The great oceanic conveyor belt

We all know there are currents in the ocean - but did you know that seawater circulates on a global scale, taking thousands of years to complete a circuit of the globe?

You may imagine a conveyor belt as the belt that moves your groceries at the check out in the local supermarket. It is a continuous, never ending loop driven by an electric motor which we can turn on or off.

Something like this occurs in our oceans, moving water around the world in a continuous three-dimensional current. Understanding the cause of this motion and the specific route is important. These currents disperse energy around our planet and are a huge influence on the world’s climate and, therefore, on the economies of nations.

Recent findings have suggested that, just like in the supermarket, this conveyor belt can be switched on and off by certain events. Where is the switch? Somewhere in the North Atlantic and the Norwegian Sea. The concern we have at the moment, while the conveyor belt works, is that some of man’s activities may be a factor in switching the belt off. This could lead to the world experiencing a mini ice-age.

The driving forces

Ocean circulation is driven by the interaction of several factors.

Firstly, there are the wind-driven surface currents. The heating effects of the sun are strong at the equator and weak at the poles. This sets up a predictable wind pattern with winds moving from the equator to the poles. These winds are often called tradewinds as they were important in determining trade routes in the days of sailing ships. One very important such wind sets up the Gulf Stream, where warm surface water from the Gulf of Mexico is blown towards northern Europe and keeps winters here relatively mild.

Left: The pattern of winds across the globe, driven by strong solar heating effects at the equator.

Secondly, temperature [thermo] differences will influence water density. The colder the water, the denser it becomes. The rule is that warm water will float on cold water [see our pages on the El Niño effect].

Thirdly, sea water is salty [haline] but not all sea water has the same saltiness or salinity. The higher the salinity, the denser the water. You can predict that fresh water will float on salty water. The water with the greatest density must be cold, salty water and the water with the least density will be warm, freshwater. To find out more aobut ocean salinity, check out the salty oceans page.

The global conveyor belt is determined by thermohaline circulation, i.e., an interaction between the effects of the water temperature and salinity on its density.

At the moment the thermo part of this interaction appears to be dominant. If you have studied convection currents you will understand the science behind the process. In the Norwegian Sea cold, salty water (very dense), sinks to great depths and pushes cold water along the ocean floor towards the equator. It gradually warms up on this journey and so rises towards the surface. At the same time, the warm surface water from the equator is forced northwards towards the pole and cools on its journey.

The direction of these deep ocean currents is affected by the shape of the seabed and the spinning of the Earth. Scientists have predicted that all of the oceans share a complex inter-oceanic circulation. Look at this simplified map of ocean circulation to see how the currents interact. This has been produced from data collected by sources including satellites and sea based sensors.

Map showing the global oceanic conveyor belt.
Blue arrows show movement of cold water; red arrows show movement of warm water.Notice the flow of warm water from the Americas towards the UK - this is the GUlf Stream,
which helps keep our winters mild.

So what is the switch?

Some scientists are suggesting that global warming is making the haline force more dominant. Far from warming up the climate of northern Europe, the overall consequence will be a much colder climate.

Why? Initial warming will cause more evaporation and rainfall, added to this will be the melting of polar ice caps, adding more fresh water to the oceans. As this is less dense than salt water, it will form a layer over the warmer, salt water thereby reducing the heat transfer from the water to the atmosphere. The fear is that this could switch off the conveyor belt and stop the water transferring heat energy to northern Europe.

Left: Cartoon illustrating how global warming could actually lead to climatic cooling in some parts of the world, such as northern Europe.
[Click on image to enlarge]

Find out more about the oceans:

Salty oceans
Oceanic conveyor belt
The El Niño effect


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February 2007