The Pacific Ocean is entering an El Niño phase, and the earliest clues may not be in temperature but in salt. European Space Agency scientists say sea-surface salinity is shifting dramatically across the tropical Pacific, and satellite data from the SMOS mission is revealing these changes in near real time.
Salt patterns move before temperatures rise
During normal conditions, strong trade winds hold a large layer of low-salinity water, called the freshwater pool, in the far western Pacific. But as El Niño develops, those winds weaken or reverse. The freshwater pool then drifts eastward, dragging a belt of heavy rainfall with it.
Satellites have captured this movement during the 2023 El Niño and La Niña events, and now again in the current emerging El Niño. The eastern edge of the freshwater pool shifts east, and saltier waters that sit just beyond it also move. Meanwhile, much of the pool itself stays relatively fresh.
Why salinity matters for forecasting
Salinity responds quickly to rainfall, evaporation, river discharge, and sea ice melt. It controls seawater density alongside temperature, which drives ocean currents that distribute heat around the planet. By tracking both temperature and salinity, scientists get a fuller picture of how the ocean and atmosphere interact during El Niño.
Regional salinity anomalies are already visible. Areas with more rainfall become fresher than normal, while drier regions become saltier due to evaporation. These contrasting patterns help researchers identify the onset of El Niño earlier than temperature-based methods alone.
The SMOS mission, launched by ESA, measures salinity from space with remarkable precision. Combined with NASA's SMAP mission, the data offers a new pathway for early detection of El Niño, which affects weather worldwide. For scientists monitoring the Pacific, salt is proving to be just as telling as heat.
