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Advances in Remote Sensing of Ocean Salinity

A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Ocean Remote Sensing".

Deadline for manuscript submissions: 15 August 2024 | Viewed by 4679

Special Issue Editor


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Guest Editor
1. Northern Gulf Institute, Department of Geosciences, Mississippi State University, Starkville, MS, USA
2. NOAA National Centers for Environmental Information (NCEI), Stennis Space Center, Hancock County, MS 39529, USA
Interests: ocean salinity; freshwater dynamics; variability of ocean currents; microplastics; air-sea interactions; tropical climate variability; ocean leadership and capacity building

Special Issue Information

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

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

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

Published Papers (3 papers)

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Research

25 pages, 6796 KiB  
Article
Comparison of Freshwater Content and Variability in the Arctic Ocean Using Observations and Model Simulations
by Emma L. Hoffman, Bulusu Subrahmanyam, Corinne B. Trott and Sarah B. Hall
Remote Sens. 2023, 15(15), 3715; https://doi.org/10.3390/rs15153715 - 25 Jul 2023
Viewed by 1634
Abstract
Freshwater content (FWC), generally characterized in the Arctic Ocean by salinities lower than 34.8 psu, has shifted in both quantity and distribution in recent decades in the Arctic Ocean. This has been largely driven by changes in the volume and salinity of freshwater [...] Read more.
Freshwater content (FWC), generally characterized in the Arctic Ocean by salinities lower than 34.8 psu, has shifted in both quantity and distribution in recent decades in the Arctic Ocean. This has been largely driven by changes in the volume and salinity of freshwater sources and the direction and magnitude of major currents. In this study, we analyze the variability in FWC and other physical oceanographic variables from 1993 to 2021 in the Arctic Ocean and Beaufort Gyre (BG) using in situ and remote sensing observations and five ocean models and reanalysis products. Generally, ocean models and reanalysis products underestimate FWC in the BG when compared with observations. Modeled FWC and sea surface height (SSH) in the BG are well correlated during the time period and are similar to correlations of the observational data of these variables. ORAS5 compares best to EN4 salinity over the entire study period, although GLORYS12 agrees well pre-2007 and SODA post-2007. Outside the BG, consistency between modeled SSH, FWC, and limited observations varies between models. These comparisons help identify discrepancies in ocean model and reanalysis products while highlighting areas where future improvements are necessary to further our understanding of Arctic FWC. As observations are scarce in the Arctic, these products and their accuracy are important to studying this dynamic and vulnerable ocean. Full article
(This article belongs to the Special Issue Advances in Remote Sensing of Ocean Salinity)
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33 pages, 9857 KiB  
Article
Characteristics of Internal Tides from ECCO Salinity Estimates and Observations in the Bay of Bengal
by Bulusu Subrahmanyam, V.S.N. Murty and Sarah B. Hall
Remote Sens. 2023, 15(14), 3474; https://doi.org/10.3390/rs15143474 - 10 Jul 2023
Viewed by 861
Abstract
Internal waves (IWs) are generated in all the oceans, and their amplitudes are large, especially in regions that receive a large amount of freshwater from nearby rivers, which promote highly stratified waters. When barotropic tides encounter regions of shallow bottom-topography, internal tides (known [...] Read more.
Internal waves (IWs) are generated in all the oceans, and their amplitudes are large, especially in regions that receive a large amount of freshwater from nearby rivers, which promote highly stratified waters. When barotropic tides encounter regions of shallow bottom-topography, internal tides (known as IWs of the tidal period) are generated and propagated along the pycnocline due to halocline or thermocline. In the North Indian Ocean, the Bay of Bengal (BoB) and the Andaman Sea receive a large volume of freshwater from major rivers and net precipitation during the summer monsoon. This study addresses the characteristics of internal tides in the BoB and Andaman Sea using NASA’s Estimating the Circulation and Climate of the Ocean (ECCO) project’s high-resolution (1/48° and hourly) salinity estimates at 1 m depth (hereafter written as ECCO salinity) during September 2011–October 2012, time series of temperature, and salinity profiles from moored buoys. A comparison is made between ECCO salinity and NASA’s Soil Moisture Active Passive (SMAP) salinity and Aquarius salinity. The time series of ECCO salinity and observed salinity are subjected to bandpass filtering with an 11–14 h period and 22–26 h period to detect and estimate the characteristics of semi-diurnal and diurnal period internal tides. Our analysis reveals that the ECCO salinity captured well the surface imprints of diurnal period internal tide propagating through shallow pycnocline (~50 m depth) due to halocline, and the latter suppresses the impact of semi-diurnal period internal tide propagating at thermocline (~100 m depth) reaching the sea surface. The semi-diurnal (diurnal) period internal tides have their wavelengths and phase speeds increased (decreased) from the central Andaman Sea to the Sri Lanka coast. Propagation of diurnal period internal tide is dominant in the northern BoB and northern Andaman Sea. Full article
(This article belongs to the Special Issue Advances in Remote Sensing of Ocean Salinity)
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18 pages, 14895 KiB  
Article
A Modified U-Net Model for Predicting the Sea Surface Salinity over the Western Pacific Ocean
by Xuewei Zhang, Ning Zhao and Zhen Han
Remote Sens. 2023, 15(6), 1684; https://doi.org/10.3390/rs15061684 - 21 Mar 2023
Cited by 3 | Viewed by 1625
Abstract
The prediction of oceanic features is always an important issue in oceanography, where deep learning has been proven to be a useful tool. In this study, we applied the improved U-net model to predict the monthly sea surface salinity (SSS) over the western [...] Read more.
The prediction of oceanic features is always an important issue in oceanography, where deep learning has been proven to be a useful tool. In this study, we applied the improved U-net model to predict the monthly sea surface salinity (SSS) over the western Pacific (WP) Ocean, and the model was designed to use the SSSs from six consecutive months to predict the SSS in the next month. The monthly satellite-based SSSs in 2015–2020 were used for model training, and the data collected after January 2021 were used to evaluate the model’s predictive abilities. The results showed that the predicted SSSs represented the general patterns of SSSs over the WP region. However, the small-scale features were smoothed out in the model, and the temporal variations were also not well captured, especially over the East China Sea and Yellow Sea (ECS&YS) region. To further evaluate the potential of the U-net model, a more specific model was conducted for the ECS&YS region (Domain 2), which successfully predicted both spatial and temporal variations in the SSSs, including the spreading and retreating of the low-salinity tongue. Based on the comparison between the two domains and sensitivity experiments, we found that the prediction biases were contributed by the spatial distributions of the SSSs, the domain size, and the filter numbers. In addition, further multi-step prediction experiments suggested that our U-net model could also be used for long-time prediction, and we have examined up to five months. Overall, this study demonstrated the great ability and potential of the U-net model for predicting SSS, even though only a few trainable data are available. Full article
(This article belongs to the Special Issue Advances in Remote Sensing of Ocean Salinity)
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

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.

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