Geoscience Reference
In-Depth Information
1.7 mm per year in the twentieth century and at a satellite-measured average rate of about
3.2 mm per year from 1993 to 2009 (Meyssignac and Cazenave
2012
), but no acceleration
has been noted in the period 1993-2013. It is unclear whether the increased rate reflects an
increase in the underlying long-term trend. Two main factors contribute to the observed sea-
level rise, notably thermal expansion from general warming of the ocean and enhanced
freshwater run-off from land-based ice due to increased melting. As such, long-term
observations of the global mean and spatial sea-level change jointly with mass changes of
the ice sheets and glaciers become of paramount importance for monitoring of the hydro-
logical cycles and constraining of the water budget between the glaciers and the ocean.
15 Sea-surface Salinity
A total of 86 % of the total global evaporation and 78 % of the precipitation occur over the
ocean (Schmitt
1995
). As such, the ocean surface salinities have proven to be a much more
reliable indicator of the water cycle than many of the land-based measurements. Salinity,
moreover, is a fundamental ocean state variable and a major determinant, along with Sea-
Surface Temperature (SST), of the density of seawater; hence it is a crucial factor in ocean
circulation, which in turn has a major impact on climate (Schmitt
2008
). Salinity vari-
ability at the sea surface also modulates or is modulated by heat, momentum and CO
2
exchange between the ocean and atmosphere. Salinity is an important constraint in ocean
models and an indicator of freshwater capping. Sea-surface salinity (SSS) is correlated
with differences between precipitation and evaporation (P-E), and improved knowledge of
P-E would provide a better estimation of latent heat flux and improve characterization of
stratification of the near-surface ocean layer. Besides, SSS variability is also related to the
freezing and melting of sea ice and to freshwater river run-off. A better understanding of
all these phenomena will be fostered by the recent availability of synoptic measurements of
SSS thanks to the ESA SMOS (Font
2010
) and NASA/CONAE Aquarius (Le Vine et al.
2010
) satellites. Concurrently with the continuous improvements in the accuracy and
reliability of these data, a routine monitoring of Sea Surface Salinity (SSS) is becoming
possible for the first time thus allowing a quantitative characterization of the above-
mentioned processes and their mutual relationships in the context of the hydrological
cycle.
16 Freshwater Discharge for Large Mid-Latitude and Tropical Rivers
Occurrences of low salinity surface patches in tropical regions are closely related to the
presence of the estuaries of the world's largest rivers in terms of fresh-water discharge
(Amazon, Congo, Orinoco, Niger) and the subsequent spreading of freshwater by the sur-
face oceanic circulation. The largest tropical river discharge regions have been studied using
satellite altimetry, SST and ocean colour, but each technique has limits in these fresh pool
regions, since salinity is the main controller of surface density in those areas. Now the
satellite sea surface salinity (SSS) missions bring the unique capability to directly detect and
track freshwater spatial gradients and lateral advection across the tropical oceans. The
spatial extent of the buoyant plumes of freshwater that form in the tropical seas due to
discharges from these world largest rivers can thus be temporally traced by SMOS/Aquarius
imagery with an unprecedented resolution (Reul et al.
2013
) In particular, river-influenced
