New Zealand marine heatwave looms following warmest winter on record

Climate scientists are warning New Zealand is on the brink of a marine heatwave after recording temperatures nearly two degrees above average in coastal waters off the North Island.

Forecasters at the National Institute of Water and Atmospheric Research (NIWA) said a marine heatwave is building, with above-average temperatures reported throughout October, following a record warm winter.

A marine heatwave is defined as an extended period of extremely warm ocean temperatures, above the 90th percentile, that can extend up to thousands of kilometres.

The warmest seas are north of the North Island, where ocean temperatures are 1.6 degrees Celsius above the November monthly average. Sea temperatures off the coast of Northland are between 18 and 21ᵒC.

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NIWA meteorologist Ben Noll said temperatures in other parts of the country were between 0.7 and 1.1 degrees above average.

“Nine times out of 10, Northland’s sea temperatures are cooler than what they are right now for the time of year. This makes what we are seeing now quite unusual,” Noll said. “We’re not even at the peak of our sea surface temperatures which typically occur over January and February.”

The news comes after climate experts in New Zealand gave their predications for the coming summer, which they said would be long, hot and dry, with an increased risk of bushfires and drought.

“High pressure systems in October brought more sunshine, warmer temperatures and less wind than normal. This pattern led to warming of the sea surface and prevented cooler water underneath from mixing to the top,” said Noll.

New Zealand is in the midst of a La Niña year, which is often linked to warmer sea temperatures on both sides of the Tasman.

In 2018, New Zealand experienced its hottest summer on record, largely propelled by a marine heatwave that saw sea temperatures rise by as much as six degrees in some areas. The average ocean temperature during January that year was 20.3C – more than three degrees above normal.

The warm seas led to a significant influx of fish and marine life usually found only in tropical climates, including the Queensland groper, whose usual home is the Great Barrier Reef.

Other marine life not usually present in New Zealand waters that have been increasingly noted by marine observers include kingfish in Dunedin Harbour, garden eels in the Kermadec Islands (1,000km north of New Zealand), sergeant major damselfish, striated frog fish and Lord Howe Moray in Northland and lion’s mane jellyfish in Wellington Harbour.

The 2018 marine heatwave also led to a boom in land-based animals, including an explosion in the rodent population.

Warmer seas provide extra energy for passing storms, Niwa said, in addition to fuelling warmer temperatures on land.

A marine heatwave this summer would mark the third in four years in New Zealand.

Warmer than average summer temperatures have been predicted for the next three months, following on from New Zealand’s hottest winter on record, which was 1.14 degrees above average.

Seven of the 10 warmest winters on record in New Zealand have now occurred since 2000. “It just showcases the trajectory that we are on,” Noll said.