Abstract
Operated since the end of 2009, the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) satellite mission is the first orbiting radiometer that collects regular and global observations from space of two Essential Climate Variables of the Global Climate Observing System: Sea Surface Salinity (SSS) and Soil Moisture. The National Aeronautics and Space Administration (NASA) Aquarius mission, with the primary objective to provide global SSS measurements from space operated from mid-2011 to mid-2015. NASA's Soil Moisture Active-Passive (SMAP) mission, primarily dedicated to soil moisture measurements, but also monitoring SSS, has been operating since early 2015. The primary sensors onboard these three missions are passive microwave radiometers operating at 1.4 GHz (L-band). SSS is retrieved from radiometer measurements of the sea surface brightness temperature (TB). In this paper, we first provide a historical review of SSS remote sensing with passive L-band radiometry beginning with the discussions of measurement principles, technology, sensing characteristics and complementarities of the three aforementioned missions. The assessment of satellite SSS products is then presented in terms of individual mission characteristics, common algorithms, and measurement uncertainties, including the validation versus in situ data, and, the consideration of sampling differences between satellite SSS and in situ salinity measurements. We next review the major scientific achievements of the combined first 10 years of satellite SSS data, including the insights enabled by these measurements regarding the linkages of SSS with the global water cycle, climate variability, and ocean biochemistry. We also highlight the new ability provided by satellites to monitor mesoscale and synoptic-scale SSS features and to advance our understanding of SSS' role in air-sea interactions, constraining ocean models, and improving seasonal predictions. An overview of satellite SSS observation highlights during this first decade and upcoming challenges are then presented.
Highlights
Ocean salinity is a key physical-chemical variable that critically contributes to the density-driven global ocean circulation and the Earth's climate (Siedler et al, 2001)
Because Soil Moisture Active-Passive (SMAP) is a real aperture radiometer, Sea Surface Salinity (SSS) in coastal areas from that sensor are less affected by land masses than Soil Moisture and Ocean Salinity (SMOS) SSS data which are affected by image reconstruction artefacts, in addition to the land-sea transition effects
This improvement was confirmed by a better matching between river plume signatures observed in SMOS SSS with those reported from SMAP data in the Bay of Bengal and the Gulf of Mexico (Boutin et al, 2018)
Summary
Ocean salinity is a key physical-chemical variable that critically contributes to the density-driven global ocean circulation and the Earth's climate (Siedler et al, 2001). Bingham et al (2002) examined the global distribution of historical (1874–1998) in situ SSS observations measured at 5 m or less in depth from the World Ocean Database 1998 (WOD98) Their results give a good indication of how poorly SSS was sampled by the end of the 1990s (Fig. 1), despite a peak period of ocean sampling (including near surface salinity) during the World Ocean Circulation Experiment (WOCE, 1990–1998). The feasibility of the measurement of SSS at L-band was demonstrated in the 1970s in a number of field campaigns from aircraft (Droppelman et al, 1970), from a bridge which spans the Cape Cod canal (Swift, 1974), and even with a satellite radiometer from the short-lived (2-week) Skylab S-194 mission (Lerner and Hollinger, 1977) It took > 40 years after the aforementioned pioneer experiments to develop and implement instruments to measure SSS regularly from space with sufficient accuracy and spatial resolution to address the GODAE recommendation. A list of all abbreviations and acronyms used in the paper is given in the Appendix B
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