Guide

El Niño, La Niña and ENSO-neutral explained

El Niño and La Niña are the warm and cool phases of the El Niño–Southern Oscillation (ENSO), a natural swing in the tropical Pacific that couples the ocean and the atmosphere. During El Niño the central and eastern equatorial Pacific is unusually warm and the trade winds weaken; during La Niña the same waters are unusually cool and the trade winds strengthen. ENSO-neutral is the state in between, when neither pattern is established. NOAA issues a dated assessment of which phase applies; the homepage shows the latest one.

The three phases side by side

Typical ocean and atmosphere conditions in each ENSO phase
Feature	El Niño	ENSO-neutral	La Niña
Central and eastern Pacific sea surface	Warmer than usual	Near the long-term pattern	Cooler than usual
Trade winds along the equator	Weaker, sometimes reversing in the west	Blowing from east to west as normal	Stronger than usual
Deep tropical rainfall	Shifts east towards the central Pacific	Concentrated over the far western Pacific	Pushed further west, over Indonesia and Australia
Warm water and thermocline	Warm layer spreads east; upwelling off South America weakens	Warm pool in the west, cool tongue in the east	Warm layer piled up in the west; upwelling strengthens
Niño 3.4 and RONI sign	Positive, typically at or above +0.5 °C	Between the ±0.5 °C thresholds	Negative, typically at or below −0.5 °C

The index rows describe typical readings, not the definition of a phase. NOAA’s assessment weighs ocean and atmosphere together, and its historical classification requires the threshold to be met for five consecutive overlapping seasons.

The ocean and the atmosphere move together

In a neutral tropical Pacific, the trade winds blow from east to west, pushing warm surface water into a deep pool near Indonesia and drawing cooler water up to the surface along the coast of South America. Warm water fuels rising air and heavy rain in the west, while the cooler east sits under sinking air. This east–west loop is the Walker circulation. El Niño begins when the loop weakens: the trades slacken, warm water sloshes eastward along the equator, the eastern Pacific warms, and the region of rising air and rainfall follows the warm water towards the central Pacific. The warmer ocean weakens the winds further, so the ocean and the atmosphere reinforce each other. La Niña is the same feedback running the other way, with stronger trades, a cooler east and rainfall pushed even further west. Neither phase is driven by the ocean or the atmosphere alone, which is why NOAA looks for both before declaring one.

Watch, advisory and current phase

NOAA’s Climate Prediction Center attaches an alert label to its monthly discussion. An El Niño Watch or La Niña Watch means conditions are favourable for that phase to develop within the next six months; it is not a statement that the phase has arrived. An El Niño Advisory or La Niña Advisory means the phase is observed and expected to continue. A Final Advisory marks the return to neutral after an event. The label is separate from the phase itself: during an advisory the phase is El Niño or La Niña, while during a watch the phase is usually still ENSO-neutral. ENSO Tracker shows the phase and the alert label as two distinct items because conflating them is the most common misreading of NOAA’s statement. The definitions are in NOAA’s ENSO FAQ ↗.

How the indices support monitoring

Sea surface temperature indices are the most visible part of ENSO monitoring because they can be summarised in one number. The weekly Niño 3.4 anomaly shows the ocean’s recent behaviour at high resolution, and the seasonal Relative Oceanic Niño Index smooths that behaviour into the three-month values NOAA uses to classify episodes. Forecasters also watch the depth of warm water below the surface, the strength of the trade winds, cloudiness and the Southern Oscillation Index of surface pressure, none of which appear on this site. An index crossing ±0.5 °C is a prompt to look closer, not a verdict. A phase is declared when the ocean signal is accompanied by the expected atmospheric response and is expected to persist.

Why the effects vary

El Niño and La Niña shift where tropical rainfall falls, and that shift nudges the jet streams and storm tracks over distant regions. The nudges are tendencies rather than guarantees. Their strength depends on the season, on how strong and where the Pacific warming or cooling is centred, and on other climate patterns acting at the same time. Two El Niño winters can bring different weather to the same city. ENSO-neutral does not mean average weather everywhere; it means the Pacific is not pushing in either direction. Regional outlooks from national meteorological services are the right place for expected local impacts. Introductory overviews from NOAA’s National Ocean Service ↗ and the World Meteorological Organization ↗ describe the global pattern of typical effects.

Which part of the tracker answers which question?

Where to look on ENSO Tracker
Question	Where to look
Are we in El Niño, La Niña or neutral right now?	The status at the top of the homepage, with NOAA’s issue date and any watch or advisory label.
How warm or cool is the Pacific this week?	The regional readings and the weekly chart, dated to the week they describe.
Is this season past the threshold?	The Seasonal RONI reading and the seasonal record, with the ±0.5 °C lines.
How does this year compare with past events?	The comparison view of either chart, aligned by calendar date or by season label.
Where do the numbers come from?	The methodology and the source list at the foot of each page.

The current phase is deliberately not repeated in this guide. It changes, and the homepage reads it from NOAA’s validated assessment each time the data are refreshed.