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The SMOS mission, launched on 2 November 2009, is a direct response to the current lack of global observations of soil moisture and ocean salinity which are needed to further our knowledge of the water cycle, and to contribute to better weather and extreme-event forecasting and seasonal-climate forecasting. Variability in soil moisture and ocean salinity is due to the continuous exchange of water between the oceans, the atmosphere and the land – the Earth's water cycle.

Soil moisture

The variability in soil moisture is mainly governed by different rates of evaporation and precipitation The importance of estimating soil moisture in the root zone is paramount for improving short- and medium-term meteorological modelling, hydrological modelling, the monitoring of plant growth, as well as contributing to the forecasting of hazardous events such as floods.

The amount of water held in soil, is of course, crucial for primary production but it is also intrinsically linked to our weather and climate. This is because soil moisture is a key variable controlling the exchange of water and heat energy between the land and the atmosphere. Precipitation, soil moisture, percolation, run-off, evaporation from the soil, and plant transpiration are all components of the terrestrial part of the water cycle. There is, therefore, a direct link between soil moisture and atmospheric humidity because dry soil contributes little or no moisture to the atmosphere and saturated soil contributes a lot. Moreover, since soil moisture is linked to evaporation it is also important in governing the distribution of heat flux from the land to the atmosphere so that areas of high soil moisture not only raise atmospheric humidity but also lower temperatures locally.

Ocean salinity

Between latitudes of 35°N and 35°S, the Earth receives more heat from the Sun than it loses to space. Poleward of these latitudes it loses more heat than it receives. The tropics would keep getting hotter and the poles would keep getting cooler if heat were not carried from the tropics by wind and ocean currents. Ocean currents are driven by temperature and salinity variations in the seawater.

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Knowledge of the distribution of salt in the global ocean and its annual and inter-annual variability are crucial in understanding the role of the ocean in the climate system. Ocean circulation is mainly driven by the water and heat flux through the atmosphere-ocean interface, but salinity is also fundamental in determining ocean density and hence thermohaline circulation. In the surface waters of the oceans, temperature and salinity alone control the density of seawater – the colder and saltier the water, the denser it is. As water evaporates from the ocean, the salinity increases and the surface layer becomes denser. In contrast, precipitation results in reduced density, and stratification of the ocean. The processes of seawater freezing and melting are also responsible for increasing and decreasing the salinity of the polar oceans, respectively. As sea-ice forms during winter, the freezing process extracts fresh water in the form of ice, leaving behind dense, cold, salty surface water.

If the density of the surface layer of seawater is increased sufficiently, the water column becomes gravitationally unstable and the denser water sinks. This process is a key to the temperature- and salinity-driven global ocean circulation. This conveyor-belt-like circulation is an important component of the Earth's heat engine, and crucial in regulating the weather and climate. Ocean salinity is also linked to the oceanic carbon cycle, as it plays a part in establishing the chemical equilibrium, which in turn regulates the CO2 uptake and release. Therefore the assimilation of sea surface salinity measurements into global ocean bio-geo-chemical models could improve estimates of the absorption of CO2 by the oceans.

Cryosphere

Observations of ice caps provide a prediction tool for the greenhouse effect since the sea ice extent responds early to altered climatic conditions. Accurate predictions of sea level rise require improved knowledge of the processes controlling the accumulation upon the ice sheets. The scarcity of accumulation rate observations, both spatially and temporally, has hindered the furthering of this understanding. Snow covers about 40 million km² of land in the Northern hemisphere during the winter season. The accumulation and depletion of snow is dynamically coupled with global hydrological and climatological processes. It is also a sensible indicator for climate change as the position of the southern boundary snow cover in the Northern hemisphere is likely to move northwards as a result of a sustained climate warming.

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