Variability and Uncertainty of Satellite Sea Surface Salinity in the Subpolar North Atlantic (2010–2019)

Lisan Yu

doi:10.3390/rs12132092

Abstract

Satellite remote sensing of sea surface salinity (SSS) in the recent decade (2010–2019) has proven the capability of L-band (1.4 GHz) measurements to resolve SSS spatiotemporal variability in the tropical and subtropical oceans. However, the fidelity of SSS retrievals in cold waters at mid-high latitudes has yet to be established. Here, four SSS products derived from two satellite missions were evaluated in the subpolar North Atlantic Ocean in reference to two in situ gridded products. Harmonic analysis of annual and semiannual cycles in in situ products revealed that seasonal variations of SSS are dominated by an annual cycle, with a maximum in March and a minimum in September. The annual amplitudes are larger (>0.3 practical salinity scale (pss)) in the western basin where surface waters are colder and fresher, and weaker (~0.06 pss) in the eastern basin where surface waters are warmer and saltier. Satellite SSS products have difficulty producing the right annual cycle, particularly in the Labrador/Irminger seas where the SSS seasonality is dictated by the influx of Arctic low-salinity waters along the boundary currents. The study also found that there are basin-scale, time-varying drifts in the decade-long SMOS data records, which need to be corrected before the datasets can be used for studying climate variability of SSS.

Highlights

Ocean salinity in the subpolar North Atlantic (SPNA, 45–70◦ N, 65◦ W–10◦ E) exhibits large variability on time scales from seasons to decades and longer time scales [1,2,3,4,5]. This variability is a result of the mixing and exchange between water masses of contrasting properties: The warm, high-salinity waters that are transported by the North Atlantic Current (NAC) from the tropical and subtropical latitudes and the cold, low-salinity waters that are transported from the Arctic latitudes by the East Greenland Current (EGC) and Labrador Current (LC) [6] (Figure 1a)
SMAP Jet Propulsion Laboratory (JPL) showed ω2 are the annual and semiannual frequencies expressed as ω1 = 2π/12 months and ω2 = 2π/6 months, large spatial variability of sea surface salinity (SSS) in the seasonal ice zone, which may be related to the inclusion of an and a1, a2, φ1, and φ2 are the estimated regression coefficients for the amplitudes and phases of the improved land/ice correction that allows SSS to be retrieved within 35 km of the coast in ice‐free annual and semiannual harmonics, respectively
For the Argo SSS field, the ice margin was the boundary of available data points, cycle of salinity have been made in several previous studies [54,55]

Summary

Introduction

Ocean salinity in the subpolar North Atlantic (SPNA, 45–70◦ N, 65◦ W–10◦ E) exhibits large variability on time scales from seasons to decades and longer time scales [1,2,3,4,5]. This variability is a result of the mixing and exchange between water masses of contrasting properties: The warm, high-salinity waters that are transported by the North Atlantic Current (NAC) from the tropical and subtropical latitudes and the cold, low-salinity waters that are transported from the Arctic latitudes by the East Greenland Current (EGC) and Labrador Current (LC) [6] (Figure 1a). Monitoring and understanding ocean salinity variability in the SPNA has long been articulated as a central element for improving modeling and prediction of the changes in subpolar water mass transformation and the Meridional Overturning Circulation (MOC) [13,14].

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