The eastern part of the equatorial band, 150°W to 90°W.The Niño 3 region is a broad band of the equatorial Pacific, spanning from 150°W to 90°W longitude and 5°S to 5°N latitude. It covers the eastern part of the equatorial Pacific, extending from the central Pacific toward the coast of South America. This area is a key zone for observing the development of El Niño because it captures the transition from the warm pool of the western Pacific to the cooler waters of the eastern Pacific.
The Niño 3 index was introduced in the 1980s by climate researchers to complement the coastal Niño 1 and 2 indices, providing a broader measure of the equatorial Pacific's state.
Sea-surface temperatures in this region are averaged from satellite and buoy measurements. During El Niño, the thermocline—the boundary between warm surface water and cold deep water—deepens here, allowing warm water to spread eastward. The temperature anomaly in Niño 3 reflects the strength of this warming and is used alongside other indices to track the spatial extent of an El Niño event.
It helps scientists distinguish between different types of El Niño (e.g., eastern Pacific vs. central Pacific events), which have different global impacts on rainfall and temperature patterns.
The Niño 3 region's temperature can swing by more than 3°C between a strong El Niño and a strong La Niña, a variation larger than the entire global warming trend of the past century.
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