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El Niño & La Niña
How the Pacific oscillation shapes New Zealand’s weather · South Island focus
El Niño Phase
El Niño
Equatorial Pacific SST anomaly: +0.5°C to +3°C above average · SOI: strongly negative
North Island
☀️
Drier & warmer than average, especially eastern NI
💨
Stronger westerlies and southwesterlies
🌊
East coast: lower-than-average rainfall
South Island
🌧
Wetter on west coast; enhanced precipitation over Alps
🌬
More frequent and stronger southwesterlies into Otago
❄️
More snowfall events on the main divide
Otago & Toko Mouth during El Niño
↑
More rain
Annual rainfall
↓
Cooler
Summer temps
SW
Dominant
Prevailing wind
↑
More likely
River floods
During El Niño, Otago typically receives more frequent southwesterly flow. For Toko Mouth this means a higher chance of winter and spring rain events, elevated river levels, and cooler-than-average summers. The swell tends to be more energetic with more regular Southern Ocean fetch.
La Niña Phase
La Niña
Equatorial Pacific SST anomaly: −0.5°C to −3°C below average · SOI: strongly positive
North Island
🌧
Wetter than average; northeast flow enhances rainfall
🌡
Warmer temperatures, especially summer
🌀
Increased risk of tropical cyclone remnants reaching NZ
South Island
☀️
Drier overall; more frequent northerlies and NW flow
🌡
Warmer and sunnier on east coast
🔥
Elevated fire risk in dry east coast regions
Otago & Toko Mouth during La Niña
↓
Drier
Annual rainfall
↑
Warmer
Summer temps
NW
Dominant
Prevailing wind
↑
More likely
Drought risk
La Niña years bring Otago its warmest and driest summers. The more frequent northwesterly flow means warm föhn conditions are common, and the Tokomairiro River can run at very low levels by late summer. Paradoxically, tropical cyclone remnants occasionally reach the top of the South Island during La Niña.
Neutral Phase
Neutral / ENSO-neutral
Equatorial Pacific SST anomaly: within ±0.5°C of average · SOI: near zero
North Island
🌤
Variable — near-average rainfall and temperatures
📊
Patterns driven more by local synoptics than ENSO
South Island
🌤
Near-average conditions; normal seasonal patterns dominate
🌬
Westerlies remain the dominant synoptic influence
Otago & Toko Mouth in Neutral Years
Neutral ENSO years produce Otago’s most “typical” weather — the climate averages you see in records. For Toko Mouth this means a relatively even spread of nor’westers, fronts, southerlies, and settled anticyclonic spells. About a third of years are neutral, and they’re often the most pleasant for the region.
Southern Oscillation Index — Illustrative Pattern
■ El Niño (negative SOI)
■ Neutral
■ La Niña (positive SOI)
