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Sea Surface Salinity

 

Salinity plays a fundamental role in the density-driven global ocean circulation, the water cycle, and climate. Therefore, salinity is considered to be an Essential Climate Variable (ECV) and something that has to be monitored along with other variables that contribute to the Climate Data Records (CDR). With remote sensing technology, Earth Observation (EO) from satellites extends our current knowledge of Sea Surface Salinity (SSS) by providing continuous and regular monitoring of this variable across the oceans.

Objectives

Through the Climate Change Initiative (CCI) program ESA aims to produce the longest SSS Climate Data Record by combining all past and present satellite missions capable of retrieving this variable from space. This will include all L-band satellites and the possibility of using C and X band radiometers to extend the SSS data set back to 2002. The resulting data set will provide an essential tool for improved monitoring methods and provide data for modelling over large temporal scales which will improve estimates of future change.

CCI SSS will provide data products that are specifically adapted to climate applications (i.e. include information on accuracy and uncertainty within the data). Furthermore, this project will explore the need to improve the performance of current SSS algorithm retrievals and directly contribute to climate science studies submitted to the next International Panel on Climate Change (IPCC) Annual Review for climate change in 2020.


About Sea Surface Salinity CCI

 

Overview

Under the CCI, the ESA seeks to make a step forward to understand the state of our warming planet, including the physical changes experienced in our oceans. Satellite observations unarguably aid the climate science community by providing quality data for a range of oceanographic applications. Due to the critical importance of studying global salinity levels ESA have funded CCI SSS Phase 1, a three-year project (2019-2021) aiming to build the longest SSS dataset record.

Salinity trends are difficult to identify using in-situ sampling methods as data collected by these observations are limited both spatially and temporally (Hegerl et al., 2018; Nguyen et al., 2018). By in-situ observations we refer to water measurements taken on site or subsequently made in a laboratory based on samples taken from a site (e.g. Argo, CTD, TSG, etc.). Sea Surface Salinity (SSS) data is collected by the following satellite missions: the European Space Agency's (ESA) Soil Moisture and Ocean Salinity (SMOS), the National Aeronautics and Space Administration's (NASA) Aquarius, Soil Moisture Active Passive (SMAP) and Salinity Processes in the Upper Ocean Regional Study (SPURS) which enables an unprecedented spatial and temporal coverage in the measurement of this variable. By combining satellite data with in-situ observations scientists have a near complete data set from which they can assess current global ocean salinity conditions.

Why Measure Sea Surface Salinity?

In 2014, the IPCC released their Fifth Annual Report (AR5), providing further evidence that our ocean’s physical state is being affected by climate change. Using ocean model simulations scientists have found that SSS is sensitive to changes in ocean evaporation and precipitation. As global temperatures have continued to rise since the 1950s, increased evaporation levels have caused high salinity regions (e.g. the north Atlantic) to become more saline, and conversely, increased precipitation has made fresh regions (e.g. the north-east Pacific) to become fresher (Lagerloef et al., 2010; Liu et al., 2018; Zika et al., 2018).

In 2018, the IPCC issued a Special Report (SR15) stating that based on current greenhouse gas emissions global warming of 1.5°C is expected by as early as 2030. This level of warming will intensify the risk of extreme weather events (e.g. droughts and floods) across the globe as well as further damage our coral reefs and expediate the loss of sea ice at our poles (Arnell et al., 2018; Rückamp et al., 2018). By studying ocean salinity levels scientists can determine changes to global evaporation and precipitation rates and model large scale climate change effects experienced in polar and subpolar regions. It is therefore critically important to understand SSS changes caused by global warming.

