Watching ENSO Through Sea Surface Salinity
A satellite salinity view of the developing 2026 El Niño
The displacement of the western Pacific fresh-pool edge provides an independent SSS-based view of El Niño/La Niña evolution.
Built on the scientific and data heritage of ESA CCI+ Sea Surface Salinity (CCI+SSS).
Status of this page
This page is a scientific demonstrator. It shows how satellite sea surface salinity (SSS) can help monitor El Niño–Southern Oscillation (ENSO) from an independent ocean-salinity perspective.
Latest situation
The SSS-based indicator shows that the eastern edge of the western equatorial Pacific fresh pool is continuing to move eastward. It is now clearly farther east than during the 2023–2024 El Niño and has reached a position comparable to the strong 2015–2016 event.
What is especially striking is the timing: in 2015–2016, a similar eastward extent was reached around December, near the usual seasonal peak of El Niño, whereas the ongoing event has already reached a similar position by August 2026. This points to a particularly rapid development of the salinity signal.
This comparison should nevertheless be interpreted with appropriate caution.
Dashboard: recent evolution
The dashboard below focuses on the most recent three years, ending with the latest available SSS field. The upper panel shows sea surface salinity in the tropical Pacific, with the 34.8-pss isohaline marking the eastern edge of the western equatorial fresh pool. The red circle/ellipse highlights the tracked fresh-pool edge between 2°S and 2°N. The lower panel shows the synchronized evolution of the SSS-based ENSO indicator.
How does the current event compare with previous El Niños?
The curves below track the same 34.8-pss fresh-pool boundary (S34.8), aligned on a common event-relative seasonal cycle to compare the timing, speed and maximum observed eastward extent of its displacement.
Key message
During El Niño development, the western Pacific fresh pool tends to expand eastward. Satellite sea surface salinity observations allow this displacement to be monitored month by month. The longitude of the 34.8-pss isohaline provides a simple way to track this movement and compare it with reference ENSO indicators.
Retrospective view: 2010 to present
The animation below provides the full retrospective view since the beginning of the salinity-satellite period used here. It combines three synchronized diagnostics:
- the map animation of sea surface salinity and the tracked 34.8-pss isohaline;
- the Hovmöller diagram showing the longitude-time evolution of equatorial SSS;
- the SSS-based ENSO indicator evolving through time.
This combined view shows how the current evolution compares with previous El Niño and La Niña episodes.
Comparison with established ENSO indicators
The salinity-based indicator is compared below with widely used ENSO indicators. The agreement between them supports the interpretation that the displacement of the salinity front captures the ENSO signal from an independent salinity perspective. Differences are also informative, because sea surface salinity reflects freshwater fluxes, ocean transport, and coupled ocean-atmosphere processes in a way that differs from indices based on sea surface temperature (SST) or multivariate ENSO indices.
MEI.v2
MEI.v2 is a multivariate ENSO index combining oceanic and atmospheric information. It provides a broad reference for coupled ENSO variability.
Niño 3.4
Niño 3.4 is a classical SST-based ENSO index in the central-eastern equatorial Pacific.
Data availability note: the NOAA Niño 3.4 series currently available here extends through 1 July 2026, so this comparison ends earlier than the MEI.v2 and RONI comparisons.
C3S Niño 3.4
This comparison uses a Niño 3.4 SST index derived directly from the Copernicus Climate Change Service (C3S) SST dataset, using a fixed 1991–2020 climatology to compute the anomaly.
The C3S-derived index provides a complementary SST-based reference for the SSS-based S34.8 indicator.
RONI
RONI is a relative Niño 3.4 index designed to account for the broader tropical warming background.
Additional diagnostics
The static figures below provide complementary views of the same SSS-based indicator. They are useful for readers who want to inspect the full-period record without replaying the animations.
Full-period SSS-based ENSO indicator
Full-period Hovmöller view
Interpretation and caveats
This SSS indicator is experimental. Like all climate indices, it depends on methodological choices and on the product version used. In particular, it is sensitive to the definition of the isohaline, the latitude band, the selected longitude domain, the treatment of uncertain satellite retrievals, the handling of near-real-time data, and the definition of thresholds.
It should be interpreted as a complementary observational indicator, not as an operational ENSO product. The analysis is work in progress.
References
- Qu, T., & Yu, J.-Y. (2014). ENSO indices from sea surface salinity observed by Aquarius and Argo. Journal of Oceanography, 70, 367–375. DOI: 10.1007/s10872-014-0238-4.
- MEI.v2: NOAA Physical Sciences Laboratory, https://psl.noaa.gov/data/timeseries/month/DS/MEIV2/
- Niño 3.4: NOAA Physical Sciences Laboratory / NOAA Climate Prediction Center, https://psl.noaa.gov/data/timeseries/month/DS/Nino34_CPC/
- RONI: NOAA Physical Sciences Laboratory, https://psl.noaa.gov/data/timeseries/month/DS/RONI/
- C3S SST: Combined sea surface and sea ice surface temperature data derived from satellite observations, https://cds.climate.copernicus.eu/datasets/satellite-ist-sst-global?tab=overview
- CCI+SSS: ESA Climate Change Initiative Sea Surface Salinity project.
- C3S: Copernicus Climate Change Service.
- C3S seasonal forecasts, https://climate.copernicus.eu/seasonal-forecasts
Credits
Scientific lead: Fabrice Bonjean — LOCEAN
Scientific and technical contributors
- Jacqueline Boutin — LOCEAN
- Roberto Sabia — ESA
- Frédéric Rouffi — ACRI-ST
- Sebastien Guimbard — OCEANOSCOPE
- Jean-Luc Vergely — ACRI-ST
- Laetitia Parc — ACRI-ST
- Nicolas Kolodziejczyk — LOPS
