Sea Surface Salinity
Measurement Types
Ocean salinity, defined as the concentration of dissolved salts in seawater, is a key variable controlling ocean circulation, climate regulation, and the ocean carbon cycle. Sea surface salinity, the salinity of seawater in the surface layer, is measurable by satellite microwave radiometry, which can be supplemented by in situ observations for a more comprehensive picture of ocean salinity beyond the surface.
Oceans cover around 71% of Earth’s surface and play a central role in climate regulation by storing heat and redistributing it from the equator to the poles, making ocean observations both indicators of climate change and essential inputs to climate models. Salinity, defined as the concentration of dissolved salts in seawater and commonly expressed in practical salinity units (psu), varies primarily due to evaporation, precipitation, river input, and sea-ice melt or formation. Higher salinity increases seawater density, contributing to sinking and thermohaline circulation, a major component of global ocean circulation that transports warm water poleward and cold water equatorward. This process regulates climate and influences the atmosphere through heat exchange. While ocean salinity has been measured in situ for centuries, advancements in remote sensing technology has allowed for the measurement of sea surface salinity at a global scale from satellites such as Aquarius (SAC-D) and SMOS (Soil Moisture and Ocean Salinity). 1) 2) 3) 4) 5)
In situ ocean salinity observations are based on direct seawater sampling, where salinity is inferred from measurements of conductivity, temperature, and depth using standard thermodynamic formulations. Electrical conductivity describes a liquid’s ability to transmit an electric current and increases with ion concentration, which is directly related to the amount of dissolved salts in seawater. In situ systems include Conductivity–Temperature–Depth (CTD) instruments, autonomous Argo floats, and measurements from research and commercial vessels. While in situ measurements provide highly accurate vertical profiles through the water column, they are limited in spatial coverage, and are hence complemented by space-based observations for measurements of salinity at the surface. 6) 7) 8) 9)
Remote sensing of sea surface salinity is based on microwave radiometry, where satellites measure sea surface brightness temperature (TB) at L-band frequencies (1 - 2 GHz). Seawater microwave emissivity depends on salinity, as higher salinity increases electrical conductivity, reducing emissivity and therefore lowering TB, allowing sea surface salinity to be inferred from calibrated observations. Dedicated missions such as SMOS, SMAP (Soil Moisture Active/Passive), and Aquarius / SAC-D use L-band radiometers to maximise sensitivity to salinity and enable measurements through cloud cover and under many weather conditions. Retrieval quality can be affected by rain, sea surface roughness, radio-frequency interference, sea ice, and land contamination. Because L-band wavelengths are relatively long, large physical or synthetic apertures are needed to achieve adequate spatial resolution from orbit. Observed TB must be corrected for contributions from sea surface temperature, surface roughness, and external radiation, through geophysical model functions. Because the salinity signal is small, reliable estimates are obtained through spatial and temporal averaging rather than single-pass observations. 10)
Space-based monitoring provides consistent, long-term, and global measurements of sea surface salinity that are not achievable with in situ observations alone. These datasets enable the detection of large-scale patterns, seasonal to interannual variability, and long-term trends, supporting climate reanalyses, ocean circulation studies, and the assimilation of salinity fields into coupled ocean–atmosphere models to improve climate prediction and assessment.
