Assessing ocean salinity retrieval using WindSAT data over the Amazone river plume and North Brazil Current retroflection

Measurement of ocean surface salinity (SSS) dynamics from space involve precise determination of the dielectric characteristics of seawater through low-noise passive microwave (MW) radiometer measurement of the oceans brightness temperature (TB), optimally performed at a low frequency near 1.4 GHz (L-band). The future SMOS mission is based on such principles and will aim at retrieving SSS with an accuracy of the order of 0.1 psu (practical salinity units). This represents numerous engineering and scientific challenges, in particular because competing terms carried in the ocean TB measurements, foremost being sea surface temperature (SST) and ocean surface roughness, must be accounted for in a new and more robust manner. Prior to launch, we analyze the sensitivity of the future measurements to these factors by looking at somewhat higher frequency C and X-band data (6.8 and 10.7 versus the L-band 1.4 GHz) that are readily available from several recent MW imagers (AMSR-E, TMI, WindSAT). The shift to C and X-bands lowers TB sensitivity to changes in salinity by a large factor of 10-20. To compensate, we performed our study over the Amazon plume region where there are large (100-200 km) and persistent salinity contrasts that exceed the 0.1 psu science salinity requirement by a factor of 10-40 [1]-[2]. This region is of great importance within the salinity mission context due to the large freshwater flux and northward propagating eddies from retroreflection of the North Brazil Current (NBC). The validity of the emissivity/scattering models developed for SMOS at L-band are analyzed at these higher frequencies and our ability to detect these world largest salinity gradients is assessed.