el does not work on a large scale. Ships have to go up and down slopes, just the same as cars or trains. The ocean topography varies considerably according to local differences in the gravitational pull of the Earth. This usually means that the mountains and valleys of the sea floor are reflected on the surface. There is a difference in height of nearly 160m (525ft) between the highest summits and lowest troughs on the ocean surface.
These highs and lows are also influenced to a lesser extent by winds, temperature and water density. This is particularly noticeable on the western side of oceans, where water tends to pile up against continents. Between the highs and lows on the ocean surface, the global range of altitudes due to these other factors is quite small, only about 2m, but this 'dynamic topography' is significant.
The highs and lows of the sea surface act like their equivalent pressure systems in the atmosphere. Massive circulations known as gyres have been mapped in great detail by Topex-Poseidon. Around their perimeters, most noticeably on the western margins, ocean currents transport millions of cubic metres of water each second around the globe. Topex-Poseidon observations made over many months clearly show large- scale variability as huge eddies spin off from these currents.
'What we really want to know is what the currents are doing,' said Dr Philip Woodworth, Topex-Poseidon principal investigator at Proudman Oceanographic Laboratory on Merseyside. 'We measure sea level because we can convert it into currents. The greater the slope of the sea, the faster the current.'
Ocean variability takes many forms. Topex-Poseidon has been able to watch how the sea level falls in winter as the ocean cools, and how it rises in summer as the ocean expands. A temperature change of 1C in the upper ocean layer alters sea level by 1cm. Maps of wave height also show how strong winter winds generate giant waves more than 6m high in the mid-latitudes, compared with much gentler winds and waves in the summer.
On a smaller scale, the satellite has been able to watch the daily tides around the world, and a small El Nino-type event when a mound of water 10cm high and several thousand kilometres across moved eastwards across the Pacific over a period of several months.
In decades to come, such satellites will be able to reveal actual changes in sea level caused by climate change, while providing the data significantly to improve computer models of ocean behaviour Although Topex-Poseidon's active life is unlikely to exceed five years, other satellites are already on the way, starting with the launch of ERS-2 at the end of this year. They will play a major role in the setting up of a Global Ocean Observing System by the end of the century.