Circular plates of new pancake ice cover the surface of the western Ross Sea in Antarctica.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Overturning section. depth ×500. routes and tracers illustrative.

Warm upper limb (amber); cold deep return (blue). Choose Density experiment to vary the parcel.

How to read these results

Overturning also needs winds and mixing. The separate parcel experiment does not calculate AMOC transport or a shutdown threshold.

Thermohaline conveyor

Explain how cooling and changes in salinity affect seawater density, then relate sinking and rising water to ocean overturning.

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Why it happens

Seawater density depends on temperature and salinity. Cooling and salinity changes can contribute to deep-water formation, connecting surface waters with slower circulation through the ocean interior.

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New sea ice forming in the Ross Sea. Freezing rejects much of the salt into the surrounding water, increasing its density and helping drive polar overturning.

Try a comparison

Open Density experiment and compare parcel density with the reference. Start at Process 0: the parcel is at mid-depth. At 4 °C and 35 g/kg, play to observe descent.

Reading the result

The circulation combines several mechanisms, including winds and mixing; it is not a single conveyor driven only by temperature and salinity.

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How Do Oceans Circulate? Crash Course Geography #9 — Crash Course.

A common misunderstanding

The ocean conveyor is one continuous pipe carrying water along a fixed route.

The conveyor image simplifies a network of branching currents, mixing and exchanges between depths. Temperature and salinity influence density, while winds and mixing also help drive circulation. A parcel can follow many routes and take a range of times to move between ocean regions.

Cases

Exam language

Thermohaline

Relating to seawater density differences caused by temperature and salinity. These contribute to ocean overturning alongside wind forcing and mixing.

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NOAA CTD water sampling. Temperature and salinity observations constrain water density and help identify deep-water masses such as North Atlantic Deep Water. The instrument photo does not show the global circulation itself.

Density

Mass divided by volume, measured in kg m⁻³. In most ocean conditions, cooling or increasing salinity makes seawater denser.

NADW

North Atlantic Deep Water: a group of deep water masses formed through cooling, mixing and sinking in the North Atlantic and neighbouring seas.

Deep water formation

The sinking of sufficiently dense surface seawater into deeper parts of the ocean. Cooling and increased salinity can contribute, especially at high latitudes. It is one part of a complex overturning circulation that also involves mixing and upwelling.

Upwelling

The movement of deeper water towards the surface, often driven by winds that move surface water away or by large-scale ocean circulation.

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Aqua MODIS, 5 May 2020. Upwelling in the Canary Current system supplies nutrients off Mauritania. The visible green water contains phytoplankton and sediment; it is not a temperature map.

Conveyor

A simplified description of interconnected deep and surface ocean circulation. Real flow involves branching currents, mixing, winds and density differences.

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NOAA Deep SOLO deployment, 2018. Profiling floats measure ocean temperature and salinity at depth, helping scientists study density structure and deep circulation. The ocean conveyor is a simplified description of a complex circulation.

Salinity

The concentration of dissolved salts in water. Evaporation and sea-ice formation can raise seawater salinity, while rainfall, river inflow and melting ice can lower it. Salinity and temperature together strongly influence seawater density.

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NASA Aquarius surface-salinity composite, August–September 2011. Colours show salt concentration; black marks missing data. This early product contains calibration artefacts and is not a map of deep-water salinity.

Process0.40ExperimentParcel temperature4.0 °CAbsolute salinity35.0 g/kg