27 Nov 2022

NZ’s ‘disjointed December’: When will summer weather arrive?

10:06 pm on 27 November 2022

By Jamie Morton of NZ Herald

A front moves over Nelson, pushing tropical heat ahead of it.

A front moves over Nelson, pushing tropical heat ahead of it (file image). Photo: RNZ / Tracy Neal

New Zealand's slide into summery weather is likely to be somewhat "disjointed", a meteorologist says, with an upcoming fine spell sitting amid an otherwise messy forecast for December.

After a November that's brought a slew of squally fronts - along with some 110,000 local lightning strikes and record monthly rainfall in a handful of spots - Niwa forecaster Ben Noll saw no sign of an instant transition to settled skies.

"I don't think it's going to be a seamless transition, so I'd caution against the idea that December is going to be this beautifully sunny month."

While Kiwis could expect a period of blue skies likely beginning from next weekend and rolling into the week after, Noll said this might not come with typically summery temperatures.

"So, I'd say our start to summer is going to be a bit disjointed," he said.

"While some spots further south might have already started trending drier, for other places more exposed to easterly and north-easterly winds - the likes of Northland and Auckland - it's going to take a little longer to get there.

"Right now, as we go through December, the chances of getting drier than normal weather are higher the further west and south you are in both islands."

That could be explained by several big drivers jostling for control of our "climate steering wheel".

Chief among them was La Niña, an ocean-driven system that had its fingerprints on what was our warmest, wettest winter on record, and which also flavoured our past two summers.

Traditionally, La Niña has delivered more north-easterly 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 are always regional and seasonal exceptions.

One was 2020, when an odd-ball La Niña event delivered a somewhat unexpected flavour - and became among four of 17 La Niña events measured since 1972 that failed to bring near or above normal rainfall for Auckland.

The 2021-22 event behaved closer to script, contributing to a record marine heatwave near the North Island, several close calls with ex-tropical cyclones, and flooding in the east and droughts in the far south.

Another key driver was the far-off ocean-atmosphere phenomenon called the Indian Ocean Dipole (IOD) - and the negative phase it's long been locked in has made for an especially soggy spring across the Tasman.

"From an oceanic perspective, the IOD has been easing pretty quickly - but it comes with a lag," Noll said.

"So, while ocean conditions have changed, the atmosphere has still stuck in IOD-mode throughout November, and that's helped intensify the subtropical jet-stream."

The third main ingredient in the mix was a notorious storm-making system called the Madden Julian Oscillation, or MJO.

Partly responsible for New Zealand's recent heavy downpours and thunderstorms, we could think of the MJO as a pulse of convective activity that intermittently circled the globe, helping to drive deluges our way when it passed near our region.

"And I do think we're going to get another pulse of the MJO at some point before the end of 2022 that'll give us another rainy spell - maybe in the middle part of the December."

Further on, however - and potentially around the New Year period - Noll said it was possible the MJO could actually help influence more settled weather here, through working in tandem with La Niña.

"At the same time, as we get into tropical cyclone season, we'll be keeping our eye out for any systems coming along, given just one of these things can bring one or two months' worth of rain in one fell swoop."

Meanwhile, we'd keep feeling the influence of above-average sea temperatures - something known to both energise visiting storm systems but also add extra warmth and humidity to conditions on land.

And background climate change - a big factor in what may prove another record-warm year - would have its hand-print on more extreme weather events and higher maximum temperatures later in summer.

Current modelling pointed to elevated odds for dryness right throughout the South Island and lower North Island throughout the season as a whole - with strong potential for similar conditions elsewhere.

"There'll be a bit of a lag before we get toward that real heat; perhaps we'll have to wait until the other side of Christmas."

* This story was first published on the NZ Herald

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