Glossary

Homepage number

Homepage phase and strength on El Niño Track come from NOAA CPC’s seasonal ENSO index product (RONI: Relative Oceanic Niño Index), stored with a timestamp and rendered from our store.

They do not come from the optional provisional monthly Niño-3.4 anomaly chart bar (detrend.nino34.ascii.txt). That monthly series is a support chart only (labeled “monthly · not final season”) and must never rewrite the homepage phase/strength sentence.

ONI remains the classic Oceanic Niño Index and is useful for historical reading and comparison; CPC documentation notes RONI is used for official monitoring and prediction.

ENSO

ENSO is short for El Niño–Southern Oscillation: a natural climate pattern in the tropical Pacific that shifts every few years among a warm phase (El Niño), a cool phase (La Niña), and near-average conditions (ENSO-neutral). Changes in tropical Pacific ocean temperature and the overlying atmosphere can tilt seasonal odds for temperature and rainfall in many regions worldwide, including parts of the United States.

Do not confuse with: A day-to-day weather forecast, a local storm name, or a guarantee that your city will see a particular winter.

Source: climate.gov/enso

El Niño

El Niño is the warm phase of ENSO: unusually warm surface waters in the east-central equatorial Pacific (especially the Niño-3.4 region), typically together with matching atmospheric changes (weaker trade winds, shifts in tropical rainfall and pressure). NOAA’s alert criteria also look for the warm anomaly to persist (or be expected to persist) across multiple overlapping three-month seasons, plus atmospheric signs consistent with El Niño.

Do not confuse with: “Any warm Pacific” or a single warm month alone; a one-month spike is not automatically a declared El Niño episode. Also not a forecast that California (or any one city) will flood.

Sources: climate.gov/enso · alert system

La Niña

La Niña is the cool phase of ENSO: unusually cool surface waters in the east-central equatorial Pacific (Niño-3.4), typically with stronger trade winds and the opposite tropical rainfall/pressure pattern from El Niño. Like El Niño, official criteria involve sustained seasonal anomalies and consistent atmospheric changes.

Do not confuse with: “Cold winter where I live”: La Niña is defined in the tropical Pacific, not by your local thermometer. Regional impacts differ from El Niño and still vary event to event.

Sources: climate.gov/enso · alert system

Phase

On El Niño Track, phase means which ENSO state CPC’s seasonal monitoring indicates right now in plain words: typically El Niño, La Niña, or ENSO-neutral (neither). Phase is the headline answer to “Is it El Niño?”, not a city forecast.

Do not confuse with: Strength (how warm/cool the seasonal index is), a Watch/Advisory alert label, or next week’s weather.

Source: climate.gov/enso · phase from store-backed seasonal CPC index (RONI)

Strength

Strength is a plain-English way to describe how large the seasonal tropical Pacific anomaly is (for example weak / moderate / strong), using thresholds CPC/climate.gov authors have published for historical El Niño episodes based on peak seasonal index values. On our site, strength wording is tied to the seasonal index we display (RONI), not to a single provisional monthly bar.

Do not confuse with: How hard it will rain in your city. Stronger events often make some regional composites more consistent historically (e.g. Southern California / Southwest precip), but impacts are still odds, not guaranteed.

Historical strength classes published using ONI (Halpert / climate.gov):

• Weak: peak ONI ≥ 0.5°C and ≤ 0.9°C
• Moderate: peak ONI ≥ 1.0°C and ≤ 1.4°C
• Strong: peak ONI ≥ 1.5°C

Our live homepage number is the seasonal RONI product; do not silently blend indexes. Source: Halpert U.S. El Niño impacts

Niño 3.4 (Niño-3.4 region)

Niño 3.4 is a boxed region of the east-central equatorial Pacific (about 5°N–5°S, 120°–170°W) whose sea-surface temperature anomaly is the ocean building block for NOAA’s main ENSO indexes (ONI and RONI). It sits between the older Niño-3 and Niño-4 boxes and was identified as especially ENSO-representative.

Do not confuse with: A place on a U.S. city map, or with Niño 1+2 (far eastern Pacific near South America); different boxes measure different parts of the basin.

