Sea surface salinity variability in the Agulhas Current region inferred from SMOS and Aquarius

Understanding of the Agulhas Current's unique retroflection and ring shedding has been greatly improved by satellite altimetry data over the last several decades, but salinity remote sensing technology has not been available to conduct similar studies with respect to regional sea surface salinity (SSS) variability. ESA's Soil Moisture and Ocean Salinity (SMOS) and NASA's Aquarius/SAC-D salinity missions can provide such data, but it remains unclear if the accuracy of these sensors is high enough to properly study this dynamic region. To address this issue, this study validates both SMOS and Aquarius against monthly in situ data from Argo floats. All available, corresponding data points over a three year period have root mean square differences less than 0.2 pss and correlation coefficients greater than 0.9. Additionally, over 80% of differences between the products fall between +/- 0.5 pss. Surface salt transports were calculated to see how well the satellite products estimate SSS advection in the Agulhas region. Zonal and meridional surface salt transports have annual maxima in austral winter and minima in austral summer, although the seasonal range is small. Using Argo, SMOS, and Aquarius SSS for this calculation produces statistically identical results, suggesting the utility of using the satellites to monitor regional salinity advection at higher frequencies and horizontal resolutions than previously possible using only sparse in situ data. SSS gradients along the subtropical front reveal that SMOS and Aquarius are much more effective tools for observing small-scale structure prevalent in the meandering of the Agulhas Return Current. Finally, we conclude that when compared to Argo floats in situ observations SMOS and Aquarius are statistically accurate and may yet prove powerful tools for observing Agulhas salinity processes on smaller time and spatial scales. (C) 2016 Elsevier Inc. All rights reserved.