General background on the oscillation itself, for context alongside the live reading above.

The number above is NOAA CPC's weekly Niño3.4 anomaly -- the fastest-updating reading this site has, and what the AI summary below draws on. RONI, shown underneath, is a 3-month running mean (e.g. "AMJ 2026" = the April-May-June average): CPC's officially-confirmed index, but slower to move. During a fast-moving swing the weekly figure will typically run ahead of RONI, and both can differ from the "CPC: ..." Alert Status badge, which CPC moves to Advisory/Watch as soon as one season crosses threshold, ahead of RONI's five-season persistence rule. All three are legitimate readings of the same event, just answering different questions: what's SST doing right now (weekly), has a multi-month state been confirmed (RONI), or has CPC flagged it operationally (Alert Status)?

What

A recurring, irregular (roughly 2–7 year) warming and cooling cycle in sea-surface temperature, driven by coupled ocean-atmosphere interaction.

Timescale

Seasonal driver – informs outlooks roughly 1–6 months ahead.

Ocean

Tropical Pacific Ocean.

Most effective

Centered on the equatorial Pacific (the Niño 3.4 region this site's RONI index tracks), with atmospheric teleconnections that reach South Asia mainly by shifting the Walker circulation and, with it, the summer monsoon.

• El Niño (warm phase) Equatorial Pacific SSTs above average; tends to weaken the South Asian summer monsoon, favoring below-normal rainfall and drought risk over India.
• La Niña (cool phase) Equatorial Pacific SSTs below average; tends to strengthen the South Asian summer monsoon, favoring above-normal rainfall.
• Neutral SSTs near average; no strong Pacific-driven monsoon signal, so other factors (IOD, MJO, local conditions) dominate.

* Tentative. RONI isn't published for a month until partway through the next one, so the most recent 1–2 months shown here are filled from NOAA CPC's weekly Niño3.4 anomaly instead -- not RONI itself, and typically higher (see the ENSO card's Info for why). The current calendar month, if shown, is doubly provisional: it's still in progress, so its average will keep shifting until the month ends.

Still Neutral by NOAA's official classification: the latest reading (1.36°C, JJA 2026) is past the ±0.5°C threshold, but an episode is only confirmed once that threshold is sustained for 5 consecutive overlapping 3-month seasons. So far 2 of the 5 required months have qualified toward an El Nino episode. Separately, CPC's real-time ENSO Alert System Status (issued 2026-09-10) already reads "El Niño Advisory." This is CPC's operational designation, which can move to Advisory or Watch as soon as one season crosses threshold with supporting atmospheric coupling, ahead of the persistence rule above.

What this index is and how it's calculated, for context alongside the line below.

What

The Relative Ocean Niño Index: a monthly ENSO index published by NOAA CPC, built from the 3-month running-mean Niño 3.4 sea-surface-temperature anomaly minus the mean SST anomaly across the full tropical belt (20°S–20°N, all longitudes), then variance-scaled to match the legacy ONI's historical spread.

How calculated

Same ±0.5°C thresholds and 5-consecutive-overlapping-season persistence rule as ONI, applied to RONI's tropical-belt-adjusted anomaly instead of the plain Niño 3.4 SST anomaly ONI uses.

Used for

CPC made RONI its primary operational ENSO index in February 2026, replacing ONI, to correct for long-term tropical-wide ocean warming that was inflating ONI's apparent trend. This site follows CPC's lead: RONI is the default ENSO classification used everywhere in this dashboard, from these status cards to the Outlook tab's composites.

How this figure is measured, for context alongside the live reading above.

Source

NOAA/PSL GODAS (Global Ocean Data Assimilation System) ocean reanalysis, potential temperature field.

Spatial span

The Niño 3.4 box: 5°S–5°N, 170°W–120°W (equatorial central-to-eastern Pacific), the same box CPC defines the Niño 3.4 SST index over. See the map below.

Method

Depth-weighted mean potential temperature from the surface down to GODAS's deepest level within the nominal 0–300m layer (~262m, via trapezoidal integration over its 25 unevenly-spaced depth levels), then area-weighted (cosine-latitude weighting) across the box above. Shown as an anomaly relative to that same calendar month's own mean over the full 1980–present GODAS record.

How this chart is measured, for context alongside the lines above.

Source

NOAA CPC, ERSSTv5 (Extended Reconstructed SST v5), 1991–2020 base period, the same fixed WMO-normal climatology CPC itself uses for these indices.

Spatial span

Four overlapping equatorial Pacific boxes, each named for the region it covers: Niño 1+2, Niño 3, Niño 4, and Niño 3.4 (the box RONI/ONI are built from). See the map below.

Method

CPC's own precomputed monthly SST anomaly for each region (downloaded directly as their published index values, not recomputed from gridded SST by this pipeline), relative to that region's 1991–2020 calendar-month mean.

How this map is measured, for context alongside the map above.

Source

NOAA CPC, ERSSTv5 (Extended Reconstructed SST v5), 2° gridded resolution.

Spatial span

30°S–30°N, 120°E–70°W, the same domain the Wind/height coupling map below shows, for visual consistency between the two.

Method

Per-grid-cell anomaly relative to that cell's own 1950–present calendar-month mean (not the 1991–2020 base the Nino-region chart above uses). Shown as a rolling 24-month window rather than full history, since a gridded map is a much larger asset per month than a single number.

Nino regions

Use the "Highlight Nino region" checkboxes below the map to overlay any of the four boxes from the chart above and see how they sit within this wider domain.

How this chart is measured, for context alongside the line above.

Source

NOAA/PSL GODAS (Global Ocean Data Assimilation System) ocean reanalysis, potential temperature field, same source as the heat content anomaly panel.

Spatial span

Two equatorial boxes (both 5°S–5°N): a western box, 120°E–160°E, and an eastern box, 160°W–100°W. See the map below.

Method

Within each box, the depth of the 20°C isotherm is linearly interpolated between GODAS's bracketing depth levels, then area-weighted (cosine-latitude weighting) across the box. The chart plots west minus east.

How this chart is measured, for context alongside the lines above.

Source

ECMWF ERA5 reanalysis (Copernicus Climate Data Store), monthly means on pressure levels, 850-hPa and 200-hPa zonal (east-west) wind fields, on a 2.5° grid.

Spatial span

5°S–5°N, 135°E–180°E, the west-central equatorial Pacific, where the low-level trade-wind response to ENSO is strongest (cf. Trenberth 1997's equatorial zonal wind indices). Unlike Niño 3.4, there's no single fixed "official" box for this diagnostic, so treat the exact boundaries as a reasonable choice rather than a universal standard. See the map below.

Method

Area-weighted (cosine-latitude weighting) mean zonal wind anomaly within the box at each level, relative to that level's own full-record (1948–present) calendar-month mean. Use the checkboxes above the chart to isolate either level.

How this map is measured, for context alongside the map above.

Source

ECMWF ERA5 reanalysis (Copernicus Climate Data Store), monthly means on pressure levels, 850-hPa wind (u, v components, drawn as arrows) and 500-hPa geopotential height (shading), on a 2.5° grid, same reanalysis as the zonal wind chart above.

Spatial span

30°S–30°N, 120°E–70°W, the same domain as the SST map above, for visual consistency between the two.

Method

Per-grid-cell anomaly relative to that cell's own full-record (1948–present) calendar-month mean. Arrow size scales with wind-anomaly speed within this panel only, not to a fixed physical reference.

Nino regions

Use the "Highlight Nino region" checkboxes below the map to overlay any of the four Nino boxes.