ENSO glossary
Short, plain-language definitions for the terms you'll run into anywhere on this site — or in NOAA's own ENSO products. Grouped loosely by topic; use your browser's find (Ctrl/Cmd+F) if you're hunting for one specific word.
The core cycle
| ENSO | El Nino-Southern Oscillation: the umbrella name for the whole recurring climate pattern — the ocean side (warming or cooling of the tropical Pacific) and the atmosphere side (shifts in tropical rainfall and pressure) acting together. El Nino, La Nina, and ENSO-neutral are the three phases of this one cycle. |
| El Nino | The warm phase of ENSO: sustained above-average sea-surface temperatures in the tropical Pacific's Nino 3.4 region, at or above the +0.5°C threshold on NOAA's index, for enough consecutive seasons to count as an event. See What is El Nino? for the full explainer. |
| La Nina | The cool phase of ENSO: sustained below-average sea-surface temperatures in Nino 3.4, at or below the −0.5°C threshold. Tends to push US winter impacts in roughly the opposite direction of El Nino, though not as a mirror image in every region. See El Nino vs. La Nina. |
| ENSO-neutral | Neither phase: Nino 3.4 anomaly sitting between roughly −0.5°C and +0.5°C. Neutral winters don't have a strong ENSO signal pulling the odds one way, so other climate patterns tend to matter more. |
Monitoring regions and indices
| Nino 3.4 region | The patch of tropical Pacific Ocean (roughly 5°N-5°S, 170°W-120°W) whose sea-surface temperature is the standard basis for ENSO monitoring. It's not the only monitoring box — Nino 1+2, Nino 3, and Nino 4 cover other stretches of the tropical Pacific and are used for more specialized analysis — but Nino 3.4 is the one behind ONI, RONI, and nearly every headline ENSO number. |
| ONI | Oceanic Nino Index: a 3-month running average of the Nino 3.4 SST anomaly, measured against a fixed 30-year climatology. NOAA's classic, longest-running ENSO index, published continuously back to 1950. |
| RONI | Relative Oceanic Nino Index: the Nino 3.4 seasonal SST anomaly minus the tropical-mean (20°N-20°S) SST anomaly. NOAA adopted RONI as its operational ENSO index on February 1, 2026, to correct for long-term basin-wide ocean warming that had been gradually inflating raw ONI values. ONI is still published too. Full explainer: ONI vs. RONI. |
| Southern Oscillation Index (SOI) | An atmospheric-pressure-based ENSO indicator: the standardized difference in sea-level pressure between Tahiti and Darwin, Australia. Sustained negative SOI tends to track with El Nino; sustained positive SOI with La Nina. It measures the "atmosphere half" of ENSO, as a companion to the SST-based indices above. |
Don't mix conventions: the Australian Bureau of Meteorology (BOM) runs its own Nino 3.4-based index and calls El Nino at a +0.8°C threshold on that index — a different convention from NOAA's +0.5°C threshold on ONI/RONI. Both are legitimate, official products, but they are not interchangeable. A reading that clears NOAA's threshold may sit below BOM's, and vice versa. Never quote a single number as if it satisfies both conventions at once — always say which index and which threshold you mean. This site uses the NOAA convention throughout (see methodology).
Alerts
| ENSO Watch | NOAA's alert level meaning conditions are considered favorable for El Nino or La Nina to develop within roughly the next six months. A Watch is not a confirmation that an event will happen — it's an early heads-up. |
| ENSO Advisory | NOAA's alert level meaning El Nino or La Nina conditions have actually been observed, and are expected to continue. An Advisory is a step up from a Watch: it reflects current conditions rather than a forecast of onset. Check the live dashboard for the current advisory status. |
Impacts and analysis
| Teleconnection | A link between a climate pattern in one part of the world (like tropical Pacific SSTs during El Nino) and weather effects somewhere far away (like a wetter Southern California winter). ENSO's US impacts are almost all teleconnections — the ocean anomaly itself is thousands of miles from the affected regions; it acts by shifting atmospheric circulation, especially the jet stream. |
| Analog year / analog comparison | A past ENSO event chosen for comparison to the current one because it followed a similar strength and development pattern. Useful for illustrating a plausible range of outcomes, not a prediction that this year will repeat any single analog. See methodology for exactly how this site aligns and scores analog comparisons. |
| Atmospheric river | A long, narrow band of concentrated water vapor transport in the atmosphere, often called a "river in the sky." Atmospheric rivers deliver a large share of West Coast winter precipitation (and flood risk) in any year, and El Nino winters tend to see the storm track — and the atmospheric rivers riding it — aimed more often at the southern US tier. |
| Subtropical jet stream | A fast, high-altitude river of wind typically found farther south, around 20-30°N. During El Nino winters it tends to strengthen and extend across the southern US, steering more Pacific storms toward California, the Southwest, and the Gulf Coast. |
| Polar jet stream | A separate fast wind current farther north, generally near the US/Canada border in winter. During El Nino it tends to sit farther north and weaken over the northern US, which is part of why the Pacific Northwest, Northern Rockies, and Northern Plains often see a warmer, drier signal. |
Organizations
| CPC | NOAA's Climate Prediction Center — the operational US government body that issues ENSO Diagnostic Discussions, ENSO Watch/Advisory alerts, and seasonal outlooks. The primary data source behind this site. See CPC's home page. |
| IRI | The International Research Institute for Climate and Society (Columbia University), which publishes ENSO forecast model plumes and probability outlooks in coordination with CPC. See the IRI ENSO Quick Look. |