Thermohaline circulation

Also called: Global ocean conveyor belt · Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"

Meaning

Thermohaline circulation (also called the global ocean conveyor belt) is a worldwide loop of ocean currents. Differences in water density drive it, and density depends on temperature ("thermo") and salt content ("haline"). Cold, salty water sinks in the North Atlantic and around Antarctica, spreads along the deep ocean, and slowly rises again in the Indian and Pacific Oceans, while warm surface water flows back to replace it.

It matters because it carries heat round the Earth and helps shape world climate [2]. It also takes oxygen and carbon down to the deep sea and brings nutrients back up. If it slows down, monsoons, including the Asian monsoon, could change.

Explanation

How it works: density makes water sink

• Density means how heavy a fixed amount of water is. Two things make seawater denser:
• Cold: cold water shrinks, so the same volume becomes heavier.

• Salt: more dissolved salt makes the same volume heavier.

• The rule: denser water sinks and lighter water rises. So ocean water moves up and down, not only sideways.

• Deep currents are different from surface currents:
• Surface currents are mostly pushed by wind.

• Deep currents are driven by density.

• Global conveyor belt: the scientist Wallace Broecker gave the loop this name (1987, 1991). Water moves through all the oceans like goods on a factory belt.

• NOAA's description:
• It is a constantly moving deep-ocean system, driven by temperature and salinity [2][3].

• It begins at the surface of the ocean near the pole in the North Atlantic [2].

• Size: it moves more than 100 times the flow of the Amazon River [2].

The two sinking zones (where deep water is made)

• North Atlantic Deep Water (NADW)
• The Gulf Stream (a warm current flowing north-east from the Gulf of Mexico across the Atlantic) carries warm, very salty water north.
• Arctic winds chill this water in the Greenland, Norwegian and Labrador Seas.
• When sea ice forms, the salt does not freeze. It is left behind in the water nearby, which becomes even saltier [2].

• The cold, salty, dense water sinks and flows south along the western Atlantic basin [3].

• Antarctic Bottom Water (AABW): the densest water in the oceans

• It forms in the Weddell Sea and the Ross Sea around Antarctica.
• Brine rejection: when seawater freezes into sea ice, most of the salt is pushed out of the ice. It leaves behind very cold, very salty water called brine.
• This water is the heaviest in the world, so it sinks to the very bottom of the ocean.
• AABW lies below NADW and spreads along the sea floor.

Feature	NADW	AABW
Where it forms	Greenland, Norwegian and Labrador Seas	Weddell and Ross Seas
Why it becomes dense	Salty Gulf Stream water is cooled	Brine rejection during freezing, plus extreme cold
Layer	Deep layer	Bottom layer (densest)
Hemisphere	Northern	Southern

The full loop and why it matters

1. Sinking: dense water sinks in the North Atlantic and around Antarctica.
2. Spreading: deep water moves south through the Atlantic, east around Antarctica, then north into the Indian and Pacific Oceans.
3. Upwelling: there, the water slowly warms, mixes and rises. Upwelling means deep, cold water rising to the surface.
4. Return: warmer surface water flows back through the Indian Ocean, around southern Africa and into the Atlantic. It then moves north to the sinking zones again. - Time: one cubic metre of water takes about 1,000 years to finish the whole journey [2]. - What it does: - Moves heat: it carries tropical heat towards the poles and helps shape world climate [2]. - Ventilation: sinking surface water is rich in oxygen, so it takes oxygen down to deep-sea life. - Stores carbon: sinking water carries dissolved CO₂ down. The CO₂ stays there for centuries, which slows its build-up in the air. - Spreads nutrients: deep water gathers nutrients from decaying matter. Upwelling brings them to the surface, where they feed plankton (tiny floating plants and animals) and support fisheries.

AMOC and the risk of slowdown

• AMOC (Atlantic Meridional Overturning Circulation): the Atlantic part of the conveyor.
• "Meridional" means north–south.
• "Overturning" means warm water flows north at the top and cold water flows south at depth.

• It keeps north-west Europe (the UK, Ireland, Norway) several degrees warmer than other places at the same latitude.

• How it is measured:

• Since 2004, the RAPID array (a line of instruments across the ocean from Florida to Africa) has given continuous estimates of the AMOC at about 26.5°N [6].
• Gulf Stream flow through the Florida Straits has been measured continuously since 1982 [6].
• Strength is measured in Sverdrups (Sv): 1 Sv = 1 million cubic metres of water per second.

• In the early RAPID record, the mean strength was 18.7 Sv, and it varied by about 4.8 Sv [6].

• Why it could weaken:

• Cause: Greenland meltwater and more rain at high latitudes add fresh water to the North Atlantic.
• Process: fresher water is lighter, so it does not sink easily.

