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Des grains de sel pour les altimètres

Image du mois - août 2003

La mer est salée, c'est bien connu, mais de combien ? La salinité (quantité qui ne comprend pas que le sel "de table", mais tous les types de sels dissous dans l'eau) n'est pas pour l'instant mesurée directement par satellite. En attendant les futures missions Smos (Esa) et Aquarius (Nasa), on peut estimer cette quantité grâce à l'altimétrie couplée à des mesures de température. En effet c'est, avec la température, ce qui détermine la densité de l'eau et influe donc sur la circulation océanique (Les effets de la salinité sur la hauteur de mer peuvent atteindre les 6 cm).

Salinity deduced from data measured over the Indian Ocean (mean over 8 years, in kg/m2), in January (winter monsoon, dry -- see Image of the Month for October 2002: Summer current, winter current), April, July (summer monsoon, wet) and October. Salinity increases with evaporation, and decreases with rains and runoff. It does not increase much in summer in the Bay of Bengal (East of India), into which many rivers flow, whereas the salinity of the Arabian Sea increases strongly during the same period.
(Credits Bulusu Subrahmanyam & James. J. O'Brien, Center for Ocean-Atmospheric Prediction Studies, Florida State University, USA)

 

La salinité peut avoir des conséquences importantes, notamment sur la formation du phénomène El Niño. Les différences de salinité sont une des raisons (avec la température) qui font que l'océan est stratifié en couche qui ne se mélangent pas, ou peu. Dans le Pacifique Tropical, une couche de salinité donnée joue un rôle de barrière. Selon sa position, la couche superficielle de l'océan où peuvent se produire des mélanges est plus ou moins profonde, et moins elle est profonde, plus la chaleur sera stockée dans un "faible" volume d'eau, d'où augmentation de la température de surface.

Quelques sites sur ce thème :

Références :

  • Shi W., B. Subrahmanyam, J.M. Morrison, 2003: Estimation of heat and salt storage variability in the Indian Ocean from Topex/Poseidon altimetry, J. Geophys. Res., 108 (C7), 7-1 to 15.
  • Maes, C. and D. Behringer, 2000: Using satellite-derived sea level and temperature profiles for determining the salinity variability: A new approch, J. Geophys. Res., 105 (C4), 8537-8547.
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