Satellite observations from the Indian Space Research Organisation’s (ISRO) Oceansat-3 spacecraft have captured a clear decline in marine productivity across the equatorial Pacific Ocean, offering fresh satellite-based evidence of the developing El Niño conditions.
The observations complement the World Meteorological Organisation’s (WMO) outlook, which reported in July that El Niño was intensifying and expected to strengthen during August–October 2026, ISRO said on September 10.
Oceansat-3, also known as EOS-06, is a third-generation Indian Earth observation satellite dedicated to oceanographic and atmospheric monitoring. It was launched aboard a PSLV in November 2022.
The developing El Niño is of particular significance for India as warmer-than-normal Pacific Ocean surface temperatures can adversely affect the Indian summer monsoon. With the monsoon season nearing its end, rainfall so far is about 15 per cent below the long-term average.
El Niño is one phase of the El Niño–Southern Oscillation (ENSO), a major climate phenomenon originating in the tropical Pacific. The cycle alternates between El Niño, characterised by unusual warming, and La Niña, marked by cooling. Changes in the tropical Pacific can influence weather patterns far beyond the region.
What the satellite observations show
During El Niño, equatorial trade winds weaken and the thermocline—a transition layer between warmer surface water and colder deeper water—deepens. This reduces the upwelling of cool, nutrient-rich water into the sunlit upper ocean.
With fewer nutrients reaching the surface, the growth of phytoplankton—microscopic, plant-like organisms that drift in oceans and seas—can decline. This, in turn, lowers concentrations of surface chlorophyll-a, an indicator of marine productivity.
To track these changes, ISRO’s National Remote Sensing Centre (NRSC) analysed chlorophyll-a measurements from the Ocean Colour Monitor-3 (OCM-3), along with ocean-surface wind vector observations from Oceansat-3. The analysis covered the tropical Pacific during April, May and June for the years 2023–2026.
A comparison of June 2024 and June 2025 observations, representing neutral ENSO conditions, with June 2026 data from the developing El Niño period showed a notable reduction in surface chlorophyll-a.
The decline coincided with a reversal in ocean-surface winds—from their usual east-to-west direction to west-to-east, according to ISRO.
“This weakening and reversal of the prevailing winds suppresses the normal equatorial upwelling across parts of the western and central equatorial Pacific. The resulting reduction in the supply of nutrient-rich subsurface waters is reflected in the observed decline in surface chlorophyll-a,” ISRO said.
The satellite observations also showed that the region of elevated chlorophyll had contracted westward between April and June 2026.
“The apparent contraction of the westward extent of the elevated-chlorophyll region between April and June 2026 further supports the weakening of equatorial upwelling as the El Niño event developed,” ISRO said.
The space agency said the combined use of multiple satellite sensors provides a clearer picture of the strong connection between physical ocean changes and biological productivity in the equatorial Pacific during the ongoing El Niño event.
If the El Niño intensifies further, as currently forecast, the low-chlorophyll region could become more pronounced in the months ahead.
“This should be further inferred with concurrent estimates of anomalies in equatorial winds, sea-surface temperature, sea-level height, thermocline depth and ocean circulation in the coming months,” ISRO said.
