The thermohaline loop and heat exchanges
"Thermo" for temperature, "haline" for salt. Cold, salty water is denser: it sinks. This engine slowly moves water through the whole world ocean and carries heat. The path on the globe is a diagram.
See a cross-section
Dive below the surface to see water change density, sink or rise.
What you see on the globe
- The ocean surface flows: the swirls move with the currents, pushed by the winds. Water turns in large whirls (gyres) and speeds up along the western coasts of the oceans: Gulf Stream, Kuroshio, Brazil Current. Where the current is fast, the water sparkles more.
- The colour of the water shows its temperature: deep blue for cold water, turquoise for warm water. Water keeps its heat far longer than air: look at the tongue of mild Gulf Stream water reaching off Norway, while along Morocco or California the water stays cool.
- The ocean is made transparent ("See below the surface" layer): the blue vein flows deep down, the orange one at the surface. Blue dots going down show cold, salty water sinking; those coming up warm up.
The great ocean conveyor belt
one loop ≈ 1000 years- 1Sinking. In the Norwegian Sea, south of Greenland and in the Weddell Sea, surface water cools. When sea ice forms, it releases its salt into the water nearby. The water becomes very dense and sinks to 2,000–4,000 m.
- 2Deep journey. These cold waters (in blue) flow down the Atlantic, join the circumpolar current around Antarctica, then reach the Indian and Pacific Oceans.
- 3Upwelling. They slowly warm by mixing and rise back towards the surface in the Indian Ocean and the North Pacific.
- 4Return at the surface. Warm currents (in red) flow back to the Atlantic via Indonesia and southern Africa, up to the Gulf Stream and the North Atlantic Drift, and the loop starts again.
Heat exchanges from ocean to atmosphere
- The ocean stores huge amounts of heat: its top 3 metres can hold as much as the whole atmosphere.
- The Gulf Stream and the North Atlantic Drift carry tropical water northwards. In winter this water gives its heat to the cold air (orange dots on the globe), sometimes several hundred W/m².
- The westerlies carry this milder air towards Europe: winters there are milder than in Canada at the same latitude.
- If surface water becomes less salty (melting ice, rain), it sinks less: the loop can slow down. This is closely studied with global warming.
Who carries the heat?
Transport towards the poles by the ocean and by the atmosphere (PW = 10¹⁵ W)
totalatmosphereocean
- Near the equator, the ocean carries as much heat as the atmosphere, or even more.
- Beyond 30°, the atmosphere takes over with the storms of the westerlies.
- Total transport peaks around 35 to 40° latitude, where the energy budget switches from gain to loss.