After three warm, wet La Niña summers in a row, El Niño is coming. The climate pattern is well known for its effect on New Zealand’s weather, but what will it mean for the sea?

Marine heatwaves have plagued our oceans since 2020. In some places, sea temperatures spiked by five degrees. Tens of millions of sponges bleached in Fiordland and 1300 tonnes of farmed salmon died in the Marlborough Sounds. This July, winter sea temperatures around Rakiura Stewart Island remained more than four degrees above average.

El Niño may bring some respite, says NIWA oceanographer Erik Behrens. Whereas La Niña winds tend northeasterly, shuttling warm air down from the tropics and heating the surface of the ocean, El Niño usually brings strong westerlies and southwesterlies. Colder winds over warm oceans make the water column unstable, mixing cool water from underneath with that on the surface, dissipating built-up heat. “We will still see heatwaves, but they shouldn’t be as intense,” says Behrens.

Still, southerly winds battered the South Island this winter, yet the marine heatwaves remained. “You’d expect a few good southerlies to stand up the oceans and make [the heatwaves] vanish,” says Behrens. That they didn’t suggests the ocean is overwarm all the way down, he says—holding a “memory” of three scorching years.