The one difference everything follows from
El Niño and La Niña are not two separate phenomena. They are opposite ends of a single measurement — the temperature of the tropical Pacific — and the thing that moves that measurement is the trade winds blowing east to west along the equator.
El Niño — winds weaken
Warm water that was being held in the western Pacific spreads back east. Upwelling off South America slows or stops. Rainfall shifts east with the warm water.
La Niña — winds strengthen
More warm water is packed into the west and more cold water is drawn up off South America. Rainfall concentrates further west than usual.
Because the tropical Pacific is large enough to move the atmosphere above it, that one change reorganises wind and rainfall far beyond the tropics — including here.
Side by side
| El Niño | La Niña | |
|---|---|---|
| In the Pacific | ||
| Trade winds | Weaker than normal | Stronger than normal |
| Warmest water sits | Central and eastern Pacific | Western Pacific, more than usual |
| Upwelling off South America | Suppressed | Enhanced |
| Oceanic Niño Index | +0.5 °C or above | −0.5 °C or below |
| Around the world | ||
| Eastern Australia | Drier, higher fire risk | Wetter, higher flood risk |
| Peru and Ecuador | Flooding; fishery collapse | Drier; fishery recovers |
| Eastern Horn of Africa | Wetter | Drier |
| Atlantic hurricanes | Fewer — more wind shear | More — less wind shear |
| In New Zealand | ||
| Prevailing summer wind | Westerly and south-westerly | North-easterly |
| North-east of the North Island | Drier | Wetter and more humid |
| West Coast and Southland | Wetter | Drier |
| Eastern districts | Drier, drought risk | Nearer normal to wetter |
| Temperature lean | Cooler south-west; hot nor'westers in the east | Warmer over much of the country |
| Ex-tropical cyclone risk | Lower | Higher |
| Behaviour | ||
| Typical duration | About a year | A year, often two or three |
| Repeats in consecutive years | Almost never | Common |
New Zealand rows follow NIWA’s description of the pattern; Pacific and global rows follow NOAA. Source: NIWA and NOAA
New Zealand, season by season
Most coverage describes the summer pattern and stops there. The two phases behave differently through the rest of the year, and NIWA describes El Niño’s effect as changing direction with the season.
| Season | El Niño | La Niña |
|---|---|---|
| Summer | Stronger westerlies. Dry in the east and north, wet on the West Coast. | North-easterlies. Humid and wet in the north-east, warmer overall. |
| Autumn | South-westerlies strengthen — a mix of the summer and winter patterns. | Warm and often humid, with ex-tropical systems still possible into April. |
| Winter | Colder southerlies tend to prevail. | Milder than normal, with less southerly outbreak. |
| Spring | Westerly quarter winds dominate; dry in the east and north. | North-easterlies return; the north-east wets up first. |
Source: NIWA — The impact of El Niño and La Niña on New Zealand's climate
How often each one happens
How fast the Pacific switches
Which is worse for New Zealand?
The question does not have a single answer, because the two phases put different places under pressure. What can be said is what each has actually done here.
El Niño’s signature risk: drought
The 1997–98 event brought one of the more severe droughts on record for eastern regions. Persistent westerlies leave Gisborne, Hawke’s Bay, Marlborough and Canterbury in a deepening rain shadow, and irrigation and stock water come under strain by late summer.
More on El Niño →La Niña’s signature risk: flooding
The Auckland Anniversary floods and Cyclone Gabrielle both fell in the closing months of the 2020–23 La Niña. Warm seas and a north-easterly flow load the atmosphere over the upper North Island with moisture, and the chance of an ex-tropical cyclone reaching the country rises.
More on La Niña →One caveat sits above both: 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. Knowing the phase tells you which way the odds have shifted. It does not tell you what next Tuesday looks like.
Common questions
What is the difference between El Niño and La Niña?
They are opposite phases of the same Pacific pattern, and the difference starts with the trade winds. In El Niño those winds weaken, so warm surface water spreads east across the tropical Pacific. In La Niña they strengthen, packing warm water into the west and pulling colder water up along South America. Everything else follows from that: rainfall moves east in El Niño and west in La Niña. For New Zealand, El Niño favours westerly and south-westerly winds with a drier east and a wetter West Coast, while La Niña favours north-easterlies with a wetter, more humid north-east and a drier lower South Island.
Which is worse for New Zealand, El Niño or La Niña?
Neither is worse overall — they carry different risks to different places. El Niño's signature problem here is drought through the east and north, as in 1997–98. La Niña's is heavy rain and flooding in the north and east of the North Island, plus a higher chance of an ex-tropical cyclone arriving; the Auckland Anniversary floods and Cyclone Gabrielle both fell in the closing months of the 2020–23 La Niña. A Gisborne orchardist and a Southland farmer would answer this question differently.
Can El Niño and La Niña happen at the same time?
No. They are opposite ends of one measurement, so the Pacific is in one, the other, or neither. The in-between state is called ENSO-neutral, and it is the most common of the three.
How quickly can the Pacific switch between them?
Faster than most people expect. The index can travel from one phase to the other inside six months when conditions line up, though a year or more is more typical. A strong El Niño is often followed by a swing towards La Niña, because the warm water displaced eastward has to redistribute and the system tends to overshoot on the way back.
Does La Niña always follow El Niño?
Not always, but the sequence is common enough to be worth expecting. What is more reliable is the asymmetry in how long they last: La Niña frequently runs for two or three years in a row, while back-to-back El Niño years are close to unknown in the record since 1950.
More on New Zealand’s climate patterns
Sources
- Oceanic Niño Index (ONI) — NOAA Climate Prediction Center. The official three-month index, and the full record from 1950 that the history chart is drawn from.
- Weekly Niño region sea surface temperatures — NOAA Climate Prediction Center. The latest weekly reading for Niño 1+2, 3, 3.4 and 4, updated each Monday.
- ENSO Diagnostic Discussion — NOAA Climate Prediction Center. The monthly advisory status and forecast probabilities for coming seasons.
- The impact of El Niño and La Niña on New Zealand's climate — NIWA / Earth Sciences New Zealand. The New Zealand wind, rainfall and regional patterns, and the note on how much of our variability ENSO explains.
- Seasonal Climate Outlook — NIWA / Earth Sciences New Zealand. The current three-month outlook for New Zealand rainfall, temperature, soil moisture and river flows.
- El Niño and La Niña — NIWA / Earth Sciences New Zealand. Background on how ENSO is monitored in the South-West Pacific.
- ENSO Wrap-Up — Australian Bureau of Meteorology. Cross-check on Pacific conditions and the Australian view of the current event.
- Get Ready — preparing for drought, storms and floods — Civil Defence — National Emergency Management Agency (NEMA). New Zealand's official preparedness guidance. WeatherNZ reports data and does not give preparedness or safety advice.
- El Niño/Southern Oscillation (ENSO) updates — World Meteorological Organization. Global impact summaries for past events.