New Zealand's coastal waters have once again warmed to reach "marine heatwave" conditions – raising the chances of another major ocean event and scorching days this summer.
Niwa meteorologist Ben Noll said water temperatures have ranged from 1.1C to 1.4C above average for November, with daily sea surface temperatures more than 3C above average around the western and northern North Island and eastern South Island over the last week.
Marine heatwave conditions, classified when the sea temperature is above the 90th percentile for at least five days, have been observed in waters offshore of all regions of New Zealand.
That was comparable to the conditions observed in November 2017, which marked the beginning of a freakish and unprecedented marine heatwave around the country and in the Tasman Sea – and helped precipitate New Zealand's hottest ever summer.
Over that period, air temperatures over the country reached between 1.7C and 2.1C above average, while sea surfaces heated up to between 1.2C and 1.9C above average.
"Frequent patterns of high pressure near and south of New Zealand during November, owing to La Niña, have caused more frequent sub-tropical, northeasterly winds than normal," Noll said.
"Reduced wind speeds through the month have prevented colder, subsurface ocean waters from getting mixed up to the surface.
"In addition, sunshine has generally been above normal, which has helped to heat the ocean surface."
Marine heatwaves are becoming increasingly frequent in a warmer climate, with 963 marine heatwave days observed in the New Zealand region between 2010 and 2019 compared with 366 between 2000 and 2009.
Scientists also warn that marine heatwaves will grow longer and more intense under climate change.
And while balmier seas might be good for beachgoers, marine heatwaves have also proven to have severe impacts on ocean ecosystems and our industries that rely on them.
In the 2017-18 event, glaciers melted as some pockets of ocean off the South Island's west coast warmed to 6C above average, while elsewhere, mussel beds suffered cascading losses and vineyards saw early harvests.
That marine heatwave itself was described by scientists as an unlikely event, even by mid-range estimates for 2050.
"Niwa's Seasonal Climate Outlook for summer will be released tomorrow, providing commentary on the likelihood of warm seas continuing through the season and their impact on the summer ahead," Noll said.
Already, Niwa was picking 2021 to be among New Zealand's five hottest years on record.
The return of La Niña conditions meant Kiwis could expect warmth everywhere this summer – but varying weather patterns, depending on location.
Unlike El Niño, La Niña drove more rain to the northeast and drier conditions to the south and southeast of the South Island.
That wasn't however the trend that played out over last summer's La Niña.
According to Niwa's recent outlook, out to January, rainfall was also most likely to be below normal in the west of the South Island, near normal in the north and east of the North Island and about equally likely to be near normal or below normal in all other regions.
Climate scientist Professor Jim Salinger said La Niña meant Kiwis could count on widespread heat over the next few months.
"We can expect it to be dry in the south and west – and if it's wet, it's going to be wet in the north and east."
As well, the balmy conditions could spell more bad news for New Zealand's shrinking glaciers.
"Although there's been plentiful seasonal snow, especially around the southern lakes area, that will melt very quickly – and then we'll see the melting eat into the permanent snow and ice later in summer," he said.
"We also might start seeing interesting things happening in the marine environment.
"Marine heatwaves can mean you get die-offs of certain seaweed."
Hotter sea temperatures also had major implications for other species like corals and fish populations.
"In 2017-18, we saw Queensland groper popping up in the Bay of Islands - some 3000km out of their normal range – and also had snapper around Fiordland," Salinger said.
"If you're getting species where they're not supposed to be, that can really upset the fishing quota."