The Need for Sea Surface Salinity Data from Satellites

Through advancements in remote sensing technology there are now two systems, the Argo array and dedicated satellites, which complement in-situ observations through respectively measuring the interior and surface salinity of the world’s oceans (Chen et al., 2018; Muller-Karger et al., 2018). By measuring global salinity levels scientists obtain crucial information regarding the hydrological cycle and thermohaline circulation, which in turn determine ocean carbon and heat storage across all ocean basins (Tatebe et al., 2018; Jaeger and Mahadevan, 2018). In addition, SSS variations can modify the vertical stratification in water density and strongly influences the air-sea exchange of gases and masses through the development of the so-called barrier layers (Pan et al., 2018; Steffen and Bourassa, 2018). Lastly, SSS is a tracer of freshwater fluxes originating from river discharges, ice melting and air-sea exchange (Evaporation minus Precipitation (E-P)) (Board, 2018).

Sea surface salinity satellite missions began with ESA’s Soil Moisture and Ocean Salinity (SMOS) which has a spatial resolution of 50 km and has provided the longest record for SSS measurements from space over the global ocean (2010 – present). SMOS carries an L-band Microwave Interferometric Radiometer with Aperture Synthesis (MIRAS), the first L-band radiometer observing from space, and crosses the equator at 0600 local time in ascending node and 1800hr in descending node along a sun-synchronous orbit (Kerr et al., 2001; Font et al., 2012). L-band radiometers collect salinity measurements within the first centimetres below the water’s surface and provide a synoptic view of the global ocean every 3-7 days (Supply et al., 2017; Mecklenburg et al., 2012). A sun-synchronous orbit places a satellite in constant sunlight, which allows the solar panels to continuously work, and is therefore the most effective orbit for imaging and weather satellites (Boain, 2004).

NASA’s Aquarius satellite successfully collected global high resolution SSS data from 2011 until the spacecraft it was positioned on ceased operating due to power failure in 2015 (Graham and Bergin, 2015). Despite this set-back, the Aquarius mission exceeded the expected life span of 36-months and presented average salinity within the upper 1-2 cm layer of the ocean across a 150 km spatial resolution (Le Vine et al., 2015; Kao et al., 2018). The Soil Moisture Active Passive (SMAP) mission replaced Aquarius in 2015 and uses a single radiometer that retrieves SSS measurements within a 40 km spatial resolution (Supply et al., 2017). It also crosses the equator at the same time as SMOS but in the opposite phase (i.e. 0600 for descending orbits and 1800 for ascending orbits) thus providing numerous correlations between the two satellites (Entekhabi et al., 2010; Supply et al., 2017).

Figure 1: Annual average Sea Surface Salinity map (salinity units using practical salinity scale, PSS) from SMOS data averaged for 2010 processed at Cersat Salinity Center (http://www.salinityremotesensing.ifremer.fr)


Project Team

 

The CCI SSS project team comprises of nine scientific and industrial partners from the United Kingdom, France, Spain, Germany and Luxemburg. The following companies make up this consortium:

  • ARGANS Ltd. (United Kingdom)
  • National Oceanography Centre (NOC) (United Kingdom)
  • ACRI-ST (France)
  • Laboratoire d'Océanographie et du Climat (LOCEAN) (France)
  • Laboratoire d'Océanographie Physique et Spatiale (LOPS) (France)
  • Institut français de recherche pour l'exploitation de la mer (IFREMER) (France)
  • OceanDataLab (ODL) (France)
  • Institut des sciences de la mer de Barcelone (ICM-CSIC) (Spain)
  • Universitat Hamburg (UoH) (Germany)
  • AdwäisEO (Luxemburg)

With additional support from the following external partners:

  • NASA's Jet Proplusion Laboratory (JPL) (U.S.A.)
  • NASA's Goddard Space Flight Center (GSFC) (U.S.A.)
  • Remote Sensing Systems (RSS) (U.S.A.)

Project Outreach

 

Details relating to the Sea Surface Salinity CCI project outreach will be added here in due course.


 

Resources

 

Latest News

News relating to the Sea Surface Salinity CCI project will be added here soon.

Meetings

Upcoming meetings will be added here shortly.

CCI SSS Documentation

This section will be populated upon acceptance of deliverables.

