Evaluation and Intercomparison of SMOS, Aquarius, and SMAP Sea Surface Salinity Products in the Arctic Ocean

Abstract

Salinity is a critical parameter in the Arctic Ocean, having potential implications for climate and weather. This study presents the first systematic analysis of 6 commonly used sea surface salinity (SSS) products from the National Aeronautics and Space Administration (NASA) Aquarius and Soil Moisture Active Passive (SMAP) satellites and the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission, in terms of their consistency among one another and with in-situ data. Overall, the satellite SSS products provide a similar characterization of the time mean SSS large-scale patterns and are relatively consistent in depicting the regions with strong SSS temporal variability. When averaged over the Arctic Ocean, the SSS show an excellent consistency in describing the seasonal and interannual variations. Comparison of satellite SSS with in-situ salinity measurements along ship transects suggest that satellite SSS captures salinity gradients away from regions with significant sea-ice concentration. The root-mean square differences (RMSD) of satellite SSS with respect to in-situ measurements improves with increasing temperature, reflecting the limitation of L-band radiometric sensitivity to SSS in cold water. However, the satellite SSS biases with respect to the in-situ measurements do not show a consistent dependence on temperature. The results have significant implications for the calibration and validation of satellite SSS as well as for the modeling community and the design of future satellite missions.

Sea Surface Salinity as a Proxy for Arctic Ocean Freshwater Changes

“…The corresponding mean difference and RMSD between the ITP and ECCO-v4 time series are −0.03, 0.08, −0.03, and 0.02 pss, and 0.32, 0.39, 0.11, and 0.23 pss, respectively (Figure 3). These values are much smaller than the overall dynamic range of the temporal variability of salinity in the Arctic Ocean reported in the literature (e.g., Fournier et al, 2019) as well as illustrated in Figure 3.…”

“…This uncertainty multiplied by the regression coefficient corresponds to a threshold of equivalent SSS change that the satellite SSHA-OBPA can help to determine, that is, too small a regression coefficient would correspond to too small an implied change of SSS for a given steric sea level signal. If the implied SSS change is smaller than the discrepancies among satellite SSS and the differences between satellite SSS with the limited in situ data in the Arctic Ocean as reported by Fournier et al (2019), then satellite SSHA-OBPA data will not be helpful in evaluating the quality of satellite SSS. In this respect, a large regression coefficient is helpful because it corresponds to a large dynamic range of implied SSS change.…”

“…In contrast, the uncertainties of satellite SSS in tropical and subtropical oceans are~0.1-0.2 pss monthly, 1 o scales (e.g., Lee, 2016). Nevertheless, salinity signals in the Arctic Ocean can be very large (Carmack, Winsor, et al, 2015), with spatial variation as large as 10 pss and seasonal variation up to 5-10 pss (e.g., Fournier et al, 2019). The large signal-to-noise ratio may render satellite SSS estimates useful in certain regions of the Arctic Ocean, especially in the shelf regions that are strongly influenced by riverine freshwater input.…”

“…L band brightness temperatures have a much lower sensitivity to salinity in polar oceans than in lower-latitude oceans (Swift & McIntosh, 1983). As a result, Arctic SSS retrievals from these missions have large uncertainties (Fournier et al, 2019;Garcia-Eidell et al, 2017;Olmedo et al, 2018;Tang et al, 2018;Xie et al, 2019). Root-mean-square differences (RMSD) between in situ and satellite salinity data range between 0.8 and 3 pss in the Arctic basin, decreasing to 0.5-1 pss for sea surface temperature above 5°C (Fournier et al, 2019).…”

“…As a result, Arctic SSS retrievals from these missions have large uncertainties (Fournier et al, 2019;Garcia-Eidell et al, 2017;Olmedo et al, 2018;Tang et al, 2018;Xie et al, 2019). Root-mean-square differences (RMSD) between in situ and satellite salinity data range between 0.8 and 3 pss in the Arctic basin, decreasing to 0.5-1 pss for sea surface temperature above 5°C (Fournier et al, 2019). In contrast, the uncertainties of satellite SSS in tropical and subtropical oceans are~0.1-0.2 pss monthly, 1 o scales (e.g., Lee, 2016).…”