A marine heatwave is to blame for hotter-than-normal weather across most of Aotearoa, according to NIWA, and it's only getting stronger.
"It's striking," said meteorologist Ben Noll on Thursday.
He said the heatwave "has caused sea surface temperatures to reach a near-record 23-24C in some coastal areas in late January", with the levels around Auckland almost tropical.
What is a marine heatwave?
A marine heatwave is a period of abnormally high ocean temperatures.
Marine heatwaves can affect the smallest sea creatures up to whole ecosystems, and can influence weather systems causing hot, humid conditions on land.
They also act as a fuel for cyclones, such as Tropical Cyclone Kirrily that is currently barrelling towards Queensland.
Is that why it's been so hot at night?
The marine heatwave is partly why night-time temperatures and humidity have been higher lately across Te Ika a Māui/North Island, said Noll.
It comes as ocean temperatures naturally hit their annual peak.
"It's stacking a 2-3C anomaly on top of the ocean warming we normally see at this time of the year," said Noll.
Swimmers and surfers would've likely noticed warm seas, he added.
The heatwave's strength could rise and fall over the next two weeks, and may intensify again from the second week of February.
The ocean warming in northern parts of the country aren't typical of El Niño, according to physical oceanographer Erik Behrens at NIWA.
"The temperatures around the South Island are more aligned with our expectations, where stronger south westerly winds as part of El Niño cause coastal upwell of cold water, reducing the likelihood for marine heatwaves."
Marine heatwaves 'have become commonplace'
Aotearoa had its first big marine heatwave in the summer of 2017 and 2018.
It caused the loss of up to 2 million Olympic swimming pools' worth of glacier ice from the Southern Alps.
Since then, they have become commonplace for New Zealand, Noll said.
"They seem to generate less and less fanfare, but this one is really standing out."
