Marine currents, the regulators of the climate

The ocean and the atmosphere together form a gigantic natural thermal machine. In effect, the ocean and the atmosphere are constantly exchanging gases, water and heat, which the ocean redistributes around the globe through marine currents.

These exchanges determine the climate around the world.

The ocean warms by absorbing solar energy and cools by evaporation and by releasing heat into the atmosphere.

However, as water has a high thermal inertia, it heats up and cools down very slowly. This means that the world's oceans store enormous quantities of heat that they release over very long periods of time.

Heat is redistributed across the planet by powerful currents that flow from the bottom to the surface and from one region to another.

Through the surface currents, the heat accumulated by the sea in the tropics will warm up the polar zones, thereby reducing the differences in temperature.

In the North Atlantic, the Gulf Stream carries heat from the tropics to Europe. It then splits into two branches that flow northwards where the water cools. Sea ice forms and salt is released into the seawater.

These waters are more salty, colder and denser and sink to the bottom. They then flow back towards the equator and after a long journey around the planet, they come back up to the surface.

The waters only sink to the depths in some very specific points around the globe, which are mainly located in the North Atlantic and the Southern Ocean. The largest source is situated in the North Sea, between Greenland and northern Europe.

Antarctica also plays an important role. Along the coasts, near Adelie Land, there is an area without any ice where warmer water flows upwards, exchanges heat and forms cold water that sinks to the bottom.

The Antarctic Circumpolar Current (ACC), which is the most powerful, largest and fastest moving current on the planet, agitates the waters of three oceans: the Atlantic, Indian and Pacific. It is 20,000 km long, 200 to 100 km wide and can go down to depths of 4,000 m. It carries 130 million m3 per second and has a surface speed of 0.9 to 3.7 km/h.

Due to its geographical position and the particularities of the ACC, the Southern Ocean plays a major role in ocean circulation around the globe and in climate regulation.

Changes in water temperature and salinity (as land and polar ice melts) affect the density of the water masses, which could slow the descent of cold waters into the deep sea. The marine currents could be affected.

Changes in current and wind patterns can cause climatic disturbances such as severe storms.

In its latest Special Report on the Ocean and Cryosphere in a Changing Climate, published in 2019, the IPCC warns that melting ice and rising temperatures are altering the workings of ocean currents, which have an extremely long global cycle.

In particular, the slowing of the North Atlantic Current reduces marine productivity, causes winter storms in Europe, and reduces rainfall in the Sahel and South Asia.