Climate outlook: hot spells, marine heatwaves possible

Forecasters are picking the potential for more "marine heatwaves" and unseasonably warm conditions later this year, which is tracking among the country's hottest ever seen. Photo / Sylvie Whinray

Forecasters are picking the potential for more "marine heatwaves" and unseasonably warm conditions later this year, which is tracking among the country's hottest ever seen.

Niwa's climate outlook for the last three months of 2021 predicted temperatures would "very likely" by above average across New Zealand.

Niwa forecaster Ben Noll said it was likely the country would see spells of "excessive heat" before the end of the year - something driven by more northwesterly winds and increased sea surface temperatures.

"We've got ocean temperatures right now sitting at 0.4C to 1C above average - and these are forecast to become warmer as work toward November and December," Noll said.

"This will set the stage for spells of warmer weather as we go through the rest of the calendar year, and will obviously have implications for how 2021 stacks up in the records."

Going by the year to date - 2021 has already seen the country's warmest winter on the books, after the previous record was set just last year - Noll expected it to finish within our five hottest.

Last year was the country's seventh warmest in around 110 years of temperature data, while six of the eight warmest years have all occurred within the past decade - a clear sign of climate change's heating influence.

Noll said it was also possible another marine heatwave could develop around the country - a phenomenon that's been observed in our coastal waters over recent summers.

"More winds coming from the northerly quarter and less coming from the south, more high pressure and sunshine and less mixing in the ocean are all factors that can lead to warmer sea conditions," he said.

"Nearly every year now, we are having at least a period of marine heatwave days - so this is something that we're definitely watching."

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.

Niwa is also closely watching the potential formation of another La Niña climate system, which could also influence weather over summer.

Oct-Dec climate outlook 🔮

💧 Soil moisture: currently wetter than this time last year

🌦️ Rain: early Oct looks unsettled, but possibly tending drier as the season unfolds

🌡️ Temps: northeasterly winds draw down warmth & humidity from the sub-tropicshttps://t.co/0S6Y7udETS pic.twitter.com/vzA8NZXbUB

— NIWA Weather (@NiwaWeather) September 30, 2021

In New Zealand, the ocean-driven phenomenon traditionally brings warmth everywhere, but 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 - and Noll said a second one could also come with an atypical flavour.

Between now and January, at least, rainfall was most likely to be near normal in the north of both islands, and about equally likely to be near normal or below normal in all other regions.

In the shorter term, he said Kiwis could expect more classic spring variability in our weather, with periods of rain and possible thunderstorms.

"September was quite a variable month, weather-wise, and we'll see that theme continue into early October," he said.

"While that will bring some rain, it'll also mean warmth - and we could be looking at temperatures reaching the low to mid-20s through a good chunk of next week, especially in the North Island, but in the south as well.

"We'll also be feeling elevated humidity at times as well. So what might happen in early October is maybe a case of what will the broader season will feel like."