Deep Circulation

Oceanography Fundamentals

Course 1 · Chapter 7 · Deep Circulation: The Slow Conveyor

In the 1980s the geochemist Wallace Broecker of Columbia University popularised a striking picture: a single "great conveyor belt" of water, sinking in the North Atlantic, creeping along the bottom of the ocean through the Atlantic, Indian and Pacific basins, and returning to the surface to flow back (check the date and the wording). The picture was a simplification, and Broecker himself said so, but it fixed in people's minds a real and important fact: the deep ocean is not still. It moves, very slowly, and it carries heat, carbon dioxide and oxygen with it. This chapter explains why the deep water moves, where it sinks, how long it takes, and why oceanographers watch it so closely.

A Different Kind of Current

Surface currents are pushed by the wind (Chapter 6). Deep currents are mainly driven by differences in density. Because density depends on temperature and salinity (Chapter 3), this is often called thermohaline circulation (from Greek words for heat and salt). Many scientists now prefer the term meridional overturning circulation, because wind and tides also contribute to the mixing that keeps the system running, and "overturning" describes what the water does: sinks in some places and rises in others (check).

Property	Surface currents	Deep circulation
Main driver	Wind	Density differences (cooling, evaporation, ice formation), plus mixing
Speed	Up to metres per second in the Gulf Stream	Typically millimetres to centimetres per second
Depth	Top few hundred metres	Thousands of metres
Time to cross an ocean	Months to years	Decades to centuries

Where Water Sinks

Water sinks when it becomes denser than what lies beneath it. This happens in only a few places, where surface water is made very cold and salty enough.

North Atlantic

Warm, salty water carried north by the Gulf Stream and North Atlantic Drift loses heat to the cold air of the Labrador, Greenland and Norwegian Seas. It becomes dense and sinks, forming North Atlantic Deep Water.

Antarctica

Around the edge of Antarctica, particularly in the Weddell and Ross Seas, freezing sea ice rejects salt (Chapter 3), leaving cold, very salty water that sinks down the continental slope. This is Antarctic Bottom Water, the densest water in the open ocean (check).

Why not the North Pacific?

The surface water there is fresher, so it is not dense enough to sink to great depth. The difference in salinity between Atlantic and Pacific is a key reason the conveyor is Atlantic-led (check).

Water Masses

As water sinks, it keeps the temperature and salinity it had at the surface, like a label. Oceanographers use these labels to trace the water as it spreads and to name it as a water mass.

Water mass	Where it forms	Where it sits
Antarctic Bottom Water	Antarctic shelf seas	Right on the bottom, spreading north into all three oceans
North Atlantic Deep Water	Labrador and Nordic seas	Above Antarctic Bottom Water, spreading south through the Atlantic
Antarctic Intermediate Water	Southern Ocean	Around 1,000 m, spreading north
Mediterranean outflow	Mediterranean, which is saltier than the Atlantic	A salty tongue at about 1,000 m, flowing out of the Strait of Gibraltar

The Loop, and How Long It Takes

Warm surface water flows north in the Atlantic, giving up heat to the air on the way (this is part of why north-west Europe is mild).
In the far north it sinks and flows south in the deep Atlantic.
Around Antarctica it joins the circumpolar current and meets the Antarctic Bottom Water.
Both spread north into the Indian and Pacific basins, where they slowly mix upward and warm.
Over a very long time they return to the surface, and a return flow carries the water back to the Atlantic.

The full circuit takes of the order of a thousand years (check; estimates range from several hundred to more than a thousand). Evidence comes from radiocarbon dating of dissolved carbon: water that sank long ago has lost carbon-14 by radioactive decay, and the oldest water, in the deep North Pacific, is of the order of 1,000 years since it last touched the atmosphere (check).

Measuring a current you cannot see Since 2004 a line of moored instruments, the RAPID array, has stretched across the Atlantic at about 26.5 degrees north, from the Canary Islands to the Bahamas, measuring the northward and southward flows (check). It finds an overturning of roughly 17 sverdrups, with large variations from year to year (check). The record is still short compared with the timescales involved, which is why scientists are cautious about claiming a trend.

Why It Matters

Heat. The Atlantic overturning carries heat from the tropics towards the north, helping to keep north-west Europe mild (but see the caution in Chapter 6).
Carbon. Sinking water carries dissolved carbon dioxide into the deep, where it can stay for centuries. The deep ocean is by far the largest active store of carbon near the surface of the planet (check).
Oxygen. Cold water holds a lot of oxygen; as it sinks it delivers oxygen to the deep, called ventilation, and without it deep water would run short of oxygen.
Nutrients. Old deep water collects nutrients from sinking debris and brings them back to the surface in upwelling regions (Chapter 6).

Could the conveyor stop? There is evidence that it has changed abruptly in the past. One idea is that a flood of meltwater into the North Atlantic about 12,900 years ago made the surface water too fresh to sink and triggered a cold snap called the Younger Dryas (check; the cause is still debated). Climate assessments say that the Atlantic overturning is very likely to weaken during this century as the climate warms, with an abrupt collapse before 2100 considered unlikely, though with low confidence (check the latest report). Disaster films exaggerate, but the question is a serious one, and it is good practice to read the actual assessment wording rather than a headline.

Try it yourself A kitchen experiment shows the principle. Fill a clear tank with water, add food colouring to a bowl of very salty water and pour it slowly down the side: the dyed salty water sinks and creeps along the bottom. Add a little ice to one end and warm water to the other, and the fluid starts to circulate. The ocean does the same, on a scale of thousands of kilometres.

Exercises

Exercise 1

The ocean has a volume of about 1.3 x 10^18 cubic metres (1.3 billion cubic kilometres). If the overturning moves 17 Sv (17 x 10^6 m3/s), estimate how many years it would take to move a volume equal to the whole ocean. Does the answer fit the thousand-year timescale in the chapter?

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Exercise 2

Explain why the formation of sea ice around Antarctica makes the water below it denser. Then say what you would expect to happen to the density if the same area received a large flow of meltwater from glaciers.

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Exercise 3

A ship takes a water sample from the deep North Pacific and from the deep North Atlantic. Which should have the older radiocarbon age, and why? What does this tell you about the direction of the deep flow?

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Quick Reference

Driver: density differences from temperature and salinity (thermohaline), plus mixing by wind and tides
Sinking regions: the North Atlantic and the Antarctic margins
Water masses: Antarctic Bottom Water (densest), North Atlantic Deep Water, intermediate waters
Timescale: decades to centuries to cross a basin; of the order of 1,000 years for the whole loop
Evidence: temperature and salinity labels, radiocarbon ages, moored arrays
Roles: heat transport, carbon storage, oxygen supply, nutrient return
Uncertainty: weakening this century is likely; an abrupt collapse is considered unlikely (check)
Check: Broecker date, RAPID figures, Younger Dryas explanation

Coming next Chapter 8 puts the ocean in the climate system: how it stores heat and carbon, the El Nino cycle in the Pacific, rising sea level, and the acidification and deoxygenation that follow from human emissions.