Currents are the central heating system of our planet and play a key role in Earth’s climate by transporting heat and moisture around the world.

The global ocean circulation can be divided into two broad systems : A wind-driven circulation that dominates in the upper few hundred meters, and a density-driven circulation in the deep ocean. The latter is called “thermohaline circulation” or THC – (thermo ~ temperature, haline ~ salty) since it is driven by variation in temperature and salinity. Both currents are shaped by the Coriolis force and the boundaries of the ocean basins.

The wind driven surface currents form large gyres. You can see these clearly in satellite data from many different sensors: Sea surface temperature (SST), sea surface salinity (SSS) and ocean colour. From altimetry measurements of sea surface height we then calculate the flow in these currents.

Except for very high latitudes the surface and deep ocean are separated by the thermocline – a layer where temperature falls rapidly with depth. This acts as a barrier to water exchange between the surface and deep ocean. Only at high latitudes in the Atlantic and Southern Ocean is it possible for surface water to become cold and dense enough to sink into the deep ocean. This has consequences for the Earth’s climate system:

• Ocean heat transport: warm water flows towards the poles in the surface ocean and cold water flows back towards the equator in the deep ocean.

• Carbon uptake and storage: cold water rich in CO2 sinks into the deep ocean in the high latitude North Atlantic, and remains there for hundreds of years

How does the deep water return to the surface?

• Turbulence in the deep ocean helps to mix water between the thermocline and deep ocean. Thermocline water can then rise to the surface in equatorial and coastal upwelling regions at the edges of the ocean gyres. The upwelling can be seen clearly in global images of ocean colour and sea surface temperature.

• Mixing between surface and deep water can easily mix in the Southern Ocean, one of the regions where stored CO2 can return to the atmosphere.

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• Dr Pierre Yves Le Traon

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