The Aquarius/SAC-D salinity mapping satellite mission: Science and measurement objectives for studying the ocean freshwater budget

The scientific motivation for measuring sea surface salinity (SSS) from satellites is to study the interactions between the Earth's water cycle, ocean circulation and climate on intra-seasonal to interannual time scales. The key science objectives are 1) to improve closure of the mean and seasonal global ocean-atmosphere freshwater balance, 2) to study the influence of SSS variability on tropical dynamics and ENSO, and 3) to monitor large scale SSS changes in high latitudes that influence ocean convective overturning circulation and long term climate variability. Aquarius/SAC-D is a dedicated salinity mapping mission being developed by the United States and Argentina for launch in 2008 and a three-year baseline mission life. Aquarius/SAC-D is designed to optimize the salinity retrieval accuracy for global climate studies. The satellite will provide global maps every 8-9 days at 100 km resolution with a three-beam push-broom configuration of 60-90 km single-beam resolution. The measurement requirements prescribe an rms uncertainty <0.2 psu globally on monthly time scales. The satellite data will be validated and jointly analyzed with the growing array of in situ salinity observations from Argo, surface drifters, moored buoys and ships of opportunity. Perhaps the most significant scientific advances to come from Aquarius/SAC-D will be understanding the atmosphere-ocean fresh water budget. In many regions, today's uncertainties are on the same order as the net fresh water flux. Simulations show that the Aquarius/SAC-D measurement accuracy will resolve the range of SSS responses in an ocean model forced with various different commonly used precipitation fields. Aquarius/SAC-D will be launched in the same time frame as the Global Precipitation Measurement (GPM) and the European Soil Moisture Ocean Salinity (SMOS) missions, providing an unprecedented opportunity to address simultaneously the atmosphere-ocean-land components of the global water cycle.