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Marine heatwave forming in parts of NZ after increase in sea temperatures, NIWA says

A marine heatwave is forming in parts of New Zealand after the sea surface temperatures increased considerably last month, NIWA forecasters say.

The warmest region is the top of the North Island, with ocean temperatures 1.6 degrees above the monthly average for November. Source: istock.com

The warmest region is the top of the North Island, with meteorologist Ben Noll saying ocean temperatures are 1.6 degrees above the monthly average for November.

The conditions extend west into the Tasman Sea and affect Northland and northern Auckland’s coastal waters.

The east coast of both the North and South Island, as well as west of the south is 1.1 degrees above average. The North Island’s west coast is 0.8 degrees above average, and the north of the South Island is 0.7 degrees above average.

Surf Lifesaving New Zealand national lifesaving manager Andy Kent said warm weather and warmer ocean temperatures could see more people paying a visit to their local beach - but they should re-acquaint themselves with important safety messages as well.

"Choose a lifeguarded beach and swim between the red and yellow flags – this is the safest place to swim," Kent said.

"We aim to start patrolling beaches as the weather heats up in different parts of the country - some Surf Life Saving clubs, such as those in Auckland and parts of the Coromandel and Bay of Plenty, start patrolling as early as Labour weekend, with the rest rolling out patrols as the weather heats up."

Aotearoa is now in La Niña which tends to bring more northerly winds and has historically been associated with warm temperatures in the Tasman Sea.

"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,” Noll said.

NIWA graphic of the warm seas around New Zealand.

“While the weather has been a bit more unsettled during early November, ocean temperatures have remained warmer than average due to prevailing northeasterly and northwesterly air flows.”

NIWA defines a marine heatwave as being an extended period of ocean temperatures being above the 90th percentile, that can extend up to thousands of kilometres.

A marine heatwave this summer would be the third in four years. The most severe was in 2017/18, when the sea was at times six to seven degrees above average.

Warmer than average seas can also provide extra energy for storms.

Mr Noll said climate modelling indicates a high likelihood for warmer than average air and sea temperatures over the next three months.

“While the next week will have some variability across New Zealand, air temperatures are most likely to lean in a warmer direction overall. Some particularly warm temperatures are possible to end the month,” he said.

SURF LIFESAVING NEW ZEALAND TIPS FOR SWIMMERS

- Always swim between the flags.

- Ask a lifeguard for advice on the beach.

- Know your limits and don't overestimate your swimming ability.

- Keep kids within arm's reach.

- Never swim, surf or fish alone - always with friends.

- If caught in a rip, relax, raise your hand for help, and ride the rip.

- If in doubt, stay out of the water.

- If no lifeguards are present and someone needs help, call police on 111.

- Don't use inflatable pool toys at the beach.