Think of the ocean on a calm day. Ignoring the rise and fall of the waves, you might imagi

ne the surface was dead flat the whole way across. You'd be wrong. Hills and valleys are as much as a feature of the sea as the land, although 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 meters. 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 (大地水准面) 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 meters.

The geoid represents the shape the sea surface would be if the oceans were net 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 variation in gravitational acceleration from several dozen satellites.

The hills and valleys of the oceans 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 Antarctic is one they are particularly interested in.

It can also predict local climate variations produced by events such as El Nino, El Nino keeps warm water that would normally move westwards close to the coast of South America, deprives Southeast Asia of its monsoon rains, and increases rainfall on the west coast of the Ametlca. Since temperature changes cause changes in sea level, geoid-watchers should be able to prepare us before it strikes.

From the first paragraph, we can learn that______.

A．the surface of the sea is a dead flat on a calm day

B．the sea waves are caused by a variety of factors

C．it's a good idea to swim uphill, sometime

D．hills and valleys only exist on land