Some parts of the ocean are higher than others
| Even taking away the waves and the tides, the oceans of the world are not entirely flat. The water level in one region can be several inches higher than the water level elsewhere. There are two reasons for this: water temperature and surface winds. When water is warmed, it expands slightly. So a region of water that is warm will expand a small amount, causing the height of the surface to rise. Steady winds can also cause a change in sea surface height by "pushing" the water and causing it to pile up against a coastline. Even small effects such as these can be detected by a satellite using an instrument called an altimeter. Oceanographers can use this information to study changes in the behavior of the world's oceans. |
Credit: NASA/TOPEX-Poseidon
For more info: http://topex-www.jpl.nasa.gov/education/altimeter-basics.html