Methods

Scientific basis

This demonstrator follows the idea that the eastern edge of the western Pacific fresh pool can be tracked through a salinity isohaline near the equator. The 34.8-pss isohaline is used as a proxy for this boundary.

Data source

The page is based on CCI+SSS satellite sea surface salinity fields, produced in the framework of ESA CCI+SSS and its transition toward ECMWF C3S.

For the Climate Data Record period, the demonstrator uses the soon-to-be-released CCI+SSS version 6 product, covering 2010–2024. An interim Climate Data Record updated in near-real time is currently in development and is expected to become operational at the end of 2026.

In the meantime, the near-real-time extension currently used for the latest months is based on the CATDS SMOS+SMAP product, which is also updated in near-real time.

Index definition

The experimental SSS-based index is based on the longitudinal position of the 34.8-pss isohaline in the equatorial Pacific.

Configuration used here:

Isohaline: 34.8 pss
Latitude band: 2°S–2°N
Longitude range: 128°E–270°E
Temporal resolution: monthly
Reference period: 2010–2024
Product version: CCI+SSS v6.3 for the CDR period, with CATDS SMOS+SMAP for the current near-real-time extension

Comparison indices

The SSS-based index is compared with MEI.v2, NOAA Niño 3.4, a C3S-derived Niño 3.4 index, and RONI. Additional reference indicators may be considered as the analysis develops.

The C3S Niño 3.4 index is derived from SST anomalies in the Niño 3.4 region in the C3S combined sea surface and sea ice surface temperature dataset, using a fixed 1991–2020 climatology.

Uncertainty and limitations

Several methodological choices influence the index:

  • the salinity product version;
  • the selected latitude band;
  • the isohaline value;
  • uncertainty in satellite retrievals;
  • treatment of near-real-time data;
  • definition of thresholds.

The index should therefore be interpreted as a complementary indicator of ENSO-related salinity variability.