Thermohaline Circulation
Why is the climate of Norway so much milder than that of Labrador, Canada, at the same latitude? Part of the answer lies in a current that starts with water sinking in the North Atlantic and takes about 1,000 years to complete one loop. In this page you will see what makes water sink, where it happens, and what could disturb it.
Density: temperature and salinity
Surface currents are pushed by wind. Deep water is not: no wind reaches it. What moves it is density. Dense water sinks, and lighter water rises or spreads on top.
The word thermohaline has two roots: thermo (heat, temperature) and haline (salt, salinity). Two properties of seawater control its density:
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Temperature. Cold water is denser than warm water, because its molecules are packed more closely.
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Salinity. Salty water is denser than fresher water, because dissolved salt adds mass without adding much volume. Salinity is measured in parts per thousand (ppt): 35 ppt means 35 g of salt in 1 kg of water.
Typical open-ocean seawater has a density near $1{,}027\ \text{kg/m}^3$, about 2.7% more than fresh water. Small changes matter a lot: a difference of just a few $\text{kg/m}^3$ is enough to make one water mass slide under another.
Use this rule of thumb for any pair of samples: colder and saltier means denser. If one property pushes up and the other pushes down, compare the sizes of the changes.
The word thermohaline* combines two Greek-derived roots. In your own words, define thermohaline circulation. Name the two properties of seawater that control it, and…
The word thermohaline combines two Greek-derived roots. In your own words, define thermohaline circulation. Name the two properties of seawater that control it, and state which physical property of the water those two properties change.
Two samples of seawater touch each other. Sample A is at $2\,^\circ\text{C}$ with a salinity of 35 ppt (35 g of salt per kg of water). Sample B is at…
Two samples of seawater touch each other. Sample A is at $2\,^\circ\text{C}$ with a salinity of 35 ppt (35 g of salt per kg of water). Sample B is at $20\,^\circ\text{C}$ with a salinity of 35 ppt. Which sample is denser, and which one would sink beneath the other? Explain in one or two sentences.
A student measures four water samples: | Sample | Temperature ($^\circ$C) | Salinity (ppt) | |---|---|---| | A | 4 | 35.0 | | B | 4 | 36.0 | | C | 20 | 35.0 | | D | 20 |…
A student measures four water samples:
| Sample | Temperature ($^\circ$C) | Salinity (ppt) |
|---|---|---|
| A | 4 | 35.0 |
| B | 4 | 36.0 |
| C | 20 | 35.0 |
| D | 20 | 36.5 |
In this range, adding 1 ppt of salt raises the density by about $0.8\ \text{kg/m}^3$, and warming by $1\,^\circ\text{C}$ lowers it by about $0.2\ \text{kg/m}^3$. Rank the samples from least dense to most dense. Then state which sample would sink to the bottom of a tank that contained all four, and justify your ranking.
Where deep water forms
Surface water sinks only where it becomes dense enough. That happens at high latitudes, in two main ways.
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Cooling. In the North Atlantic, near Greenland and Iceland, cold winter air chills the surface water. The Gulf Stream and North Atlantic Drift bring salty water here, so it is cold and salty. It sinks and forms North Atlantic Deep Water.
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Brine rejection. When seawater freezes, the ice crystals are almost pure water. Most of the salt is left behind in the liquid just under the ice. This process is called brine rejection. That water becomes saltier, so denser, and sinks. Around Antarctica, it forms very cold, dense Antarctic Bottom Water, which spreads along the ocean floor.
When seawater freezes near Antarctica or in the Arctic, most of the salt is left behind in the liquid water instead of becoming part of the ice. Name this process, and…
When seawater freezes near Antarctica or in the Arctic, most of the salt is left behind in the liquid water instead of becoming part of the ice. Name this process, and explain how it affects the salinity and density of the water just under the ice.
The global conveyor belt
Sinking water must be replaced. Dense water spreads along the ocean floor, slowly rises elsewhere (mostly in the Indian and Pacific Oceans), then returns near the surface toward the North Atlantic. Linking surface and deep flow gives the global conveyor belt.