Example Products
SMOS Level 2 (L2) Ocean Salinity (OS) Product
The SMOS Level 2 Ocean Salinity product is derived from the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) instrument aboard the SMOS satellite and provides swath-based maps of sea surface salinity. Salinity is retrieved by inverting L-band brightness temperature measurements, with corrections applied for land-sea contamination, surface roughness, radio-frequency interference, and ancillary geophysical effects, and anomalies are referenced to the World Ocean Atlas 2009 climatology. The dataset has been produced routinely since early 2010, with a consistently reprocessed archive from 2010 to at least 2024 and a latency of 8 - 12 hours per orbit. It supports ocean and climate research, operational forecasting, and the generation of higher-level gridded salinity products and long-term climate data records used to assess changes in ocean freshwater content. 11) 12) 13)
SMAP Version 3 (V3) Level-3 (L3) Sea Surface Salinity Products
The SMAP Version 3 Level-3 sea surface salinity (SSS) products are derived from L-band brightness temperature observations acquired by the radiometer onboard the SMAP satellite and are produced by remote sensing systems. SSS is retrieved using a geophysical model function with ancillary wind and environmental data, followed by quality control and mapping via a Backus–Gilbert optimum interpolation scheme onto a fixed 0.25° Earth grid, with products provided at 40 km and 70 km effective spatial resolution. The dataset delivers near-global ocean coverage from April 2015 as 8-day running means and monthly averages, supporting studies of ocean circulation, freshwater fluxes, air–sea interaction, climate variability, and operational model evaluation and data assimilation. 14) 15)
Aquarius Sea Surface Salinity Products
The Aquarius Sea Surface Salinity products are derived from L-band brightness temperature measurements acquired by the Aquarius instrument aboard the SAC-D (Satélite de Aplicaciones Científicas‑D) satellite, combining three passive radiometers with a scatterometer to correct for surface roughness effects. Salinity is retrieved from calibrated radiances and provided as Level-3 gridded products at 1° spatial resolution using temporal averaging to reduce noise and achieve a target accuracy of around 0.2 psu. The dataset covers the period from August 2011 to June 2015, with weekly and monthly global products supporting studies of ocean freshwater variability, large-scale circulation, and climate processes. 16) 17) 18)
Climate Change Initiative (CCI) Sea Surface Salinity Project
The Climate Change Initiative (CCI) Sea Surface Salinity (SSS) project provides global Level-4 sea surface salinity datasets produced by the ESA-led CCI consortium through the integration of multiple satellite missions. The products are derived from L-band microwave radiometer observations from SMOS, Aquarius, and SMAP, combined using multi-sensor merging and reprocessing techniques to generate a spatially and temporally consistent climate data record. Version 5.5, released March 2025, provides weekly and monthly global fields at approximately 50 km spatial resolution on a 0.25° EASE grid, covering the period from 2010 to 2023, with improvements including updated forward models, enhanced radio-frequency interference mitigation, improved high-latitude calibration, and expanded quality control information. These datasets support climate monitoring and modelling, assessment of long-term changes in the ocean freshwater cycle, evaluation of ocean circulation variability, and improved understanding and forecasting of climate phenomena such as the El Niño–Southern Oscillation (ENSO). 19)
Related Missions
Several satellite missions contribute to the observation of sea surface salinity through dedicated L-band microwave radiometry or by providing complementary variables required for retrieval, validation, and interpretation. The missions listed below illustrate the progression from pioneering research satellites to operational support for global salinity monitoring, climate data records, and integrated studies of ocean circulation and the water cycle.
Aquarius
Launched on 10 June 2011, the Aquarius mission provided NASA’s first global observations of sea surface salinity and operated until June 8, 2015. Aquarius was a NASA-built instrument flown aboard Argentina’s CONAE SAC-D spacecraft and comprised three 1.4 GHz L-band microwave radiometers and a 1.26 GHz scatterometer, which together measured ocean brightness temperature and surface roughness to enable salinity retrieval, supporting studies of ocean circulation and climate variability. 21)
SMOS
Launched on November 2, 2009, the Soil Moisture and Ocean Salinity (SMOS) mission is an ESA Earth Explorer satellite dedicated to L-band microwave observations of sea surface salinity and soil moisture. SMOS carries the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), a passive interferometric radiometer that measures natural microwave emissions from the ocean surface, enabling global monitoring of salinity variability to support studies of ocean circulation, the water cycle, and climate processes. 20)
SMAP
Launched on January 31, 2015, NASA’s Soil Moisture Active/Passive (SMAP) mission is an Earth observation satellite launched with L-band active radar and passive radiometer instruments, although the radar ceased operations in 2015. While primarily designed for soil moisture retrieval, the passive radiometer also enables measurements of sea surface salinity, supporting studies of the global water cycle, ocean circulation, sea-level variability, and climate processes.