• Result: less deep water forms, and the conveyor may slow down.

• IPCC AR6 (2021):

• The AMOC will very likely decline over the 21st century under all SSP scenarios (SSPs are the IPCC's future emission scenarios) [4].
• There is medium confidence that it will not collapse abruptly before 2100 [4].

• There is low confidence in how large the decline will be, and in AMOC changes during the 20th century [4].

• Tipping point: a climate tipping point is a limit. Once a system crosses it, the system shifts to a new state that is hard to reverse. An AMOC collapse is one candidate.

• Past warning: the Younger Dryas (about 12,900–11,600 years ago)
• It was a sudden cold spell. Northern Europe and eastern North America cooled by about 4 to 8 °C [5].
• It is linked to a shutdown of the Atlantic overturning after meltwater floods freshened the North Atlantic.

Where it occurs

• North Atlantic sinking zone: the Greenland, Norwegian and Labrador Seas.
• The Labrador Sea lies between Canada and Greenland.
• The Greenland and Norwegian Seas lie between Greenland and Norway.

• NADW forms here.

• Antarctic sinking zone: the Weddell Sea (Atlantic side of Antarctica) and the Ross Sea (Pacific side of Antarctica). AABW forms here.

• Gulf Stream path: from the Gulf of Mexico, through the Florida Straits, north-east across the Atlantic. It feeds warm, salty water to the North Atlantic sinking zone.
• Measurement line: the RAPID array at 26.5°N, stretching from Florida to Africa [6].
• Indian and Pacific Oceans: the main zones of upwelling, where deep water rises and warms.
• India and the Indian Ocean:
• The Asian landmass closes the northern Indian Ocean, so there is no polar sinking zone there.
• The Indian Ocean mainly receives deep water from the Southern Ocean. It is a region of upwelling and return flow.

• The return surface flow passes through the Indian Ocean, around southern Africa, into the Atlantic.

• North-west Europe (UK, Ireland, Norway): the region whose mild climate depends most on the AMOC.

Don't confuse with

• Surface currents (e.g. the Gulf Stream): mostly wind-driven and found in the upper ocean. Thermohaline circulation is density-driven and works mainly in the deep ocean. The Gulf Stream only feeds water into the conveyor.
• AMOC vs thermohaline circulation: the AMOC is only the Atlantic north–south part of the global conveyor. Thermohaline circulation is the whole worldwide loop through all oceans.
• NADW vs AABW: NADW forms in the Northern Hemisphere (Greenland, Norwegian, Labrador Seas) and occupies the deep layer. AABW forms in the Southern Hemisphere (Weddell, Ross Seas), is the densest water and lies at the very bottom.
• Sinking vs upwelling: sinking happens at high latitudes (North Atlantic, Antarctica). Upwelling, the rise of deep water, happens mainly in the Indian and Pacific Oceans.

Prelims Hooks

• Thermohaline: "thermo" means temperature and "haline" means salinity. Deep currents are driven by density, not wind. "Global conveyor belt" is Wallace Broecker's term.
• NADW: Greenland, Norwegian and Labrador Seas. AABW: Weddell and Ross Seas. AABW is the densest water mass and forms mainly by brine rejection.
• One full circuit takes about 1,000 years. Flow is more than 100 times the Amazon's [2].
• AMOC is measured in Sverdrups (1 Sv = 10⁶ m³/s) by the RAPID array at 26.5°N, running since 2004 [6].
• Trap: the northern Indian Ocean has no deep-water sinking zone, because Asia closes it. Deep water reaches it from the south.
• IPCC AR6: the AMOC will very likely decline this century, and a collapse before 2100 is judged unlikely (medium confidence) [4]. Younger Dryas (about 12,900–11,600 years ago) is linked to an Atlantic overturning shutdown [5].

Mains Points

• Role in the climate system (GS-I):
• It moves heat from the equator to the poles [2], which explains the mild climate of north-west Europe.
• It takes oxygen and carbon to the deep sea and stores CO₂ for centuries.

• Upwelling brings nutrients up, which supports plankton and rich fishing grounds.

• Climate-change threat (GS-III):

• Greenland melt and more high-latitude rain freshen the surface, so less water sinks and the AMOC weakens.
• IPCC AR6 says a decline this century is very likely [4]. The Younger Dryas shows that a past shutdown cooled northern Europe and eastern North America by 4–8 °C [5].

• Long-term observing lines such as RAPID (since 2004) [6] feed IPCC assessments and give early warning of tipping points.

• Why India should care (GS-I/III):

• An AMOC collapse could push the tropical rain belt south and weaken the Asian monsoon [4].
• This would threaten Indian farming, water security and food prices.
• So India has a direct interest in global emission cuts and in supporting ocean monitoring.

Read more

• The thermohaline circulation: the deep-ocean conveyor