The loop is very slow. One full trip covers roughly $20{,}000\ \text{km}$ in about 1,000 years.
The conveyor belt also matters for climate. It moves heat, dissolved gases (like CO₂) and nutrients around the planet. Warm surface water flowing north in the Atlantic releases heat to the air, which helps keep northwestern Europe milder than places at the same latitude in Canada.
Scientists estimate that one full trip around the global "conveyor belt" of thermohaline circulation takes about 1,000 years and covers a path of roughly…
Scientists estimate that one full trip around the global "conveyor belt" of thermohaline circulation takes about 1,000 years and covers a path of roughly $20{,}000\ \text{km}$. Use $1\ \text{year} \approx 3.15 \times 10^{7}\ \text{s}$.
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Calculate the average speed of the deep water in km per year.
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Convert that speed to cm/s.
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Compare it with a typical surface current speed of $0.5\ \text{m/s}$ and explain why the deep circulation is called "slow."
Disturbing the belt
The conveyor belt depends on dense water sinking at high latitudes. Anything that makes surface water there lighter can slow the sinking. Two changes matter:
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Freshwater input. Melting of the Greenland ice sheet and Arctic sea ice adds fresh water, which lowers salinity and density.
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Warming. Warmer surface water is less dense.
A weaker sinking could slow the conveyor belt. Then less warm water would be pulled north by the Gulf Stream and North Atlantic Drift, so less heat would reach northwestern Europe, and that region could cool. Because the loop is so long, the full response to a change could take centuries.
Melting of the Greenland ice sheet adds large volumes of fresh water to the surface of the North Atlantic. Explain, step by step, how this freshwater input changes the…
Melting of the Greenland ice sheet adds large volumes of fresh water to the surface of the North Atlantic. Explain, step by step, how this freshwater input changes the salinity and density of surface water. Then explain how those changes could affect the sinking of water in the North Atlantic. Finally, predict what would happen to the heat that the Gulf Stream and North Atlantic Drift carry toward northwestern Europe.
Near Greenland, the density of surface seawater can be approximated by $$\rho = 1027.8 + 0.78\,(S - 35) - 0.15\,(T - 4)\ \ \text{kg/m}^3,$$ where $S$ is salinity in ppt…
Near Greenland, the density of surface seawater can be approximated by
$$\rho = 1027.8 + 0.78\,(S - 35) - 0.15\,(T - 4)\ \ \text{kg/m}^3,$$where $S$ is salinity in ppt and $T$ is temperature in $^\circ\text{C}$.
Cold, salty water of $S = 35.2$ ppt and $T = 4\,^\circ\text{C}$ currently sinks here.
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Calculate its density.
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Meltwater lowers the salinity to 34.0 ppt at the same temperature. Calculate the new density.
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Separately, suppose the water warms by $2\,^\circ\text{C}$ with no change in salinity. Calculate that density.
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Compare the two changes. Decide which has the larger effect on sinking in this scenario, and explain what each implies for thermohaline circulation and for the long-term transport of heat.
Antarctic Bottom Water forms in the Southern Ocean when very cold, salty water sinks along the edge of the continent and then spreads along the ocean floor. Suppose that…
Antarctic Bottom Water forms in the Southern Ocean when very cold, salty water sinks along the edge of the continent and then spreads along the ocean floor. Suppose that over several decades the sea ice around Antarctica shrinks, and meltwater from land ice also enters the surface ocean.
Write a claim-evidence-reasoning (CER) argument that predicts how these changes would affect (a) the formation of bottom water, (b) the structure (layering) of the deep ocean, and (c) the timing of thermohaline circulation. Use the idea that one loop takes about 1,000 years to explain why the full effects of changes today could appear over many centuries.
Back to the start: Norway is milder than Labrador because the North Atlantic Drift brings warm water north. That flow is part of the conveyor belt, which depends on cold, salty water sinking near Greenland.