Maarg Manthan · Topic 6.11

Atlantic Meridional Overturning Circulation (AMOC)

Geography › Oceanography · Topic 6.11

The Atlantic Meridional Overturning Circulation, or AMOC, is the large system of ocean currents that carries warm surface water north in the Atlantic, including the Gulf Stream and North Atlantic Drift, and returns cold, dense water south at depth. In the North Atlantic the water cools, becomes dense and sinks, forming North Atlantic Deep Water. The AMOC moves heat northward and keeps north-west Europe far warmer than other places at the same latitude. Studies suggest it has weakened, and its future under climate change is being closely studied.

5 min read · UPSC Prelims · Geography · Oceanography

At a Glance

Surface flow Warm water flows north in the Atlantic
Deep flow Cold water returns south at depth
Sinking regions Labrador and Nordic Seas
Heat role Warms north-west Europe
Concern Weakening under climate change
Exam link UPSC Prelims, Geography: Oceanography

Where Does It Fit?

This topic follows the topic on the major ocean currents and takes up the Atlantic part of the global thermohaline circulation. It links ocean currents to climate change, which is a growing theme in the Prelims and Mains. Previous topic: Major Ocean Currents of the World and Their Effects. Next topic: Resources from the Ocean: Mineral Resources on the Deep Sea Floor, Energy Resources, Biotic Resources.

How Does the AMOC Work?

In short: the AMOC is a conveyor of warm water north near the surface and cold water south in the deep.

Northward flow: warm, salty water from the tropics moves north in the Gulf Stream and the North Atlantic Drift.
Heat loss: in the North Atlantic, strong winds and cold air take heat from the water, which warms the air over Europe.
Sinking: the cooled water, already salty because of evaporation in the subtropics, is dense, and sinks in the Labrador and Nordic Seas.
Southward return: the dense water, called North Atlantic Deep Water, flows south at depths of 2 to 4 km along the western side of the Atlantic.
Upwelling: it returns to the surface in the Southern Ocean and other seas, closing the loop with the global circulation described in Density of Ocean Waters.

The flow of the AMOC is measured in sverdrups, where 1 sverdrup is a million cubic metres of water per second. At about 26 degrees north, the RAPID-MOCHA array of instruments, in operation since 2004, shows a flow of roughly 17 sverdrups.

Why Does the AMOC Matter for Climate?

Warm Europe: the AMOC carries a large amount of heat northward, which is why Britain, Norway and north-west Europe are much milder than places at the same latitude in Canada.
Rainfall: a weaker AMOC shifts the tropical rain belt, with possible drying of the Sahel and changes in the monsoons of Africa, South Asia and South America.
Sea level: a weaker flow lets water pile up along the eastern coast of North America, raising local sea level.
Carbon and nutrients: it takes carbon dioxide and oxygen to the deep sea and controls fisheries of the North Atlantic.
Storms: changes in sea temperature can affect Atlantic hurricanes and European winter storms.

Is the AMOC Weakening?

This is an active field, and the conclusions should be read with caution.

Observed changes: several studies suggest the AMOC has weakened since the mid-20th century, by amounts that differ between studies, and a cool patch of ocean south of Greenland, called the cold blob, is often linked to it. Direct measurements since 2004 are too short to be sure of a long-term trend.
Cause: warming of the surface water and extra fresh water from melting Greenland ice and heavier rain make the surface water lighter, so it sinks less.
Projections: the Intergovernmental Panel on Climate Change judged that the AMOC is very likely to weaken during the 21st century, but had medium confidence that it will not abruptly collapse before 2100. Some recent studies suggest a higher risk of a collapse, possibly in the middle of this century, while others find it unlikely in this century. The question is unsettled.
Past: ice-core and sediment records show that the AMOC shut down or slowed in the last ice age, causing rapid cooling in the North Atlantic, as in the Younger Dryas about 12,900 years ago.

