Niño 3.4 is a sea surface temperature (SST) index measuring temperature anomalies in a specific region of the equatorial Pacific Ocean. The region is bounded by 5°N–5°S latitude and 170°W–120°W longitude [1][2][3]. It quantifies how much the sea surface temperature in this area deviates from a long-term climatological average, typically calculated using a 3-month running mean.

The index serves as a primary indicator for monitoring the El Niño–Southern Oscillation (ENSO) cycle [4][2]. When the 3-month running mean SST anomaly reaches approximately +0.5°C or higher and is sustained over multiple overlapping 3-month periods, it indicates El Niño conditions; conversely, anomalies of −0.5°C or lower indicate La Niña conditions [2][3]. Stronger events are often discussed when anomalies exceed ±1.0°C or more. The Oceanic Niño Index (ONI), which NOAA uses as its official standard for declaring El Niño and La Niña episodes, is directly calculated from the 3-month running mean of Niño 3.4 SST anomalies [2].

Niño 3.4 is crucial for climate forecasting because SST changes in this region trigger "teleconnections"—large-scale atmospheric shifts that influence weather patterns worldwide, including rainfall, drought, and hurricane activity across multiple continents [4][2]. Different data products (such as NOAA ERSST, OISST, and HadISST) and different climatological base periods (commonly 30-year normals like 1981–2010 or 1991–2020) are used to compute anomalies, which can produce small variations in reported values [5][6]. Despite these technical variations, Niño 3.4 remains the most reliable single metric for assessing seasonal climate variability and predicting global weather impacts.