Validation of SMOS and Aquarius Salinity data in the Agulhas Current System

Western boundary currents are important to study because they influence regional climates and may impact climate change. The Agulhas Current, in particular, is vital to the transport of heat and salt from the Indian Ocean to the Atlantic, specifically through Agulhas rings. On average, six Agulhas rings are shed per year from the current into the Atlantic Ocean, each being approximately 320 km in diameter. In order to better understand and assess the role of these rings in the global climate system, it is essential to make accurate measurements of salinity in this region. This study validated data within the Agulhas Current System from the NASA Aquarius/SAC-D salinity mission and ESA's Soil Moisture and Ocean Salinity (SMOS) mission using Argo float data and HYbrid Coordinate Ocean Model (HYCOM) simulations, which served as the best in situ and model comparisons to the satellite data respectively. There are approximately 100 Argo floats in the Agulhas Return Current that contribute to the monthly average. In situ observations, such as ones by Argo floats, are important because of their accuracy; however, these observations lack in being able to provide complete spatial and temporal coverage of the Agulhas Current System. This form of in situ observations is also limiting in that the measurements are not taken within the top five meters of the surface. Therefore, the satellite data provides the coverage that Argo floats cannot. Because of this, the satellite data must be as accurate as possible within the Agulhas Current in order to analyze the role of salinity and to determine the significance of the Agulhas rings. The four data sources (the two satellites, the model, and the in situ) were compared at four box regions of 11° latitude x 11° longitude, two inside and two outside of the current. The boxes overlaying a portion of the current were compared to an area of high salinity and an area of low salinity, both located outside of the current between Africa and Australia. The average annual salinity of the box encompassing the Agulhas Current and the Agulhas Return Current was approximately 35.34 – 35.50, as determined by each of the data sources. The time period September 1, 2011 to July 30, 2012 was used to determine the specific accuracies of each box. Results showed that Aquarius salinity measurements tended to be higher as compared to SMOS measurements as well as both observed (Argo) and modeled (HYCOM) values, except in areas of low salinity. In these regions of low salinity, Aquarius salinity measurements tended to be lower than the SMOS, Argo, and HYCOM measurements. This was true for both the daily and monthly data. Monthly data showed an improved accuracy over daily data and met the mission goal for each satellite of a global accuracy of 0.2 psu. In this study, the accuracy was determined through a comparison to Argo and HYCOM data. Additional accuracies were analyzed for the daily Aquarius data using the individual ascending and descending passes. A seasonal pattern was detected indicating that during the austral fall, the ascending pass measured a higher salinity than the descending pass. The opposite occurred during the austral spring. Lastly, the satellite measurements of sea surface salinity and sea surface temperature from the Advanced Very High Resolution Radiometer (AVHRR) were compared within each of the box regions. Through the analysis of the salinity measurements within these regions, this study determined the accuracy of salinity measurements within the Agulhas region from the NASA Aquarius/SAC-D salinity mission and ESA's SMOS mission to be within the acceptable range of the mission goal.