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In this paper, we selected illustrative examples to review how the first SSS products
derived from the SMOS sensor can readily help to better characterize some of the key
processes of the marine branch of the global hydrological cycle. First, we illustrated the
new monitoring capabilities for some of the world's largest oceanic freshwater pools
generated by the discharge of very large tropical rivers. In particular, we show how SMOS
SSS traces the freshwater signals from the Amazon-Orinoco and Congo river plumes.
River runoff is an important variable in oceanography as their freshwater affects SSS and
the buoyancy of the surface layer, and they represent a source of materials exotic to the
ocean and highly important to biological activity. Obviously, they are key hydrologic
components of the freshwater exchanges between the atmosphere, land, and ocean. Despite
this importance, tracing river freshwater transport over large distances has not been
straightforward previously principally because of a lack of SSS data. Tracing those very
large rivers over great distances now become an important endeavor, as sufficient data are
available from the SMOS and Aquarius sensors that can be further combined with satellite-
derived surface geostrophic current data.
Second, we evidenced key oceanic precipitation signatures in the SMOS SSS signal.
Satellite radiometry at L-band provides for the first time a global measure of the salinity at the
ocean-atmosphere interface (within the upper centimeters). Rain events induce freshening of
the ocean surface and are responsible for a high temporal variability in the SSS, consistently
detected by both in situ and space-borne sensors. Because of the vertical haline gradient
generated by the rain-induced freshening in the upper ocean, fresher surface waters are,
however, systematically found from space in rainy area compared with the 1-10-m depth
in situ data. These differences challenge calibration/validation activities of the satellite SSS in
high precipitation regions. Nevertheless, satellite SSS data certainly provide new information
about ocean-atmosphere interfacial freshwater fluxes in these conditions. This was evi-
denced by comparing spatial patterns and amplitudes of the large-scale SSS anomalies
estimated from the SMOS data and the net evaporation minus precipitation fluxes in the
tropical Atlantic. Under the Inter Tropical Convergence Zone and sufficiently far away from
the river runoff signals, residual SSS anomalies were shown to be highly correlated with the
Evaporation minus Precipitaion (E-P) anomalies. In particular, SSS anomalies become
increasingly negative as the precipitation anomalies progressively exceed the evaporation
anomalies. This demonstrate the importance of monitoring SSS from space in rainy regions,
suggesting that the interfacial SSS values might be a good large-scale oceanic rain gauge of
the global hydrological water cycle.
The interfacial character of the space-borne measurements also offers new information
of interest for ocean circulation models in the perspective of better constraining oceanic
precipitation forcing terms.
Finally, the SSS observations from SMOS satellite were used to reveal new aspects of
the main tropical fresh pool evolution and interaction with wind-driven atmospheric pro-
cesses. SMOS imagery thus captures how the large eastern Pacific fresh pool is system-
atically eroded at the end of the boreal summer on its eastern side by the wind-driven
Panama upwelling, which brings cold and salty waters to the surface. Prior to SMOS data
availability, the few existing studies of the eastern Pacific describing seasonal variations of
SSS did not investigate their cause beyond rainfall (e.g., Fiedler and Talley 2006 ). Thanks
to the new SMOS data, SSS variability associated with wind-driven processes in that
region, such as the Panama upwelling signal recently evidenced by Alory et al. ( 2012 ), can
now be characterized more deeply.
Because of the buoyant character of the freshwater that forms at the ocean surface due to
large river discharges or intense local precipitation, the upper ocean stratification in several
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