What Happened
- The U.S. National Oceanic and Atmospheric Administration (NOAA) has, effective February 1, 2026, retired its traditional Oceanic Niño Index (ONI) and adopted a new standard called the Relative Oceanic Niño Index (RONI) — the first major change to ENSO classification in 75 years.
- The change was driven by the rapid rise in global ocean temperatures: the traditional ONI compared sea surface temperatures (SSTs) in the Niño 3.4 region against a fixed 30-year historical average, but as baseline ocean temperatures have risen due to climate change, this method increasingly misclassified ENSO phases.
- RONI instead measures SST anomalies in the Niño 3.4 region relative to other tropical ocean basins (20°S–20°N) rather than against a historical fixed baseline, removing the long-term warming trend from the index signal.
- Under the new RONI system, the 2024–2025 and 2025–2026 winters — previously classified as ENSO-neutral under ONI — are now designated definitive La Niña events. The data is unchanged; only the method of interpreting it has been revised.
- RONI is expected to generally dampen the measured strength of El Niños and amplify La Niñas compared to ONI, more accurately capturing the climate system's actual behaviour relative to warming baselines.
Static Topic Bridges
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
Connection to this news: The ONI's fixed historical baseline was calibrated to a cooler pre-warming ocean; as the entire tropical ocean warms, the baseline itself shifted upward, making relative cooling events (La Niña) invisible under the old index. RONI corrects this by anchoring the metric to relative, not absolute, temperature differences.
El Niño and the Indian Monsoon
India's Southwest Monsoon (June–September) is strongly modulated by ENSO. El Niño years are associated with suppressed monsoon rainfall — weakened pressure gradients reduce moisture transport from the Arabian Sea. La Niña years, conversely, tend to enhance monsoon activity, leading to above-normal rainfall. Approximately 60% of India's agriculture is monsoon-dependent, and ENSO-linked monsoon variability directly affects food security, reservoir storage, hydropower generation, and drought/flood risk. The Indian Ocean Dipole (IOD) acts as a modulating factor — a positive IOD can partially offset El Niño's negative impact on Indian rainfall.
- El Niño effect on Indian monsoon: typically suppresses rainfall (historically, 8 of 14 major droughts in India linked to El Niño)
- La Niña effect: generally enhances monsoon (above-normal rainfall risk)
- Modulating factor: Indian Ocean Dipole (IOD) — positive IOD can offset El Niño impact
- India Meteorological Department (IMD): uses ENSO state as primary input for Long Range Forecast (LRF) of monsoon
- RONI implication: winters previously counted as ENSO-neutral may now be reclassified — affecting retrospective monsoon analysis
Connection to this news: Reclassifying past ENSO-neutral winters as La Niña under RONI will require India's meteorologists and climate modellers to revisit historical monsoon correlations, potentially revising our understanding of which years were rainfall-surplus due to La Niña influence.
Climate Change and Baseline Drift in Meteorological Indices
As global mean temperatures rise, many meteorological indices calibrated against historical baselines become unreliable because the baseline itself is shifting. This is called "baseline drift" — the signal becomes contaminated by the long-term trend. The WMO standard 30-year climate normal (currently 1991–2020) is updated every decade, but rapid warming means even a 10-year-old baseline may be outdated for detecting anomalies. RONI addresses this for ENSO by removing the tropical-mean warming signal entirely, making the index responsive only to relative Pacific-Atlantic-Indian ocean temperature differences.
- WMO climate normal: 30-year average, updated every decade (current: 1991–2020)
- Baseline drift problem: rapid warming inflates anomaly magnitudes measured against older baselines
- ONI limitation: fixed 30-year comparison period failed to track rising mean ocean temps
- RONI innovation: subtracts tropical mean SST anomaly, isolating ENSO-specific signal from long-term trend
- Broader implication: other climate indices (e.g., drought indices, heat wave thresholds) face similar baseline revision needs
Connection to this news: The NOAA RONI transition is a landmark case of climate science institutionally acknowledging that rapid warming has outpaced the assumptions embedded in decades-old measurement frameworks, with direct implications for how India's meteorological agencies calibrate seasonal forecasts.
Key Facts & Data
- New index: Relative Oceanic Niño Index (RONI), adopted February 1, 2026
- Replaced: Oceanic Niño Index (ONI), in use for approximately 75 years
- Adopting body: NOAA Climate Prediction Center (CPC)
- RONI methodology: Niño 3.4 SST anomaly minus tropical belt (20°S–20°N) mean SST anomaly
- Key effect: RONI dampens El Niño strength, amplifies La Niña strength vs. ONI
- Reclassification: 2024–25 and 2025–26 winters now La Niña under RONI (previously ENSO-neutral under ONI)
- Earth's average monthly temperature took a notable upward jump in early 2023, continuing through 2025
- Three-quarters of the recent change in Earth's energy imbalance attributed to human-caused warming plus the shift from La Niña to El Niño cycle