Ocean	Atlantic Ocean
Known for	Global heat and freshwater transport

Atlantic Meridional Overturning Circulation. It is a major component of the global ocean conveyor belt, responsible for transporting vast amounts of heat, salt, and nutrients around the planet. This large-scale system involves the northward flow of warm, salty surface water in the Atlantic Ocean and the southward return flow of colder, deep water. Its stability is a critical factor in regulating the climate of regions like North America and Western Europe.

Overview

The Atlantic Meridional Overturning Circulation is a key part of the Earth's climate system, moving water masses across entire ocean basins. It is primarily driven by differences in water density, which are controlled by temperature and salinity—a process known as thermohaline circulation. The system connects the Southern Ocean and the Arctic Ocean, with significant formation sites in the Nordic Seas and the Labrador Sea. Major research programs like RAPID and OSNAP are dedicated to monitoring its behavior. Understanding its dynamics is crucial for projects like the Intergovernmental Panel on Climate Change assessment reports.

Physical mechanism

The circulation begins with warm, salty water from the Gulf Stream and the North Atlantic Current moving northward. As this water travels into high latitudes, it loses heat to the atmosphere, cooling and increasing in density. Evaporation in subtropical regions like the Sargasso Sea further increases salinity. In critical convection sites such as the Greenland Sea and near Iceland, the now-dense surface water sinks, forming North Atlantic Deep Water. This deep water then flows southward at depth, crossing the Equator and eventually upwelling in the Southern Ocean and the Pacific Ocean. The complete loop is closed by surface currents, including the Antarctic Circumpolar Current.

Role in climate

This circulation acts as a central heating system for the Northern Hemisphere, redistributing enormous amounts of equatorial heat poleward. It is responsible for the relatively mild climates of Scandinavia and the British Isles compared to other regions at similar latitudes, such as Labrador. By transporting warm water northward, it releases heat into the atmosphere, influencing weather patterns over North America and Europe. Furthermore, it plays a vital role in the global carbon cycle by sequestering atmospheric carbon dioxide into the deep ocean. Its interaction with sea ice in the Arctic also affects albedo and freshwater balance.

Measurement and observation

Direct continuous measurement began in 2004 with the RAPID array, a series of moorings deployed along the 26th parallel north across the Atlantic Ocean. This was supplemented by the OSNAP array observing the subpolar North Atlantic. Historical data comes from ship-based hydrographic sections like those conducted during the World Ocean Circulation Experiment. Key monitoring proxies include sea surface temperature patterns, subsurface salinity measurements, and satellite observations of sea surface height from missions like TOPEX/Poseidon. Institutions such as the National Oceanic and Atmospheric Administration and the Woods Hole Oceanographic Institution lead these efforts.

Variability and potential changes

The system exhibits natural variability on decadal to centennial timescales, linked to phenomena like the Atlantic Multidecadal Oscillation. However, climate models from the Intergovernmental Panel on Climate Change consistently project a weakening over the 21st century due to anthropogenic climate change. This weakening is primarily driven by increased freshwater input from melting of the Greenland Ice Sheet and increased precipitation, which reduces water density and inhibits sinking. Paleoclimate evidence from events like the Younger Dryas and the 8.2-kiloyear event shows the circulation has undergone abrupt changes in the past. Current research focuses on potential tipping points, with studies published in journals like *Nature* indicating possible early warning signs.

Impacts of weakening or collapse

A significant slowdown or collapse would have profound climatic consequences. It would lead to a stark regional cooling across the North Atlantic, particularly affecting Western Europe, while other parts of the globe, like the Southern Hemisphere, would experience additional warming. Major disruptions to precipitation patterns would occur, potentially shifting the Intertropical Convergence Zone and affecting monsoons in regions like the Sahel and India. Sea level along the North American coastline, particularly around New York City, would rise significantly. Ecosystem productivity in the North Atlantic would be altered, impacting fisheries. Such a shift could also accelerate the melting of adjacent ice sheets, including the Antarctic ice sheet, through complex feedbacks.

Category:Ocean currents Category:Climate Category:Atlantic Ocean