Oceansat-3 Detects Chlorophyll Decline in Pacific Amid Developing El Niño

The equatorial Pacific Ocean is normally governed by strong trade winds blowing from east to west. These winds push warm surface water westward, allowing cooler, nutrient-rich water to rise from the deep ocean along the South American coast, a process called upwelling. This upwelling is the engine of some of the world's most productive marine ecosystems. The world's richest fishing grounds are found precisely where warm and cold oceanic currents meet, or where upwelling brings cold, nutrient-dense water to the sunlit surface layer. Phytoplankton, microscopic marine plants, thrive on these nutrients and form the base of the entire ocean food web. Their abundance is measured through chlorophyll-a concentration, the green pigment used in photosynthesis, which satellites can detect from space.

El Niño disrupts this system fundamentally. During an El Niño event, the normal easterly trade winds weaken or even reverse to westerlies. Warm water that was pushed westward slumps back eastward across the Pacific. This suppresses upwelling along the South American coast and across the equatorial Pacific, cutting off the nutrient supply to surface waters. Without nutrients, phytoplankton cannot grow, chlorophyll concentrations fall, and marine primary productivity collapses. The same mechanism that makes El Niño years catastrophic for Peruvian anchovy fisheries, historically one of the world's largest, operates across the broader equatorial Pacific. Sustained nutrient depletion of this kind can produce low-oxygen, low-productivity zones in the ocean, analogous in effect (though different in cause) to the anthropogenic dead zones created by eutrophication, where marine life is similarly starved of the conditions needed to survive.

El Niño is part of the larger ENSO (El Niño-Southern Oscillation) cycle, which also includes La Niña (anomalous cooling) and neutral phases. The Southern Oscillation refers to the see-saw in atmospheric pressure between the central Pacific and the Australian/Indian Ocean region. Strong ENSO events drive large-scale weather disruptions globally: suppressed monsoon rainfall over South Asia, droughts in Australia and southern Africa, and flooding along the South American Pacific coast. For India, El Niño's primary impact is through weakening of the Walker Circulation, which reduces moisture transport to the subcontinent and leads to deficient southwest monsoon seasons.