What El Niño is

El Niño is the warm phase of a recurring shift in the tropical Pacific known as the El Niño–Southern Oscillation. Every two to seven years, on no fixed schedule, the trade winds that normally hold warm surface water in the western Pacific weaken, and that warm water spreads back east towards South America. The tropical Pacific is large enough that moving the warmest water rearranges rainfall and wind patterns across much of the world. For New Zealand, an El Niño summer usually means stronger westerly winds, a drier east coast and a wetter West Coast.

Events normally begin between April and June, reach their peak between November and January, and fade through the following autumn. The name comes from Peruvian fishermen, who used el niño — the Christ Child — for the warm current that arrived off their coast around Christmas, long before anyone knew it was part of a Pacific-wide pattern.

How it works

In an ordinary year the trade winds blow from east to west along the equator, dragging surface water with them. Warm water piles up around Indonesia, where sea level sits roughly half a metre higher than off South America, and cold water wells up along the South American coast to replace what has been pushed away. That upwelling carries nutrients with it, which is why the waters off Peru support one of the largest fisheries anywhere.

When the trade winds slacken, the warm water that was being held in the west spreads east and the upwelling weakens or stops. Rainfall follows the warm water: the storms that normally sit over Indonesia and the western Pacific shift towards the central and eastern Pacific, and because those storms drive the winds of the wider atmosphere, weather patterns move with them well beyond the tropics.

The change reinforces itself for a time — weaker winds allow more warming, and the warming weakens the winds further. It ends when the deeper ocean has given up enough heat for the trade winds to re-establish, which is why events run for roughly a year rather than settling in permanently, and why a strong El Niño is often followed by a swing towards La Niña.

El Niño, La Niña and neutral

El Niño and La Niña are opposite phases of the same oscillation, with a neutral state in between that the Pacific spends roughly half its time in. For New Zealand the two phases tend to produce close to opposite seasons.

El NiñoNeutralLa Niña
Trade windsWeaker than normalNear normalStronger than normal
Warmest Pacific waterCentral and eastern PacificWestern PacificWestern Pacific, more so than usual
NZ summer windMore westerly and south-westerlyVariableMore north-easterly
NZ east coastDrier, higher drought riskNear normalWetter, more humid
NZ west coastWetterNear normalDrier
NZ temperature leanCooler south-west, hot nor'westers in the eastNear normalWarmer and more humid overall

For the full comparison — how often each occurs, how fast the Pacific switches, and the two phases season by season — see El Niño vs La Niña.

New Zealand rows follow NIWA’s description of the pattern. Source: NIWA — The impact of El Niño and La Niña on New Zealand's climate

How strength is measured

The standard measure is the Oceanic Niño Index: the sea surface temperature anomaly in the Niño 3.4 region of the central tropical Pacific, averaged over three months. NOAA declares El Niño conditions once the index reaches +0.5 °C and La Niña conditions at −0.5 °C, and an event is recognised when the index holds beyond the threshold for five consecutive overlapping three-month periods.

Weak0.5 – 0.9 °C
Moderate1.0 – 1.4 °C
Strong1.5 – 1.9 °C
Very strong2.0 °C and above

Because the index averages three months, it lags the ocean. Weekly readings for the same region move first and run further, which is why the weekly figure at the top of this page can sit well above the official three-month index during a fast-developing event.

Source: NOAA Climate Prediction Center — ONI

Every event since 1950

The index has been running since 1950, so the record now covers around two dozen El Niño events of varying size. Strong events cluster irregularly, and the gap between them has ranged from two years to fifteen.

What El Niño means for New Zealand

NIWA describes the New Zealand pattern as stronger or more frequent winds from the west in summer, which raises the chance of drier-than-normal conditions on the east coast and more rain than normal in the west, because the Southern Alps and the main North Island ranges block the flow. In winter the wind tends to come more from the south and is colder. Spring and autumn fall between the two, with stronger or more frequent south-westerlies.

One qualification matters more than anything else on this page: on NIWA’s figures, ENSO accounts for less than 25 percent of the year-to-year variance in seasonal rainfall and temperature at most New Zealand locations. El Niño shifts the odds across a season. It does not determine the weather on any given day, and other influences — the Southern Annular Mode, the temperature of the Tasman Sea — can outweigh it for weeks at a time.

