“These two drivers are working together to lead to this ridge signal that develops closer to Christmas,” Noll said.
“Right now, I’d say there’s at least moderate confidence that this will develop in the lead-up to Christmas, but what I’m less confident about is, how long will it last?”
One scenario, he said, was that high pressure hung over New Zealand from Christmas, right through to the New Year period.
Another was that we received an unwelcome visit, closer to the end of the month, from a system swinging in from the north or north-west.
“The Coral Sea has been generally unsettled with plenty of storm activity, so there’s always a chance that things could go in the other direction for us,” he said.
“Still, we’re now looking at a good chance of a spell of settled conditions.”
It didn’t help forecasters that our third consecutive La Niña summer was continuing to bring its usual variability, with rainy fronts intermingled with summery temperatures.
Traditionally, La Niña has delivered more northeasterly winds that bring rainy conditions to North Island’s northeast, and drier conditions to the south and south-west of the South Island.
Thanks to the north-easterly winds, warmer temperatures also tended to play out over much of the country during La Niña, although there were always regional and seasonal exceptions.
After what was the warmest November on record – and also the wettest for many locations – December began with a slightly cooler flavour.
This followed a marked drop in sea surface temperatures around the country observed on November 25, pushing pause on what’s been two years of marine heatwave conditions.
“We’ve seen colder waters just as summer started, with surface temperatures dropping from marine heatwave conditions around many parts of the country to temperatures nearer normal conditions,” Dr Joao de Souza, of MetOcean Solutions, said.
“The sharp drop in sea surface temperatures was likely caused by the cold, windy weather we’ve seen in many parts of the country over the last couple of weeks.
“This cooled down the surface water, bringing an end to sea surface marine heatwaves.”
However, that dip was likely to be short-lived.
“Although there are no marine heatwaves right now, the sea surface temperatures around the country are still higher than average in many places,” he said.
“With settled weather and some sunshine, the marine heatwaves will likely reappear.”
Noll said Niwa was still observing sea surface temperatures in some areas ranging between 0.5C to 1.5C above average – particularly in and around Wellington and Golden Bay.
Meanwhile, an abundance of moisture over coming days, combing with forecast heat, could make for a sticky week for most regions.
“We’re expecting rounds of showers and thunderstorm activity at times, with air flows pretty much straight out of the tropics,” Noll said.
“This means we could see day-time temperatures of 30C in some places over the next 10 days, while nights are likely to be quite muggy and gooey.
“These conditions are likely to stick around for a week or so and should be a noticeable change from what we’ve experienced so far this summer.”
After New Zealand’s second-warmest opening six months on the books, along with its warmest winter and ninth-warmest spring, 2022 is likely to finish up among our hottest years on the books – and quite possibly another record-breaker.
