Week 1: Oceans and Climate > Topic 1d - Large scale ocean currents

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:

How does the deep water return to the surface?

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Geostrophic currents in m/s

The ocean currents and their speeds (in cm/s) derived from GOCE data.

schematic of the global ocean circulation (NB! This is not a current map). It shows how water sinks in the North Atlantic and Southern Ocean and travels round the ocean basins. The Southern Ocean links the other oceans.

Global Thermohaline circulation