Another marine heatwave is forming around parts of New Zealand after sea surface temperatures warmed considerably last month, Niwa forecasters say.
Meteorologist Ben Noll said the warmest region was the north of the North Island, where ocean temperatures were 1.6C above the November monthly average.
In this region, marine heatwave conditions were affecting Northland and northern Auckland's coastal waters, and extended west into the Tasman Sea.
In the east and west of the South Island, temperatures were 1.1C above average, he said.
"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.
"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."
Noll said New Zealand was now in a La Nina climate regime which tended to bring more northerly winds and had historically been associated with warm Tasman Sea temperatures.
A marine heatwave is an extended period of extremely warm ocean temperatures that can extend up to thousands of kilometres.
These temperatures must be above the 90th percentile — or the value above which 90 per cent of historical observations occur.
"In other words, nine times out of 10, Northland's sea temperatures are cooler than what they are right now for the time of year," he said.
"This makes what we are seeing now quite unusual.
"The actual temperatures are between 18C and 21C.
"We're not even at the peak of our sea surface temperatures which typically occur over January and February."
He said a marine heatwave this summer would be the third in four years.
The most severe was in 2017-18 when the sea was 6C to 7C above average, at times.
A key climate driver and contributor to New Zealand's hot start in 2019 was also the presence of above-average ocean temperatures.
Marine heatwave conditions persisted in the Tasman until March. Warmer than average seas can also provide extra energy for passing storms.
Noll said climate modelling indicated a high likelihood for warmer than average air and sea temperatures over the next three months.