The Great Oceanic Conveyor Belt



© Participe Futur

Since a few days now, the sailing boat Alcyon arrived off the Norwegian coasts. There, the crew could feel the effects of the Gulf Stream, the great oceanic current. Everybody knows that there are currents in the oceans. But we don't imagine what an efficient and fantastic network it is: a circuit where oxygen and nutriments are transported, and which regulates temperatures and climate throughout the earth!

How is oxygen transported to the great depths ?

Nearly all the marine animals need oxygen to breath, just like you and I. Well, of course, there is about 30 times less oxygen in the water than in the air... This is why fish have a much more performing respiratory system than ours, in order to extract the oxygen molecules from the water: they have gills.

Well, this means that if there are fish living at great depths (and there are), there must be oxygen there. Where does it come from?

The ocean breaths through its surface

The oxygen itself comes from the surface of the oceans, where waves and tides mix the oxygen into the water. Even though, this o­nly works for the first 150 to 200 meters (deep) from the surface, while the oceans have an average depth of 3.7 kilometres and a maximum depth of 11 kilometres!

If you figure yourself the ocean to be a bathtub, you will then have two layers of water : o­n the surface, a very thin layer of well oxygenated water, and, underneath, all the rest of the water deprived of oxygen. But then, how does the oxygen get to the bottom?

The great circuit: the oceanic currents

In fact, there is a real circulation network which runs through all the oceans, with highways, roads and small streets: the highways are the great oceanic currents which run around the globe at speed of 4 m/h to more than 8 km/h. The roads and the small streets are the regional currents, specific of each coast, of each creek.

Not o­nly do these great oceanic currents flow horizontally, but they also flow vertically, at certain precise spots (some currents flow upwards, and some downwards).

Two main rules for water circulation

To understand this oceanic circulation, there are two main rules to learn:

  1. Cold water is denser than hot water (in this context, « denser » can be translated more or less by "heavier")
    What does this mean ? It means, for example, that if you pour hot and cold water into a glass, the cold water will immediately sink at the bottom of the glass because it is "heavier" than the hot water (you'll find a small and easy experiment to show this at the bottom of this article);
  2. Salty water is denser than fresh water. And the saltier the water is, the denser it gets (small and simple experiment also described at the bottom of this article).

Sea water (or ocean water) is salted. But there are places where it is saltier than in others. In fact, in the oceans, there are great masses of water that have different characteristics: warm or cold (depending o­n the climate of the place), and salty or saltier... And the combination of these different characteristics will decide which mass of water will stay at the surface or which will sink to the bottom.

Hop o­nto the "Great Conveyor Belt" !

The circuit of the great oceanic currents is often called « the Great Conveyor Belt ».
We are going to hop o­nto it and make the whole tour, in order to understand how this circuit works. (The currents are described here in a simplified way, because the oceanic currents are, in fact, much more complex). The map with the current is available at each paragraph, because having a look o­n it makes things easier to understand.

What creates the Gulf Stream?

We depart from a very nice and warm area: the Caribbean. The water is very warm there and very salty because the weather is hot and it doesn't rain often. Therefore, a great quantity of water evaporates and the salt concentrates more and more in the water which is left.

Pushed by the winds, these great masses of warm and salty water become a current which flows in the direction of the North Atlantic: the Gulf Stream. It is a surface current, even if the water is very salty, because the warm water is not as heavy as the cold waters surrounding it.

The Gulf Stream flows at a speed of 3 to 8 km / hour. o­n its way, it warms up the climate of England and of all the countries o­n the western coast of Europe: thanks to him, we have milder winters than Canada: Quebec is at the same latitude than Nantes (France) and it gets 4 months of snow per year for o­nly a few days of snow in Nantes... This illustration shows clearly the impact of the Gulf Stream, by showing the difference of the water temperatures o­n the European coasts and the Canadian coasts.

Once they arrive in the North Atlantic, the warm waters of the Gulf Stream begin to cool down. And for a same temperature, this water is saltier than the surrounding (and therefore denser)... they are going to sink straight to the bottom!

Water that sinks straight to the bottom !

It is off Norway (more precisely, in the Norway Sea, the Labrador Sea and the Groenland Sea) that this water sinks and becomes a deep current, called the North Atlantic Deep Water. This phenomenon is o­ne of the motors of the actual oceanic circulation. The North Sea is the o­nly place in the whole world which offers all the necessary conditions for this phenomenon to happen.
In Antarctica, a similar phenomenon also allows the surface water to sink to the depth of the oceans, but in lesser quantities: 10 millions m3 of water sink every second in Antarctica, while 20 or 30 million m3 / second sink in the North Atlantic.

So it is in the area that Alcyon crosses at the moment that the main part of the earth's deep currents is created. They "stir" the oceanic waters and transport oxygen to the great depths.

The North Atlantic is a key area for the actual oceanic circulation and many scientific people are afraid that the climate changes might destabilise the fragile equilibrium, which allows this phenomenon to happen. If any modification of the oceanic currents should happen in this area, it would mean important changes of the climate of all the European countries!

The Circumpolar Current, the most powerful current in the world

The cold and deep current which is created off Norway flows back in the direction of the South, through all the Atlantic, towards Antarctica, where it feeds the Circumpolar Current.

The Circumpolar Current flows all around the earth, near Antarctica, and is the most powerful of the globe! The father of "Nemo", the hero of Walt Disney's cartoon, swims into this incredible current with a Green Turtle that has an Australian accent: it is a deep current which flows all around Antarctica, with 180 million cubic meter of water per second, from the West towards the East ! (the flow of the Rhône river in its delta is "only" 2300 cubic meters per second)

The Circumpolar Current distributes cool water in all the oceans of the world, amongst which the Atlantic Ocean and the Caribbean.

Golden areas for fish... and for fishermen !

There are some areas, where small effluents of the deep and cold currents we just described climb along the coast, guided by the local topography, and finally reach the surface. Huge quantities of fish live in these places, because the cold currents bring nutriments to the surface, that they "picked up" in the ocean depths. These areas, well-known by the fishermen, are called "upwelling zones".

This cold water, o­nce it reached shallow areas in hot countries, warms up and becomes saltier and saltier... The loop is buckled!

Two simple experiments:

To show how cold waters sink to the bottom and create a water circulation

  • Take a transparent rectangular aquarium.
  • Put ice-cubes at o­ne end of the aquarium (just like o­n the picture).
  • Wait 3 or 4 minutes.

Then pour some colouring (syrup for example) at the other end of the aquarium and look at what happens: the syrup will cross the surface towards the ice cubes and then sink under the ice cubes to the bottom!

If you click o­n the picture, on this website, you'll see a short film showing the whole experiment.
In fact the ice-cubes create cold water that sinks to the bottom of the aquarium, thus creating a water circulation around the aquarium.

To show that salty waters are « heavier » (denser) than fresh water

  • Take two transparent glasses and pour 1/10 of litre in each of them.
  • In o­ne of the glasses, and pour a few drops of red syrup (to colour the water).
  • In the other glass, put 5 small spoons of salt and mix.
  • Pour very slowly the coloured water into the glass of salty water.

Take care: to avoid making swirls, hold the two glasses nearly in horizontal position.

If there haven't been too many swirls, the two masses of water will not mix. The salty water (white) stays at the bottom because it is denser than the fresh water (red).

For those wishing additional information :

O­n this website (only available in french) you'll find a program, which allows you to place a particle wherever you wish in the North Atlantic and see what road it will follow in the next years. (You'll see that oceanic currents are a lot more complex than described in this article!).

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