Think of the ocean on a calm day. Ignoring the rise and fall of the waves, you might imagine the surface was dead flat the whole way across. You’d be wrong. Hills and valleys are as much a feature of the sea as the land, albeit on a much smaller scale.
These undulations have a variety of causes. Tides, currents, eddies, winds, river flow and changes in salinity and temperature push the sea level up in some places and down in others by as much as 2 metres. Ever tried swimming uphill?
How do we map these oceanic hills and valleys? First, we need to know what the planet would look like without them. This is where the geoid comes in. It is a surface where the Earth’s gravitational potential is equal and which best fits the global mean sea level. It is approximately an ellipsoid, though uneven distribution of mass within the Earth means that it can vary from this ideal by up to 150 metres.
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The geoid represents the shape the sea surface would be if the oceans were not moving and affected only by gravity. Thus it can be used as a reference to measure any deviations in the ocean surface height that aren’t caused by gravity – the hills and valleys, for instance, or any regional increase in sea level.
So how do you measure the geoid and the ocean’s irregular topography? It’s complicated. Geophysicists calculate the geoid using data on variations in gravitational acceleration from several dozen satellites. The most recent one to tackle this, known as Grace and run by NASA and the German space agency DLR, was launched earlier this year. It helps to define the geoid to an accuracy of 20 centimetres. Now the European Space Agency is building another satellite, the Gravity Field and Steady State Ocean Circulation Explorer (GOCE), which is due to be launched in early 2006 and will help determine the geoid to an accuracy of 1 centimetre.
The hills and valleys of the ocean are all very interesting, but can the geoid tell us anything more significant about the state of the planet? It certainly can. Knowing accurately where the geoid lies and how the ocean surface deviates from it will help meteorologists spot changes in ocean currents associated with climate change. The circumpolar current around Antarctica is one they are particularly interested in.
It can also predict local climate variations produced by events such as El Niño – that’s the name given to the occasional reversal of the trade winds over the Pacific. El Niño keeps warm water that would normally move westwards close to the coast of South America, deprives South-East Asia of its monsoon rains, and increases rainfall on the west coast of the Americas. Since temperature changes cause changes in sea level, geoid-watchers should be able to prepare us before it strikes.