Data Sources

Data pertaining to this project is sourced by the CCI Open Data Portal and visual analysis of the data is provided through the CCI Toolbox.

Useful Links

Websites related to the Sea Surface Salinity CCI project will be added here in due course.

Other SSS Data Sources:

Websites related to ESA’s SMOS mission:

  • ARGANS SMOS [Level 2]
  • Barcelona Expert Center (BEC) [Level 3+]
  • Centre Aval de Traitement des Données SMOS (CATDS) [Level 3+]

Websites related to NASA’s Aquarius, SMAP and SPURS missions:

  • Physical Oceanography Distributed Active Archive Center (PO.DACC) [Level 1+]

References

Arnell, N.W., Lowe, J.A., Lloyd-Hughes, B. and Osborn, T.J., 2018. The impacts avoided with a 1.5° C climate target: a global and regional assessment. Climatic change, 147(1-2), pp.61-76.

Boain, R.J., 2004. AB-Cs of sun-synchronous orbit mission design.

Board, O.S. and National Academies of Sciences, Engineering, and Medicine, 2018. Sustaining Ocean Observations to Understand Future Changes in Earth's Climate. National Academies Press.

Chen, G., Peng, L. and Ma, C., 2018. Climatology and seasonality of upper ocean salinity: a three-dimensional view from argo floats. Climate dynamics, 50(5-6), pp.2169-2182.

Entekhabi, D., Njoku, E.G., O'Neill, P.E., Kellogg, K.H., Crow, W.T., Edelstein, W.N., Entin, J.K., Goodman, S.D., Jackson, T.J., Johnson, J. and Kimball, J., 2010. The soil moisture active passive (SMAP) mission. Proceedings of the IEEE, 98(5), pp.704-716.

Font, J., Boutin, J., Reul, N., Spurgeon, P., Ballabrera-Poy, J., Chuprin, A., Gabarró, C., Gourrion, J., Guimbard, S., Hénocq, C. and Lavender, S., 2013. SMOS first data analysis for sea surface salinity determination. International Journal of Remote Sensing, 34(9-10), pp.3654-3670.

Fu, L.L., Lee, T., Liu, W.T. and Kwok, R., 2019. Fifty Years of Satellite Remote Sensing of the Ocean. Meteorological Monographs.

Graham, W. and Bergin, C., 2015. Farewell Aquarius as SAC-D spacecraft concludes its mission – NASASpaceFlight.com [Online]. Available at https://www.nasaspaceflight.com/2015/06/farewell-aquarius-sac-d-spacecra... [Accessed 26 March 2019].

Hegerl, G.C., Black, E., Allan, R.P., Ingram, W.J., Polson, D., Trenberth, K.E., Chadwick, R.S., Arkin, P.A., Sarojini, B.B., Becker, A. and Dai, A., 2018. Challenges in quantifying changes in the global water cycle. Bulletin of the American Meteorological Society, 99(1).

Kerr, Y.H., Waldteufel, P., Wigneron, J.P., Martinuzzi, J.A.M.J., Font, J. and Berger, M., 2001. Soil moisture retrieval from space: The Soil Moisture and Ocean Salinity (SMOS) mission. IEEE transactions on Geoscience and remote sensing, 39(8), pp.1729-1735.

Jaeger, G.S. and Mahadevan, A., 2018. Submesoscale-selective compensation of fronts in a salinity-stratified ocean. Science Advances, 4(2), p.e1701504.

Lagerloef, G., Schmitt, R., Schanze, J. and Kao, H.Y., 2010. The ocean and the global water cycle. Oceanography, 23(4), pp.82-93.

Le Vine, D.M., Dinnat, E.P., Lagerloef, G.S.E., de Matthaeis, P., Abraham, S., Utku, C. and Kao, H., 2014. Aquarius: Status and recent results. Radio Science, 49(9), pp.709-720.

Liu, T., Schmitt, R.W. and Li, L., 2018. Global search for autumn‐lead sea surface salinity predictors of winter precipitation in southwestern United States. Geophysical Research Letters, 45(16), pp.8445-8454.

