Watching over the oceans
Predicting climate change
The oceans absorb most of the solar radiation reaching the planet, making them an enormous storehouse of heat. By redistributing this heat via their currents, the oceans attenuate temperature contrasts between equator and poles, enabling us to enjoy temperate climates.
Ocean currents also transport many dissolved substances: gases such as carbon dioxide, and chemical products including nitrates and phosphates. Some scientists estimate that the oceans slow down the greenhouse effect, and consequently global warming, by absorbing a large part of the carbon dioxide produced by human activities.
The oceans thus play a decisive, yet poorly quantified role in the changes of temperature on our planet. Ocean-atmosphere interaction also influences winds and precipitation.
Understanding ocean currents
Ocean currents regulate the planet's temperature. The oceans receive considerable amounts of heat in the equatorial zones. This heat is stored in the water and then gradually transported toward the poles by the currents.
Understanding these currents and their interaction with the ocean itself is essential. The sea level is very unstable in regions with intense currents, varying up to 30 centimetres, compared to only 6 or 7 centimetres where currents are weak. Knowledge of the main currents, winds, waves and tides, is also necessary to protect economic activities, since 98% of world trade takes place by sea.
The ocean is perpetually moving: on the surface, movements may take a few hours to several years, while movements at greater depths can take many centuries. Crédits : CNES/ill.D.Ducros
Unlocking the secrets of tides
Thanks to altimetry, we can now forecast tides in mid-ocean to within 2 cm.Crédits : Legos
In this complex phenomenon, deformations are not proportional to lunar or solar attraction, because of the ocean's multiple responses. This complexity, due mainly to the fluidity of the ocean, is accentuated by friction on the ocean floor, the extreme irregularity of the coasts, undersea topography, and so on.
The Topex/Poseidon satellite makes it possible to produce global maps of the oceans' tides accurate to within about 2 cm.
Analysing oceanic seasons
Like the atmosphere, oceans also have their seasons. In the northern hemisphere, for example, the ocean receives more of the sun's heat in the summer, causing it to expand.
One of Topex/Poseidon’s discoveries concerns the extent to which these seasons affect water levels. Measurements via satellite have enabled researchers to observe an average variation of more than 15 cm between the warm and cold seasons in the Atlantic, and 20 cm in the Mediterranean.
By analysing Topex/Poséidon data from the Mediterranean, researchers have, for the first time, been able to characterise changes in its mean sea-level. Variations are considerable, ranging up to 20 cm. Crédits : CLS/DOS
Precise mapping of the ocean floor
From April 1994 to March 1995, ERS-1, the European altimetry satellite, was placed in a special orbit that allowed high-density observation of the oceans and consequently the mapping of undersea topography (since there is a relationship between this and ocean surface level). The accuracy of the Topex/Poseidon measurements (thanks particularly to its Doris tracking system) has improved the ERS-1 measurements taken on this new orbit.
It has consequently been possible to map the entire ocean bed with a resolution of a few kilometres, revealing the great complexity of the undersea depths. In addition to formations which we already knew, such as the oceanic ridges, or the main zones of fracture or volcano alignment, the satellite also revealed formations previously unknown. Along with the major structures of which we were already aware, such as the mid-ocean ridges, major fault areas or lines of volcanoes, the satellite revealed other previously unsuspected structures.
