Last month, beleaguered Aucklanders were doused with nearly half the rainfall they usually get in a year, or eight-and-a-half times what a typical January would bring.
Further south, soils in rain-starved regions had dried to the point that parts of Otago just entered a meteorological drought.
Following a bout of cooler temperatures and widespread rain this week, Niwa Weather’s Chris Brandolino said most of New Zealand could expect a period of more settled conditions.
But beyond that – around late February and early March – forecasters were predicting the tropics to become more active, boosting the potential for another decent rainfall event.
That was down to what’s called the Madden Julian Oscillation (MJO) – a pulse of rain and thunderstorms that circles the globe every 30 to 60 days – passing above New Zealand.
“The message is this: in the first week or two of March, we’ve got to be watching the tropics to our north and north-west,” Brandolino said.
“That’s going to elevate chances for more organised rainfall, showers and thunderstorms.”
For the risk of another big deluge for New Zealand, Brandolino said the MJO helping tropical moisture pool together was just one half of the equation, as it required a low-pressure system to carry it here.
The historic storm that brought Auckland’s wettest day on January 27 happened to coincide with a visiting MJO – but also a low that formed an “atmospheric river” back to the tropics and a freakish combination of other local factors.
“But this period definitely bears watching.”
As for the possibility of another tropical cyclone forming up and travelling down here?
In the immediate term, that risk was considered low, particularly with Gabrielle having helped churn up, and temporarily cool, the waters where it formed.
MetService is due to publish its next tropical cyclone outlook tomorrow.
Still, as Brandolino pointed out, February and March tended to be the most active period of our November-to-April season – as also highlighted by Bola (March 1988), Fehi and Gita (February 2018) and Dovi (February 2022).
“The fact that we have also have a waning La Niña in the mix adds up to an elevated risk for cyclones for the tropics, certainly through that early autumn period,” he said.
“And we can still have a significant influence from the tropics with an atmospheric river – it’s not strictly tropical cyclone-dependent.”
La Niña - an ocean-driven climate phenomenon much to blame for the dramatic wet-and-dry contrast between New Zealand’s north and south this summer – was predicted to fade out to a “ENSO-neutral” state around mid-year.
It was La Niña’s influence – alongside other key factors like climate change and a negative phase of another regional driver called the Indian Ocean Dipole – that made 2022 New Zealand’s wettest year on the books, not to mention its warmest.
Brandolino explained that, much like a light dimmer, rather than a switch, La Niña wouldn’t exit the stage abruptly, but gradually.
That meant the transition into Enso-neutral conditions – and then likely El Niño later in the year – could come with plenty more warmth and rain for the north and east.
“That pivot isn’t going to be smooth ... it’s likely to be very lumpy.”
Over three years, and against a background of global heating, La Niña had helped pile up warmth across a vast swathe of ocean called the “Western Pacific Warm Pool”, which had repeatedly offered a potent source of moisture for rain-makers that’ve travelled our way.
As well, pressure systems moving about our neighbourhood had been unfortunately positioned by a predominantly positive Southern Annular Mode.
So long as this ocean-atmosphere set-up stayed in place, Niwa climate scientist Dr Sam Dean said the potential for energetic storms coming our way stayed higher.
“Even though we’re starting to transition to the end of summer, sea surface temperatures across our region have only just passed their peak now,” Dean said.
“They’re still basically very warm – and until they’ve cooled down, there’s certainly potential for more events.”
