The Aquarius/SAC-D mission: Designed to meet the salinity remote-sensing challenge

In an Oceanography article published 13 years ago, three of us identified salinity measurement from satellites as the next ocean remote-sensing challenge. We argued that this represented the next " zeroth order " contribution to oceanography (Lagerloef et al., 1995) because salinity variations form part of the interaction between ocean circulation and the global water cycle, which in turn affects the ocean's capacity to store and transport heat and regulate Earth's climate. Now, we are pleased to report that a new satellite program scheduled for launch in the near future will provide data to reveal how the ocean responds to the combined effects of evaporation, precipitation, ice melt, and river runoff on seasonal and interannual time scales. These measurements can be used, for example, to close the marine hydrologic budget, constrain coupled climate models , monitor mode water formation, investigate the upper-ocean response to precipitation variability in the tropical convergence zones, and provide early detection of low-salinity intrusions in the subpolar Atlantic and Southern oceans. Sea-surface salinity (SSS) and sea-surface temperature (SST) determine sea-surface density, which controls the formation of water masses and regulates three-dimensional ocean circulation. The 1995 Oceanography paper evidently marked a turning point for salinity remote sensing. Until that time, there was no organized program for developing a satellite capability to measure this important yet sparsely observed ocean variable, in contrast to the established satellite programs to observe surface temperatures, winds, sea level, and ocean color. The paper demonstrated that newly evolving technology could provide global salinity measurements at a similar, scientifically useful accuracy and spatio-temporal resolution, and it came at a time of growing scientific awareness of the need for the data. Vigorous efforts begun in the late 1990s by the community of scientists interested in salinity measurements have led to the development of the Aquarius/SAC-D mission (Figure 1), now targeted for launch in mid 2010. This mission's principal scientific objective is to provide monthly global measurements of SSS. Here, we describe the Aquarius/SAC-D mission and how its capabilities are designed to meet the salinity remote-sensing challenge, providing the ocean research community with a preview of the measurement characteristics and scientific applications of this pioneering mission.