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, Niño 3.4, and RONI. Additional reference indicators may be considered as the analysis develops.

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.