Warm and cold gulfstreams
Oceans cover approximately 70% of the surface of the Earth. The volume of the sea is large, and it contains a lot of heat. Part of this heat is carried by sea currents from warm areas near the Equator to colder areas to the north and south. This heat transport is essential to climate and weather conditions in the colder areas as they receive much of their heat from sea currents. Climate and weather conditions are generally determined by the oceans.
The Gulf Stream carries heat to the North Atlantic and has a considerable impact on European climate. The salinity of the Gulf Stream is a precondition of deep water formation in the Norwegian Sea and the Greenland Sea.
The Gulf Stream and the climate in Europe
The climate and weather in Europe are very dependent on the Gulf Stream, which carries heat from the sea off the coast of Florida to Northern Europe, past Iceland, the Faroe Islands and the north of Norway. In these parts of the Atlantic Ocean, the atmosphere is warmed by the Gulf Stream. The warm air blows across Europe in westerly winds. Consequently, Europe has milder winters.
Why are the warm waters carried so far north? The explanation is found in deep water formation in the North Atlantic.
In the Norwegian and Greenland Seas, surface water descends several kilometres down to the bottom of the ocean where it flows back southwards. The descent of surface water draws new surface water from the south. This flow from the south is in fact the Gulf Stream.
Deep water formation
Warm surface water flows from the south to the north
Deep water formation in the North Atlantic is a gigantic pump, which is believed to power a global deep sea current that flows underneath well-known surface currents. The deep sea waters from the North Atlantic surface and reappear in different places in the Pacific Ocean many hundreds (maybe even thousands) of years after they have descended.
But why do water masses decend in the North Atlantic? What keeps the pump going?
The illustration shows how warm surface water flows from south to north. It descends at the polar ice front and flows back south.
The descent is accelerated when the surface water freezes to ice. Salt in the surface water is liberated from the ice and is added to the water just below the newly formed ice. The increase in the salinity of the water leads to an increase in the density of the water. So the increase in salinity is the reason why the water descends.
The salt pump draws warm, saline water from the south (the Gulf Stream) and pushes cold saline deep sea water towards the south.
The descent is due to two things:
The sea surface water freezes to ice and thaws again
The sea surface water under the ice contains a higher salinity than the deeper layers
The North Atlantic has low salinities, so new salt has to be supplied by the Gulf Stream to make the pump work - hence the name 'the salt pump'. The pump also works by the alternating freezing and thawing of the water. The mechanism can be described in the following five steps:
1. Warm saline surface water is supplied
2. The surface water freezes and becomes ice, and salt is liberated in the water under the ice
3. Cold surface water of higher salinity descends to the bottom
4. Fresh saline water is drawn from the south
5. Ice melts in warm periods
It is essential that the depth of the sea is quite substantial for the salt pump to be powerful. So it is important that the ice shelf is situated near the deep areas of the North Atlantic, that is the Greenland Sea and the Norwegian Sea.
Warmer climate and a new Ice Age?
Well-known cold and warm sea surface currents
The average temperature of the atmosphere has been steadily rising for many years. This has led to the study of global warming and the degree of responsibility that can be attributed to humans.
Climatologists and oceanographers are worried that a warmer climate may have unexpected consequences for deep water formation in the North Atlantic.
The well-known cold and warm sea surface currents. Note the cold south-flowing currents at the ocean floor. They are powered by the salt pump.
Higher temperatures in the atmosphere will move the polar ice front to the north and closer to land. The ice front moves away from the deep seas between Greenland and Norway and into shallower waters where the salt pump may be substantially weakened.
If the salt pump is weakened, the supply of warm saline water to the North Atlantic by the Gulf Stream will be proportionately reduced. The salinity of the water will decrease and the salt pump will be further weakened.
The consequence could be a swift and permanent freezing over of the waters north of Iceland and the Faroe Islands. Then the salt pump and the Gulf Stream would stop carrying heat to the waters north of Iceland. The temperatures over these areas would decrease to a lower level than at present, leading to a climate in northern Europe that would be significantly colder, perhaps even comparable to a Glacial Age.
At the moment, this is only theoretical, but scientists have found that the most recent Glacial Age started very suddenly 130,000 years ago. It may have started like this. However, nobody can tell for sure.