Very strong El Niño raises weather and coastal questions for New Zealand

New Zealand faces a very strong El Niño in the tropical Pacific, with forecasters expecting it to intensify later in 2026. Earth Sciences New Zealand’s October–December outlook points to stronger westerly winds and contrasting rainfall patterns across the country. Victoria University of Wellington researchers said on 8 October that the effects on coastal waters and marine life are much less certain.

The World Meteorological Organization says the event is firmly established and strengthening rapidly. It forecasts an October–December sea-surface temperature anomaly of about 3.7°C in the central Pacific’s Niño 3.4 region, with a peak around December. That figure is a forecast, not a temperature already observed.

What New Zealand’s El Niño outlook says

Earth Sciences New Zealand says very strong conditions were already present when it issued its outlook on 1 October. It expects more frequent westerly airflows, stronger gusts and more fronts in western and southern areas. Across most of the country, temperatures are more likely to be above average during October–December, although swings between warm and cold spells may be greater than usual.

The rainfall outlook varies sharply by region. Northland, Auckland, Waikato and Bay of Plenty have a 45% chance of below-normal rainfall for the three-month period. For Gisborne, Hawke’s Bay and Wairarapa, that probability is 55%. Parts of the North Island already had below-normal soil moisture and river flows, and further dry spells could challenge sectors that depend on water.

Earth Sciences New Zealand rates its confidence in both temperature and rainfall forecasts as medium. It says atmospheric influences, including a positive Southern Annular Mode, could partly counter the circulation normally associated with El Niño. Its forecast paths also differ on when the event will peak and how quickly it will fade.

Why a strong Pacific event matters for coastal waters

El Niño begins with changes in Pacific atmospheric pressure and trade winds. Weaker or reversed easterly winds reduce the usual rise of cold, nutrient-rich water off western South America, while warmer surface water accumulates farther east. The ocean response around New Zealand is less straightforward than the broad Pacific signal, according to the Victoria University researchers.

Their account says El Niño commonly brings slightly cooler-than-average sea-surface temperatures around New Zealand. That could give heat-stressed marine ecosystems temporary relief. Persistent westerlies could also increase upwelling along the west coast, bringing nutrients that support phytoplankton and kelp and, in turn, species that rely on them for food or habitat.

The same weather pattern could carry a cost. Heavier western rainfall may send more freshwater and sediment into coastal waters, particularly from heavily logged or farmed catchments. The researchers say sediment can reduce light and oxygen, cover bottom-dwelling animals and make it harder for filter feeders such as cockles, mussels and sponges to feed. These are possible pathways, not measured damage from this El Niño.

What past marine heatwaves can and cannot tell us

Cooler coastal water is not guaranteed. The university researchers note that a marine heatwave can coincide with El Niño: a Tasman Sea heatwave during the 2015–16 event lasted 251 days and reached 2.9°C above normal. They link past heatwaves with salmon deaths and damage to kelp and sponges around New Zealand. Those historical losses do not establish that similar harm is occurring now or will occur this summer.

The scale of the developing Pacific event is unusually large. Earth Sciences New Zealand reports that its 30-day Relative Oceanic Niño Index reached +2.14°C through 28 September, having crossed its +2°C very-strong threshold on 19 September. The WMO says its forecast peak would exceed the previous Niño 3.4 anomaly record of +2.6°C from November 2015 to January 2016. Neither measure directly predicts a particular outcome for a New Zealand coast or fishery.

What remains uncertain for New Zealand’s coast

The university researchers say the event’s precise influence on regional currents and lingering ocean heat remains unknown, as do possible effects farther north of New Zealand. They call for data collection and adaptive management informed by mātauranga Māori. For now, the established development is a strengthening El Niño and a seasonal weather outlook; the ecological consequences depend on how local ocean and weather conditions unfold.