The Aquarius/SAC-D Mission: Designed to Meet the Salinity Remote-Sensing Challenge

Abstract

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.

Estimating satellite salinity errors for assimilation of Aquarius and SMOS data into climate models

“…To be consistent with the reported statistics from Lagerloef et al [2008], in this section, instead of data error variance, we work in terms of the standard deviation of data error e D . Recall also that e 2 D can be an overestimate of observational error variance, as it also includes contributions from representation errors defined in terms of the model solution under consideration here.…”

“…A noticeable exception is the values of in situ error in the Arctic, where in situ e D could be as large as the satellite errors, likely due to the ice impact and general shortage of in situ data in high latitudes. The zonally averaged values of e D are consolidated between the hemispheres in Table 1 and are shown in reference to the total allocation errors for the Aquarius mission accuracy requirements [Lagerloef et al, 2008] As a final issue, if possible one would like to be able to separate different sources of errors and to estimate how much of e D are representation errors and how much are attributed to instrument noise. Notice, for example, relatively large values of the zonally averaged in situ errors in the equatorial band 10 S-10 N in Figure 12, with the band-average value reaching 0.17.…”

Observations of rain‐induced near‐surface salinity anomalies

“…However, it is known that vertical salinity gradients in the upper meter of the ocean can form due to precipitation [Soloviev and Lukas, 1996] and at low wind speeds these gradients can persist for many hours [Boutin and Martin, 2006]. Determining the conditions under which a precipitationinduced near-surface vertical salinity gradient (in this paper near-surface is interpreted to be depths ranging from a few centimeters below the surface salinity to a few meters) is present and whether the salinity change as a function of depth is large enough to affect microwave radiometric salinity measurements is important in understanding the performance of Aquarius and SMOS [Lagerloef et al, 2008;Boutin et al, 2013].…”

“…Res. : Oceans, 2014) using nearsurface ARGO surface temperature/salinity profiling drifters (ARGO-STS) [Lagerloef et al, 2008] has shown that negative salinity anomalies were observed in the top few meters of the tropical and subtropical ocean 1.3% of the time. Anderson and Riser (submitted manuscript, 2014) found the mean salinity difference between a depth of 4 m and the surface was 0.05& (i.e., the salinity at 4 m depth minus the salinity at the surface), although they provide evidence that much larger near-surface salinity gradients are possible.…”

“…Satellite-mounted L-band microwave radiometers such as Aquarius and Soil Moisture and Ocean Salinity (SMOS) provide monthly global maps of sea surface salinity (i.e., salinity measured in the ocean surface layer defined by the L-band penetration depth of approximately 0.01 m) [Swift, 1980] with an accuracy of 0.1 psu and a spatial resolution of 100 km [Font et al, 2004;Lagerloef et al, 2008]. However, there is evidence that in areas with frequent precipitation, salinity measured by Aquarius or SMOS is biased low by a few tenths of a psu compared to salinity measured by ARGO profiling drifters [Boutin et al, 2013;Lagerloef, 2013].…”