RegionRainfall leanNotes
NorthlandDrierWesterly flow leaves the north-east sheltered; soil moisture tends to run low by late summer.
AucklandDrierBelow-normal rainfall is the usual lean, with water restrictions more likely in a strong event.
WaikatoDrierDrying through summer, with pasture growth the first thing to feel it.
Bay of PlentyDrierSheltered from a westerly flow by the ranges to the west.
GisborneDrierAmong the regions most exposed to El Niño drought in past events.
Hawke's BayDrierThe east coast rain shadow deepens under a persistent westerly.
TaranakiDrierDrier on the whole, though the westerly exposure moderates it against the east coast.
Manawatū-WhanganuiDrierWindier than normal, with rainfall leaning below average.
WellingtonDrierStronger and more frequent wind; the Wairarapa side dries the most.
NelsonDrierSheltered from the westerly, and dry spells extend.
TasmanMixedDrier towards Nelson, wetter on the Buller side as you move onto the West Coast.
MarlboroughDrierOne of the drier regions in an El Niño summer.
CanterburyDrierNor'westers cross the Alps dry and warm; irrigation demand rises.
OtagoMixedEastern and coastal Otago lean drier; inland and Central Otago can pick up rain from the west.
SouthlandWetterExposed to the south-westerly flow, so rainfall leans above normal.
West CoastWetterThe Alps wring out the westerly flow here, and heavy rain events shift this way.
Chatham IslandsNo clear signalWell east of the main islands, with no consistent ENSO rainfall signal.

Leans are the tendency across past El Niño events, not a forecast for a particular season. For the current three-month outlook, see NIWA. Source: NIWA Seasonal Climate Outlook

What past El Niño events did

Each of these seasons is described as El Niño contributing to what happened rather than causing it. The oscillation loads the dice over a season across a wide area; pinning any single drought or flood on it alone would claim more than the science supports.

1982–83

Peak ONI +2.1 °C · DJF 1983 · Eastern Pacific

At its peak: Niño 1+2 +2.7 °C off Peru, Niño 4 +0.4 °C near the Date Line.

The strongest event measured up to that point, and the one that turned El Niño from a regional curiosity into a subject of global forecasting. It went largely undetected until it was well developed, because the satellite record was thin and one of the main monitoring buoys had been reading warm water as instrument error.

Peru & Ecuador
Warm water displaced the cold, nutrient-rich Humboldt Current, collapsing the anchoveta catch. Rain fell on coastal desert that normally receives almost none, and rivers flooded.
Australia
Severe drought across the south-east ran into the Ash Wednesday bushfires of 16 February 1983, which killed 75 people in Victoria and South Australia.
Indonesia
Failed wet season and drought, with extensive forest fires in Borneo.
New Zealand
A dry summer through eastern districts, with the westerly pattern that typifies El Niño here well established.

1997–98

Peak ONI +2.4 °C · NDJ 1997 · Eastern Pacific

At its peak: Niño 1+2 +3.9 °C off Peru, Niño 4 +0.6 °C near the Date Line.

Forecast months ahead — the first event where the warning arrived early enough to act on — and still the benchmark for a fast-developing eastern Pacific El Niño. 1998 became the warmest year measured at the time.

Indonesia & South-East Asia
Drought and forest fires produced haze that hung over the region for months, closing airports and schools across Malaysia, Singapore and Indonesia.
Peru & Ecuador
Coastal flooding and landslides, with fishery losses repeating the pattern of 1982–83.
East Africa
Heavy rain and flooding across Kenya and Somalia — the eastern Horn tends to get wetter during El Niño while southern Africa dries.
United States
A wet, storm-heavy winter across California and the southern states; an unusually mild one across the north.
New Zealand
One of the more severe droughts on record for eastern regions, running through the 1997–98 summer.

2015–16

Peak ONI +2.6 °C · NDJ 2015 · Mixed

At its peak: Niño 1+2 +2.0 °C off Peru, Niño 4 +1.6 °C near the Date Line.

The highest ONI in the record that begins in 1950. Unlike 1982–83 and 1997–98 the warmest water sat in the central Pacific rather than against South America, so Peru was spared the worst of it while the atmospheric effects were felt widely. It landed on an already-warming baseline, and 2016 became the warmest year measured until 2023.

Southern Africa
Drought through the 2015–16 growing season across Zimbabwe, Zambia, Malawi and South Africa, with widespread crop failure and food shortages.
Ethiopia
Successive failed rains drove one of the country's largest humanitarian responses in decades.
India
A below-average 2015 monsoon, following a weak 2014 season.
Coral reefs
Sustained ocean heat triggered a global bleaching event, including mass bleaching on the Great Barrier Reef in early 2016.
New Zealand
A drier-than-normal summer in the north and east, though the event's effect here was milder than its size in the Pacific suggested.

