Dear Colleagues,

Salinity plays an important role in the global ocean as it influences water mass formation, density and circulation, heat storage, air–sea interactions, and the hydrological cycle. Understanding salinity variability is, therefore, paramount in understanding global climate. In the past, salinity measurements have been sparse. The launches of the soil moisture and ocean salinity (SMOS), Aquarius, and soil moisture active passive (SMAP) satellites initiated a new era for investigating the surface salinity of global oceans, which has improved our understanding of salinity variability and dynamics, among others. The scientific value of data collected by these salinity satellites is contributing to both oceanographic and climate-related studies. These data have been further augmented by improved ocean observations from Argo floats, drifters, and gliders, along with numerical modelling and data assimilation.

The aim of this Special Issue is to highlight recent salinity studies. This includes the successes, applications, and impacts of satellite-derived sea surface salinity measurements on oceanographic research. This Special Issue also highlights several ongoing innovative, synergetic uses of other satellite-derived parameters (e.g., SST, altimetry, scatterometry, ocean color), in situ measurements, data assimilation, and numerical models to further our understanding of the global earth system, especially ocean variability, dynamics, and air–sea interactions.

The topics of interest include, but are not limited to, the following:

• Salinity studies in the aquatic environment;
• Inter-comparison, evaluation, and validation of salinity products ;
• Successes and challenges of satellite-derived sea surface salinity missions;
• Improvements in sea surface salinity retrieval and products;
• Improving retrieval techniques for coastal sea surface salinity;
• Effects of rain on satellite salinity retrieval;
• Understanding salinity variability using satellite(s), in situ observations, and ocean models;
• Ocean salinity budgets, fluxes, and transports;
• Salinity-influenced stratification and air–sea interactions;
• Use of salinity information in understanding freshwater plumes;
• Data assimilation of salinity to improve ocean studies, climate variability, and forecasting;
• Role of salinity in understanding ocean and climate change;
• Using salinity products to improve understanding of the hydrological cycle;
• Novel applications of satellite-derived sea surface salinity products.

Dr. Ebenezer Sackitey Nyadjro
Guest Editor

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• salinity
• sea surface salinity
• in situ salinity
• SMAP
• SMOS
• Aquarius
• remote sensing
• data assimilation
• freshwater dynamics
• ocean climate

Title: Salinity fronts in the South Atlantic
Authors: Igor M Belkin; Xin-Tang Shen
Affiliation: Zhejiang Ocean University
Abstract: Seasonal climatology of salinity fronts in the South Atlantic has been created from satellite SMOS sea surface salinity (SSS) measurements, 2011-2019, processed at the Barcelona Expert Center of Remote Sensing (BEC) and provided as high-resolution (1/20°) SSS data. The SSS fronts are identified with narrow zones of enhanced horizontal gradient magnitude (GM) of SSS computed with the Belkin-O’Reilly algorithm. Seasonal climatologies are generated for large-scale open-ocean SSS fronts and for low-salinity regions maintained by the Rio de la Plata discharge, Magellan Strait outflow, Congo River discharge, and Benguela upwelling. To facilitate feature recognition in satellite imagery, the salinity gradient has been log-transformed, which improved visual contrast of gradient maps, revealing new features. A 2000-km-long triangular area in the eastern tropical-subtropical Atlantic is filled with regular quasi-meridional mesoscale striations that form a giant ripple field with a 100-km wave length extending from Africa toward the NE Brazil. South of the Tropical Front, within the subtropical high-salinity pool, a trans-ocean quasi-zonal narrow linear belt of SSS maximum is documented. The Smax belt shifts north-south seasonally while retaining its well-defined linear morphology, which is suggestive of a mechanism that maintains this feature. In the SW Atlantic, the Rio de la Plata plume expands in winter (June-July), reaching across the South Brazilian Bight, up to Cabo Frio (23°S) and beyond. The Plata estuarine front moves in and out seasonally. Farther south, the Magellan Strait outflow expands northward in winter up to 39-40°S to nearly join the Plata outflow. In the SE Atlantic, the Congo River plume spreads radially from the river mouth, with the spreading direction varying seasonally and interannually. The plume is often bordered from the south by a quasi-zonal front along 6°S. The diluted Congo River water spreads southward seasonally down to the thermal Angola-Benguela Front at 16°S. The Benguela upwelling is delineated by a meridional front, which extends north alongshore up to 20°S, where the low-salinity Benguela upwelling water meets the high-salinity tropical water (“Angola water”) to form a salinity front, which is separate from the thermal Angola-Benguela Front at 16°S. The Angola water thus forms a wedge between the low-salinity waters of the Congo River outflow in the north and Benguela upwelling in the south. This high-salinity wedge is bordered by salinity fronts that migrate north-south seasonally.

Title: Satellite-derived variability of sea surface salinity and geostrophic currents off western Patagonia
Authors: Gonzalo Saldías; Pedro Figueroa; David Carrasco; Diego Narváez; Iván Pérez-Santos; Carlos Lara
Affiliation: University of Bío-Bío
Abstract: The coastal ocean off western Patagonia is one of the main coastal regions with high freshwater inputs from rivers, rain, and glaciers in the southern hemisphere. This study provides an analysis of the seasonal and interannual variability of sea surface salinity, and meridional geostrophic transports (i.e. Cape Horn Current) in the coastal ocean off western Patagonia using improved satellite products of Sea Surface Salinity (SSS) and geostrophic velocities covering 11 years of data. Our results reveal a clear salinity minimum in a coastal band between 42-54°S associated with the highest freshwater content described in previous studies. The average geostrophic currents are stronger south of 49°S, in line with the location of the Cape Horn Current. The average salinity minimum tends to disappear south of 54°S, with salinity values increasing slightly southward. The seasonal cycle of salinity shows the most pronounced minimum in summer. The greatest variability in salinity (standard deviation of salinity fields) occurs in the southern region of the Cape Horn Current. Hovmöller plots reveal two cores of minimum salinities observed in spring and summer. The freshwater off the Gulf of Penas contributes to the northern core. The meridional geostrophic transport differs between the northern and southern sections, with transports predominantly towards the equator (pole) north (south) of about 47-48°S during spring-summer. There is a marked seasonal variability in the magnitude and northern limit of the southward-flowing Cape Horn Current, being extended further north during winter and with a maximum average magnitude during summer-fall. On interannual scale, a major drop in surface salinity occurred off northern and central Patagonia during 2018-2019. Finally, a potentially persistent freshening trend was identified for the coastal region off southern Patagonia (south of 52°S), nonetheless, longer time series are required to confirm a long-term freshening pattern.