ENSO
The El Niño–Southern Oscillation System
The El Niño–Southern Oscillation (ENSO) is the periodic alternation of warming (El Niño) and cooling (La Niña) of sea surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in atmospheric pressure patterns (the Southern Oscillation). ENSO operates on a 2–7 year cycle and is the single most important driver of inter-annual climate variability globally. The Niño 3.4 region (5°N–5°S, 120°W–170°W) is the standard monitoring zone for SST anomalies. An anomaly of ≥+0.5°C for five consecutive overlapping three-month periods indicates El Niño; ≤−0.5°C indicates La Niña.
- ENSO cycle period: 2–7 years
- Key monitoring region: Niño 3.4 (central-eastern Pacific)
- ONI threshold (traditional): ±0.5°C sustained over 5 consecutive 3-month periods
- RONI effective: February 1, 2026 (replaces ONI after 75 years)
- RONI method: SST anomaly in Niño 3.4 minus average SST anomaly across entire tropical belt (20°S–20°N), adjusted to match ONI variability scale
● Tracked since February 25, 2026 · last seen June 12, 2026 · updates as the daily brief publishes
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