Sources: ONI explainer · why so many ENSO indexes

ONI (Oceanic Niño Index)

The Oceanic Niño Index (ONI) is NOAA’s classic index for the ocean side of ENSO: a three-month running mean of sea-surface temperature anomalies in the Niño-3.4 region, compared with a centered multi-decade base period. Values of about +0.5°C or higher indicate oceanic El Niño conditions; about −0.5°C or lower indicate oceanic La Niña conditions, with episode coloring historically requiring the threshold for several overlapping seasons.

Do not confuse with: A single calendar-month Niño-3.4 anomaly, RONI (relative version), or our homepage phase sentence if the product lock says we display RONI. ONI remains essential for reading many historical tables and climate.gov explainers.

Sources: ONI explainer · CPC ONI table · ONI ASCII

RONI (Relative Oceanic Niño Index)

The Relative Oceanic Niño Index (RONI) is CPC’s seasonal ENSO index that starts from the same Niño-3.4 idea as ONI, then subtracts the broader tropical-ocean anomaly and adjusts variance. In plain English: it asks whether Niño-3.4 is warm or cool relative to the tropics as a whole, which can make classification more consistent as tropical oceans change. CPC publishes RONI as overlapping three-month seasons; thresholds of about ±0.5°C with multi-season persistence are used similarly for historical warm/cold episode coloring.

Do not confuse with: The monthly Niño-3.4 anomaly file used for a provisional support bar (detrend.nino34.ascii.txt), or with monthly “Relative Niño-3.4” (Rnino34.ascii.txt). Homepage phase/strength on El Niño Track use seasonal RONI, not that provisional monthly bar.

Sources: CPC RONI · RONI info PDF · RONI ASCII

Watch / Advisory / Final (CPC ENSO Alert System)

NOAA CPC (with IRI) issues an ENSO Alert System status with monthly updates. In climate.gov’s plain wording:

• Watch: conditions are favorable for El Niño or La Niña to develop within the next six months.
• Advisory: El Niño or La Niña conditions are observed and expected to continue.
• Final advisory: issued after El Niño or La Niña conditions have ended.
• Not Active: neither El Niño nor La Niña are observed or expected in the coming six months.

Do not confuse with: National Weather Service severe weather watches/warnings (tornado, flood, etc.). ENSO Watch/Advisory is a seasonal climate status label; we only show it if store-backed from CPC, and we never invent yellow-triangle “ENSO danger” theater.

Sources: alert system · climate.gov/enso

Anomaly

An anomaly is a departure from a long-term average: for example, sea-surface temperature in Niño-3.4 that is +0.5°C means half a degree Celsius warmer than the reference average for that season or month. ENSO indexes are built from anomalies, not from raw temperatures alone.

Do not confuse with: “Weird weather today,” a forecast error, or a city temperature “feels like.” On our site, anomalies refer to official ocean (or composite climate) departures from average.

Source: ONI explainer

Composite

A composite is an average of many past maps or seasons that share a condition (for example, all El Niño winters). Composites show the typical pattern of above- or below-average temperature, precipitation, or snowfall (useful for “what it usually means” language), but they are not a promise for the next season.

Do not confuse with: A forecast for this winter, a city-specific official meter, or a single famous year (1997–98 is one event inside a composite, not the composite itself).

Source: CPC ENSO composites

Teleconnection

A teleconnection is a climate link between distant places: for ENSO, changes in the tropical Pacific can shift jet streams and storm tracks and tilt seasonal odds thousands of miles away (for example, wetter Gulf Coast winters during many El Niño events). Teleconnections describe typical remote patterns, not guaranteed local weather.

Do not confuse with: Instant same-day weather “connected” to the Pacific, or a claim that every El Niño produces the same outcome in your town.

Sources: what are teleconnections · cascade of global impacts

ENSO-neutral

Neither El Niño nor La Niña criteria are met; tropical Pacific near average.

Source: climate.gov/enso

Southern Oscillation

The atmospheric pressure “seesaw” side of ENSO (historically Tahiti vs Darwin and related indexes).

Source: why so many ENSO indexes

Monthly Niño-3.4 anomaly (provisional chart only)

Single-month SST anomaly in Niño-3.4 from CPC’s monthly ERSST series: support chart, not homepage phase.

Source file: detrend.nino34.ascii.txt