The ocean currents themselves are in Major Ocean Currents of the World and Their Effects. As an analytical point, the AMOC is one of the climate ‘tipping elements’, a system that could shift abruptly, and it shows why changes in the Arctic and Greenland can affect the weather and monsoons far away.

Why Is This Topic Important?

Climate Explains the warmth of Europe
Tipping point Climate change topic
Link Ocean circulation

What Came Next?

The next topic covers the resources of the oceans: minerals, energy and living resources. Next topic: Resources from the Ocean: Mineral Resources on the Deep Sea Floor, Energy Resources, Biotic Resources.

Key People and Terms

AMOC Atlantic overturning current system
North Atlantic Deep Water Dense water that sinks in the North Atlantic
Sverdrup One million cubic metres per second
Gulf Stream Warm current off North America
Tipping element System that may shift abruptly
Cold blob Cool patch south of Greenland

Exam Corner

Points to Remember

The AMOC is the Atlantic part of the thermohaline circulation.
Water sinks in the Labrador and Nordic Seas.
One sverdrup is a million cubic metres per second.
The AMOC is very likely to weaken this century, says the IPCC.
Collapse before 2100 is uncertain.

Do Not Confuse With

Gulf Stream and AMOC A warm current versus the whole overturning system
AMOC and thermohaline circulation The Atlantic branch versus the global loop
Weakening and collapse Slowing versus shutdown

Memory Hook

Warm goes north, cold sinks deep; the conveyor turns and Europe keeps warm.

Mains Angle

Questions link the AMOC with climate change and tipping points. These points add depth.

Mechanism Density-driven sinking
Impacts Europe, rainfall, sea level
Uncertainty Weakening and collapse risk

A Question You May Face

An original practice question, not a past paper question.

“What is the Atlantic Meridional Overturning Circulation? Discuss the likely consequences of its weakening.”

How to Answer

Define AMOC and its loop.
Role Heat transport.
Risk Weakening, rain and sea level.
Caution Uncertainty of collapse.

GS Relevance

Prelims: what AMOC is, its effects, risks of weakening. GS Paper 1: oceanography; GS Paper 3: climate change.

Frequently Asked Questions

What is the AMOC?

The Atlantic Meridional Overturning Circulation is the system of currents in the Atlantic Ocean that carries warm surface water north, including the Gulf Stream, and returns cold, dense water south at depth. It forms part of the global thermohaline circulation and transports large amounts of heat towards the North Atlantic.

Why is Europe warmer than other places at the same latitude?

North-west Europe is warm mainly because the AMOC and the North Atlantic Drift carry warm water and heat from the tropics to the North Atlantic. The ocean gives this heat to the air, and prevailing westerly winds carry the warmth over Britain and Norway, so they are milder than Labrador in Canada.

Why is the AMOC weakening?

Studies suggest the AMOC is weakening because warming surface water and extra fresh water from melting Greenland ice and increased rain make the surface water lighter. Lighter water sinks less in the Labrador and Nordic Seas, so the overturning slows. The extent of the weakening is still debated.

What would be the effects of a weaker AMOC?

A weaker AMOC could cool north-west Europe relative to other regions, shift tropical rain belts, bringing drier conditions to parts of the Sahel and changes in monsoon rainfall, raise sea level along the eastern coast of North America, and disturb Atlantic fisheries and storm patterns.

Will the AMOC collapse?

Scientists are not agreed. The Intergovernmental Panel on Climate Change judged that the AMOC is very likely to weaken this century but had medium confidence that it will not abruptly collapse before 2100. Some newer studies suggest a higher risk, so the latest research should be checked.

PYQ Practice — Statement Analysis

1 The AMOC carries warm surface water northward in the Atlantic.

True

Cold deep water returns south.

2 Deep water in the AMOC sinks in the tropical Atlantic.

False

It sinks in the Labrador and Nordic Seas.

3 The Gulf Stream is a part of the AMOC.