Mecklenburg, S., Drusch, M., Kerr, Y.H., Font, J., Martin-Neira, M., Delwart, S., Buenadicha, G., Reul, N., Daganzo-Eusebio, E., Oliva, R. and Crapolicchio, R., 2012. ESA's soil moisture and ocean salinity mission: Mission performance and operations. IEEE Transactions on Geoscience and Remote Sensing, 50(5), pp.1354-1366.

Muller-Karger, F.E., Miloslavich, P., Bax, N.J., Simmons, S., Costello, M.J., Sousa Pinto, I., Canonico, G., Turner, W., Gill, M., Montes, E. and Best, B.D., 2018. Advancing marine biological observations and data requirements of the complementary essential ocean variables (EOVs) and essential biodiversity variables (EBVs) frameworks. Frontiers in Marine Science, 5, p.211.

Nguyen, P.T., Koedsin, W., McNeil, D. and Van, T.P., 2018. Remote sensing techniques to predict salinity intrusion: application for a data-poor area of the coastal Mekong Delta, Vietnam. International journal of remote sensing, 39(20), pp.6676-6691.

Pan, L., Zhong, Y., Liu, H., Zhou, L., Zhang, Z. and Zhou, M., 2018. Seasonal variation of barrier layer in the Southern Ocean. Journal of Geophysical Research: Oceans, 123(3), pp.2238-2253.

Rückamp, M., Falk, U., Lange, S., Frieler, K. and Humbert, A., 2018. The effect of overshooting 1.5° C global warming on the mass loss of the Greenland ice sheet. Earth System Dynamics, 9(4), pp.1169-1189.

Steffen, J. and Bourassa, M., 2018. Barrier Layer Development Local to Tropical Cyclones based on Argo Float Observations. Journal of Physical Oceanography, 48(9), pp.1951-1968.

Supply, A., Boutin, J., Vergely, J.L., Martin, N., Hasson, A., Reverdin, G., Mallet, C. and Viltard, N., 2018. Precipitation estimates from SMOS sea‐surface salinity. Quarterly Journal of the Royal Meteorological Society, 144, pp.103-119.

Tang, W., Yueh, S., Yang, D., Fore, A., Hayashi, A., Lee, T., Fournier, S. and Holt, B., 2018. The Potential and Challenges of Using Soil Moisture Active Passive (SMAP) Sea Surface Salinity to Monitor Arctic Ocean Freshwater Changes. Remote Sensing, 10(6), p.869.

Tatebe, H., Tanaka, Y., Komuro, Y. and Hasumi, H., 2018. Impact of deep ocean mixing on the climatic mean state in the Southern Ocean. Scientific reports, 8(1), p.14479.

Zika, J.D., Skliris, N., Blaker, A.T., Marsh, R., Nurser, A.G. and Josey, S.A., 2018. Improved estimates of water cycle change from ocean salinity: the key role of ocean warming. Environmental Research Letters, 13(7), p.074036.


Support

 

Contact Information

 

Project Leaders:

  • Scientific Leaders: Jacqueline Boutin and Nicolas Reul.
  • Project Manager: Rafael J. Catany (RCatany@argans.co.uk).

ESA Technical Officers:

 

Frequently Asked Questions

  • What is SSS?
    • SSS stands for Sea Surface Salinity. For more information please refer to the Useful Links section above.
  • Where can I find SSS data?
    • Links to data provided by this project can be found in the Data section above. Alternative data sources can be found in the Useful Links section above.
  • What is CCI?
    • CCI stands for the Climate Change Initiative. The CCI is a programme initiated by ESA. For more information please refer to the CCI home page.
  • What is ECV?
    • ECV stands for Essential Climate Variable. For more information please refer to the ECV Inventory page.
  • What is ESA?
    • ESA stands for the European Space Agency. For more information please refer to the ESA home page.
  • What is EO?
    • EO stands for Earth Observation.