CIMR
The Copernicus Imaging Microwave Radiometer (CIMR) is a two-satellite Copernicus Sentinel Expansion mission designed to provide multi-frequency microwave observations of sea surface salinity, sea surface temperature, and sea-ice concentration. CIMR will carry wide-swath, conically scanning passive microwave radiometers, including L-band 1.4 GHz channels that enable sea surface salinity measurements, supporting long-term monitoring of the ocean freshwater cycle, polar ocean processes, sea-ice conditions, ocean circulation, and climate variability. 22)
References
1) ESA, “Understanding ocean salinity,” URL: https://www.esa.int/Enabling_Support/Preparing_for_the_Future/Space_for_Earth/Blue_worlds/Understanding_ocean_salinity
2) EO Handbook, “Ocean salinity,” URL: https://database.eohandbook.com/measurements/measurements.aspx?measurementTypeID=27
3) ESA, “Frosty frontiers and salty seas: Climate data from ESA,” 06 November 2024, URL: https://earth.esa.int/eogateway/eo-insights/frosty-frontiers-and-salty-seas-climate-data-from-esa
4) NASA, “Sea surface salinity,” 12 October 2009, URL: https://svs.gsfc.nasa.gov/10504/
5) NASA, “Sea surface salinity,” 12 March 2013, URL: https://svs.gsfc.nasa.gov/11193/
6) Ocean Sciences, “Science Spurs Why,” URL: https://salinity.oceansciences.org/science-spurs-why-05.htm
7) Frontiers in Marine Science, “Satellite Salinity Observing System: Recent Discoveries and the Way Forward,” URL: https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2019.00243/full
8) Argo, “Argo Float Program,” URL: https://argo.ucsd.edu/
9) Phionics, “Linear Conversion Conductivity to Salinity,” 03 December 2020, URL: https://www.phionics.com/2020/12/03/linear-conversion-conductivity-to-salinity/
10) N. Reul et al., “Sea surface salinity estimates from spaceborne L-band radiometers: An overview of the first decade of observation (2010–2019),” Remote Sensing of Environment, 1 June 2020, URL: https://www.sciencedirect.com/science/article/pii/S0034425720301395
11) Ocean Sciences, “Ocean Salinity 2022: Science Advisory Board report,” URL:> https://salinity.oceansciences.org/docs/ocean_salinity_2022/osc22_sabia.pdf
12) ESA, “SMOS science products,” URL: https://earth.esa.int/eogateway/catalog/smos-science-products
13) ESA Dragon Programme, “SMOS sea surface salinity Level 2 retrievals,” URL: https://ftp.itc.nl/pub/Dragon4_Lecturer_2018/D1_Mon/PM%20Sea%20Surface%20Salinity%20Retrievals/L2/D1L2-DRAGON_OTC18_PPT_final_Sabia_SMOS_SSS_final.pdf
14) Ocean Sciences, “SMAP Salinity,” URL: https://salinity.oceansciences.org/smap-salinity.htm
15) Ocean Sciences, “SMAP V3 Sea Surface Salinity Data,” URL: https://salinity.oceansciences.org/data-smap-v3.htm
16) AGU Publications, “Aquarius: Status and recent results,” URL: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014RS005505
17) Data.gov, “Aquarius Official Release Level-3 Sea Surface Salinity Standard Mapped Image (Ascending),” URL: https://catalog.data.gov/dataset/aquarius-official-release-level-3-sea-surface-salinity-standard-mapped-image-ascending-mis-8aaa7
18) NASA PO.DAAC, “Aquarius Mission Objectives and Data,” URL: https://podaac.jpl.nasa.gov/Aquarius?tab=mission-objectives§ions=about%2Bdata
19) ESA Climate Change Initiative, “Sea Surface Salinity Record Extended,” URL: https://climate.esa.int/en/news-events/Sea-Surface-Salinity-Record-Extended/
20) ESA, “SMOS data,” URL: https://earth.esa.int/eogateway/missions/smos/data
21) NASA Jet Propulsion Laboratory, “Aquarius,” URL: https://www.jpl.nasa.gov/missions/aquarius/
22) ESA, “CIMR,” URL: https://www.esa.int/Applications/Observing_the_Earth/Copernicus/CIMR