The Soil Moisture and Ocean Salinity mission: the science objectives of an L band 2-D interferometer

The main objective of the Soil Moisture and Ocean Salinity (SMOS) mission is to deliver crucial variables of the land surfaces: soil moisture, and of ocean surfaces: sea surface salinity fields. The mission should also deliver information on root zone soil moisture, vegetation, and biomass, and lead to significant research in the field of the cryosphere. The main aspects of the baseline mission are presented in this paper. Over land, water and energy fluxes at the surface/atmosphere interface are strongly dependent upon soil moisture (SM). Evaporation, infiltration and runoff are driven by SM while soil moisture in the vadose zone governs the rate of water uptake by vegetation. Soil moisture is thus a key variable in the hydrologic cycle. Soil moisture, and its spatiotemporal evolution as such, is an important variable for numerical weather and climate models, and should be accounted for in hydrology and vegetation monitoring. For the oceans, sea surface salinity (SSS) plays an important role in the Northern Atlantic sub polar area, where intrusions with a low salinity influence the deep thermohaline circulation and the meridional heat transport. Variations in salinity also influence the near-surface dynamics of tropical oceans, where rainfall modifies the buoyancy of the surface layer and the tropical ocean-atmosphere heat fluxes. SSS fields and their seasonal and inter-annual variabilities are thus tracers and constraints on the water cycle and on the coupled ocean-atmosphere models. Even though both SM and SSS are used in predictive atmospheric, oceanographic, and hydrologic models, no capability exists to date to measure directly and globally these key variables.