2023–24

Peak ONI +2.0 °C · NDJ 2023 · Mixed

At its peak: Niño 1+2 +2.0 °C off Peru, Niño 4 +1.5 °C near the Date Line.

A strong event that fell just short of the very strong band, with warmth spread fairly evenly across the monitoring regions rather than concentrated at either end. NIWA declared El Niño conditions in September 2023, after the three-year La Niña ended that March. 2023 and then 2024 each set global temperature records.

Panama
Drought lowered Gatún Lake, which feeds the canal locks, and the canal authority cut the number of daily transits for much of 2024.
Amazon basin
Rivers fell to record lows in late 2023, stranding river communities that depend on boats for supplies.
Southern Africa
Drought through the 2024 growing season, with national emergencies declared in Zambia, Zimbabwe and Malawi.
New Zealand
The clearest illustration of how loosely El Niño binds here. The westerly pattern did establish over the South Island and lower North Island, but the expected dry east did not follow: NIWA recorded rainfall above or well above normal from Waitomo east to Hawke's Bay and Gisborne over summer 2023–24, the opposite of the textbook signal. It was the ninth-warmest summer on record.

Impact summaries draw on the World Meteorological Organization’s event reporting, NOAA’s historical ENSO record and NIWA’s New Zealand climate summaries. Source: WMO, NOAA and NIWA

Common questions

What is El Niño?

El Niño is the warm phase of a Pacific-wide pattern called the El Niño–Southern Oscillation. The trade winds that normally push warm surface water west across the tropical Pacific weaken, and that warm water spreads back east. Because the tropical Pacific is large enough to move the atmosphere above it, the change in where the ocean is warmest shifts rainfall and wind patterns worldwide. El Niño events typically develop between April and June, peak between November and January, and fade by the following autumn.

Were the Auckland floods and Cyclone Gabrielle caused by El Niño?

No. Both happened in late January and February 2023, during the closing months of a three-year La Niña, and NIWA did not declare El Niño conditions until September 2023. NIWA associates the flooding in the north and east of the North Island with that La Niña phase, together with unusually warm water in the Coral Sea and a favourable pulse of the Madden–Julian Oscillation, which supplied the heat and moisture those systems drew on. Heavy rain in the warm, humid north-east is closer to the La Niña pattern than the El Niño one, which tends to dry those areas out.

How is El Niño measured?

The main yardstick is the Oceanic Niño Index, which is the sea surface temperature anomaly in the Niño 3.4 region of the central tropical Pacific, averaged over three months. NOAA declares El Niño conditions when the index reaches +0.5 °C and La Niña conditions at −0.5 °C. An event is classified as weak at 0.5 to 0.9, moderate at 1.0 to 1.4, strong at 1.5 to 1.9, and very strong at 2.0 or above.

Does El Niño mean a hot summer in New Zealand?

Not by itself. El Niño tends to bring stronger westerly and south-westerly winds, which can hold temperatures down even while the east coast dries out. Canterbury and other eastern areas can still see high temperatures when the wind comes across the Alps as a hot, dry nor'wester. Background warming has also lifted the baseline, so recent El Niño summers have been warmer than earlier ones of similar strength.

How much of New Zealand's weather does El Niño actually explain?

Less than most coverage suggests. NIWA puts ENSO at under 25 percent of the year-to-year variance in seasonal rainfall and temperature at most New Zealand locations. It shifts the odds across a season rather than setting the weather on any given day, and other drivers — the Southern Annular Mode, the state of the Tasman Sea — can override it for weeks at a time.

Where does the name El Niño come from?

From Peruvian fishermen, who used el niño — the Christ Child — for the warm current that appeared off their coast around Christmas. The name was later applied to the whole Pacific-scale pattern once its global reach was understood.

Official guidance

WeatherNZ reports forecast and climate data, and nothing on this page is preparedness or safety advice. For what to keep at home and what to do during a drought, storm or flood, the official source is Get Ready, run by Civil Defence — the National Emergency Management Agency and the regional Civil Defence Emergency Management groups. Warnings and watches come from MetService, and the three-month outlook from NIWA.

More on New Zealand’s climate patterns

La Niña

The cool phase — north-easterly winds, a wetter north-east, and the 2020–23 event behind the Auckland floods and Cyclone Gabrielle.

El Niño vs La Niña

The two phases side by side: what differs, how often each occurs, and which one the Pacific is in right now.

Sources

Everything on this page traces back to one of the following. The live figures come from NOAA’s published indices; the New Zealand interpretation follows NIWA, which is the authority for this country.