ESA Says Ocean Salinity Maps From SMOS May Flag El Niño Earlier Than Temperature

The European Space Agency says satellite measurements of how salty the ocean surface is may give forecasters an earlier signal of El Niño than sea-surface temperature alone. In an update on its SMOS mission, the agency reports that as the Pacific moves into an El Niño phase, salinity observations are tracing the redistribution of freshwater and salt across the tropical Pacific — and that researchers are now working on an ENSO-specific salinity index to sit alongside the temperature indices that define the phenomenon.

According to ESA, sea-surface salinity — the concentration of dissolved salts in the upper ocean layer — responds quickly to rainfall, evaporation, river discharge and, in polar waters, the formation and melting of sea ice. The agency describes it as both a driver of ocean circulation and a tracer of water masses and surface currents. Because salinity and temperature together set the density of seawater, ESA argues that observing both gives a fuller account of how the coupled ocean–atmosphere system behaves during El Niño, the warm phase of the El Niño–Southern Oscillation.

The freshwater pool moves east

ESA describes a specific mechanism it is now watching from orbit. Under neutral conditions, the agency says, strong trade winds hold a large layer of low-salinity surface water — the so-called freshwater pool — in the far western Pacific. During El Niño, the easterly trade winds weaken or even reverse, removing the force that normally confines that warm western Pacific pool.

The result, ESA says, is that the eastern edge of the freshwater pool shifts eastwards, and the belt of deep atmospheric convection and heavy rainfall associated with it moves with it. The agency singles out one feature as particularly striking: the eastward displacement of the saltier waters lying immediately east of the freshwater pool, while salinity within much of the pool itself stays comparatively low. ESA attributes this evolution to the combined influence of changing rainfall patterns and ocean surface currents as the trades slacken.

An animation published with the update, drawing on data from ESA's SMOS, NASA's SMAP mission and other sources, illustrates the movement of the freshwater pool during the 2023 El Niño and La Niña events as well as during the current emerging El Niño. ESA also publishes a view of Pacific Ocean salinity for June 2026.

The redistribution of warm surface water and rainfall produces distinct regional salinity anomalies, according to the agency: areas receiving more rain turn fresher than normal, while regions with less precipitation and stronger evaporation become more saline. ESA says these contrasting patterns trace the movement of atmospheric moisture and underline the close coupling between the ocean and the global water cycle.

A companion variable, and a possible new index

"Salinity acts as a fingerprint of the water cycle," said Klaus Scipal, ESA's SMOS Mission Manager. "Where temperatures tell us how much heat the ocean contains, salinity gives us a first approximation where freshwater is entering or leaving the ocean. Together, these measurements allow us to better understand the exchange of heat and water between the ocean and atmosphere, improving our ability to monitor and predict climate variability."

ESA is careful about the hierarchy of variables: sea-surface temperature, it says, remains the defining characteristic of El Niño, with salinity described by scientists as an indispensable companion. The agency reports that researchers are developing an ENSO-specific sea-surface salinity index to complement conventional temperature indices, with the aim of establishing whether spatial patterns of surface salinity can improve forecasts of the timing and intensity of ENSO events by providing a signal earlier than temperature alone. That work is framed as an open question rather than a settled result. Related background on measurement methods is collected on our how El Niño is measured pages.

The instrument behind the data

SMOS — Soil Moisture and Ocean Salinity — was launched in 2009 and is designed to map sea-surface salinity from space while also measuring soil moisture over land, ESA says. It carries an imaging microwave radiometer able to detect small variations in the natural microwave emissions from Earth's surface, which the agency says allows subtle changes in ocean salinity to be monitored.

More than 15 years after launch, ESA reports, the mission continues to extend a long-term record supporting research into the global water cycle, ocean circulation and climate variability. The full update is available from ESA.