As the Pacific Ocean moves into an El Niño phase, scientists are monitoring changes in sea-surface temperature, but also sea-surface salinity – which is another key indicator of this phenomenon.

Measurements from satellites such as the European Space Agency's SMOS mission are revealing how freshwater and salt patterns shift across the Pacific as this year’s El Niño develops – offering a potential new pathway for early detection.

Sea-surface salinity, the concentration of dissolved salts in the upper layer of the ocean, is much more than a measure of how salty the sea is. It responds rapidly to changes in rainfall, evaporation, river discharge and, specifically in polar regions, the melting and formation of sea ice. Salinity is both an important driver of ocean circulation and a tracer of the movement of water masses and surface currents.

Together with temperature, salinity controls the density of seawater, triggering ocean currents that redistribute heat around the planet. Observing both variables gives scientists a much more complete picture of how the coupled ocean–atmosphere system behaves during El Niño – the warm phase of the El Niño–Southern Oscillation (ENSO).

ENSO is a single climate phenomenon, but it has three phases; the two opposite phases, El Niño and La Niña, and a neutral phase in the middle of the continuum.

During the current emerging El Niño, satellites have detected notable changes in the tropical Pacific.

During normal, or neutral, conditions, strong trade winds trap a large layer of low-salinity surface water, known as the freshwater pool, in the far western Pacific. However, during El Niño, the easterly trade winds weaken or even reverse, removing the force that normally confines this warm western Pacific freshwater pool.

Drawing on data from ESA’s SMOS and NASA’s SMAP missions and other data sources, the animation above illustrates the movement of the freshwater pool during the 2023 El Niño and La Niña events, and the current emerging El Niño.

Here, the eastern edge of the western Pacific freshwater pool shifts eastwards, together with the associated belt of deep atmospheric convection and heavy rainfall.

One of the most striking features is the eastward displacement of the saltier waters immediately to the east of the freshwater pool, while salinity within much of the pool itself remains relatively low. This evolution reflects the combined influence of changing rainfall patterns and ocean surface currents as the trade winds weaken during El Niño.

This redistribution of warm surface waters and rainfall creates distinct regional salinity anomalies. Areas experiencing more rainfall become fresher than normal, while regions with less precipitation and greater evaporation become more saline. Together, these contrasting patterns trace the redistribution of atmospheric moisture and highlight the close coupling between the ocean and the global hydrological cycle.

Measurements of changing sea-surface temperature and salinity are helping researchers better understand the processes driving the El Niño and improve predictions of the phenomenon’s far-reaching impacts on weather, climate and marine ecosystems around the world.

ESA’s SMOS Mission Manager, Klaus Scipal, said, “Salinity acts as a fingerprint of the water cycle.

“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.”

Although sea-surface temperature remains the defining characteristic of El Niño, scientists say salinity has become an indispensable companion variable – offering a clearer picture of how heat and freshwater move through the coupled ocean-atmosphere system.

Researchers are developing an ENSO-specific sea-surface salinity index to complement traditional sea-surface temperature indices. The aim is to determine whether spatial patterns of sea-surface salinity can improve forecasts of the timing and intensity of ENSO events by providing an earlier signal than sea-surface temperature alone.

As the current El Niño unfolds, satellites are providing a unique, global perspective on one of Earth's most influential climate phenomena.

Launched in 2009, ESA's SMOS, which stands for Soil Moisture and Ocean Salinity, mission is specifically designed to map sea-surface salinity from space while also measuring soil moisture over land.

Using an imaging microwave radiometer, SMOS can detect tiny variations in the natural microwave emissions from Earth's surface, allowing scientists to monitor subtle changes in ocean salinity.

More than 15 years after launch, the mission continues to add to the long-term record that is advancing our understanding of the global water cycle, ocean circulation and climate variability.