Satellite Salinity Observing System: Recent Discoveries and the Way Forward

Abstract

Advances in L-band microwave satellite radiometry in the past decade, pioneered by ESA's SMOS and NASA's Aquarius and SMAP missions, have demonstrated an unprecedented capability to observe global sea surface salinity (SSS) from space. Measurements from these missions are the only means to probe the very-near surface salinity (top cm), providing a unique monitoring capability for the interfacial exchanges of water between the atmosphere and the upper-ocean, and delivering a wealth of information on various salinity processes in the ocean, linkages with the climate and water cycle, including land-sea connections, and providing constraints for ocean prediction models. The satellite SSS data are complimentary to the existing in situ systems such as Argo that provide accurate depiction of large-scale salinity variability in the open ocean but under-sample mesoscale variability, coastal oceans and marginal seas, and energetic regions such as boundary currents and fronts. In particular, salinity remote sensing has proven valuable to systematically monitor the open oceans as well as coastal regions up to approximately 40 km from the coasts. This is critical to addressing societally relevant topics, such as land-sea linkages, coastal-open ocean exchanges, research in the carbon cycle, near-surface mixing, and air-sea exchange of gas and mass. In this paper, we provide a community perspective on the major achievements of satellite SSS for the aforementioned topics, the unique capability of satellite salinity observing system and its complementarity with other platforms, uncertainty characteristics of satellite SSS, and measurement versus sampling errors in relation to in situ salinity measurements. We also discuss the need for technological innovations to improve the accuracy, resolution, and coverage of satellite SSS, and the way forward to both continue and enhance salinity remote sensing as part of the integrated Earth Observing System in order to address societal needs.

Statistical Assessment of Sea-Surface Salinity from SMAP: Arabian Sea, Bay of Bengal and a Promising Red Sea Application

“…This study refers to these data pairs (SMAP, in-situ) as collocated observations. These pairs, however, are not strictly collocated because of the intrinsic differences between satellite and in situ observations [4,33,34].…”

“…The National Aeronautics and Space Administration's (NASA) Soil Moisture Active Passive (SMAP) satellite launched on 31 January 2015 was designed as a soil-moisture mission, but expanded to retrieve sea-surface salinity (SSS) from space continuing the legacy of the Aquarius mission (2011)(2012)(2013)(2014)(2015), the NASA pathfinder ocean-salinity satellite [1][2][3][4]. SMAP spinning antenna allows it to observe Earth in nearly full 360°scans in few seconds, and overlapping loops create a 1000 km wide swath.…”

“…Ocean salinity is far from being the primary factor contributing to the received signals by satellite L-band radiometers. There are several unwanted signals that must be removed to retrieve salinity [1,2,4]. These unwanted signals include extraterrestrial contributions (e.g., galaxy, moon, sun), antenna-radiation emissions, Faraday rotation in Earth's ionosphere, atmospheric attenuation, sea-surface temperature and roughness, and land contamination near coastlines (i.e., leakage of emissivity from the land surface into the radiometer receiver [3]).…”

“…Moreover, there is variability within the satellite footprint that is captured by in situ measurements but not by the satellite, in which SSS can be interpreted as a spatial-average over the footprint [33][34][35]. Because of the inconsistency between satellite and in situ measurements, the present work uses the term "difference" instead of "error" to refer to it (see [4] for a discussion on this matter). Figure 1 shows the mean SSS field for four years (September 2015 to September 2019) based on Argo in situ observations mapped into a 1°× 1°grid [36], and the mean fields from the two SMAP products that are evaluated in the present work.…”

“…A difference in the maps of Figure 1 is the absence of in situ observations in marginal seas (the Red Sea and the Arabian/Persian Gulf in the western, and the Andaman Sea in the eastern). Marginal seas are usually not included in Argo-gridded products [4,36] because the amount of in situ observations is much reduced and even nonexistent in these regions, such as in the very shallow Arabian/Persian Gulf (maximum depths of 110-160 m [44]). In the SMAP fields of Figure 1, there are also defined SSS values closer to the coast than in the Argo SSS mean-field as float-parking depth is around 1000 m, with few observations in shallow waters (<1000 m) in the standard Argo program [36].…”