Even after the wettest winter on record, forecasters say dryness is looking increasingly likely for many regions this summer. Photo / Andrew Warner
Even after the wettest winter on record, forecasters say dryness is looking increasingly likely for many regions this summer. Photo / Andrew Warner
Even after the wettest winter on record, forecasters say dryness is looking increasingly likely for many regions this summer. Jamie Morton explains.
Why are forecasters looking out for drying?
Take a drive across New Zealand right now and you'll see scant sign of dryness – and our national drought indexshows the only dry conditions are limited to the south-west tip of the country.
For most of Aotearoa, soil moisture levels are running at either healthy levels, or with excess water in the ground, after a slew of deluges that contributed to our wettest winter in about 50 years of records.
But with La Niña beginning to play a more dominant role in our climate – and the past fortnight marking a transition to warmer temperatures that have been so far elusive over spring – that picture's been changing.
Niwa monitoring showed soil moisture levels were now leaning toward the drier side in large parts of Canterbury and Central Otago, along with areas in Marlborough.
"We've been watching drier conditions developing across the South Island – and it's the same story in the lower North Island," Niwa meteorologist Ben Noll said.
"This has come with more frequent easterly and north-easterly winds and high-pressure systems that are consistent with a La Niña signal that we do expect to continue for the next three months."
What's the outlook for summer?
Noll said current modelling pointed to elevated odds for dryness right throughout the South Island and lower North Island over summer – with strong potential for similar conditions elsewhere.
"In general, I'd say that, for perhaps all regions except for perhaps the eastern North Island, we are signalling some elevation in the potential for drier-than-normal conditions," he said.
"You're talking about a big chunk of the country that should have their antennas up."
🌡️ Nov 2022-Jan 2023 climate outlook: heating up! 📈
🌴 Above average temps (air & sea), humidity favoured
🚜 Decreasing soil moisture, elevated odds for dryness in the south, parts of the north
The past two summers – which also happened to coincide with forming La Niña systems – also brought some dryness to northern regions including Waikato, Northland and Auckland.
"With these places, it's always a tough call with La Niña, as this can come with tropical plumes of moisture moving down the country," he said.
"Oftentimes, the northern North Island is the first place to see these systems: but if they miss, then that dry pattern gets reinforced."
At the end of summer 2022, soil moisture levels in most of Northland, Auckland and northern Waikato finished up below average – a trend notably out of step with La Niña's historic tendency to bring more rain to the north and east, and dryness to the south and west.
In Auckland, particularly, a 37-day dry spell that stretched through the holiday period and beyond proved the second-longest dry spell since records began in 1943.
Noll added there was always potential for weather over the next three months to break from typical La Niña patterns, in periods of what was called intra-season variability.
"With a brief change to westerly winds in early November, we may see one of those periods later next week, with the potential for quite a heavy rain event in the western South Island."
Only the southern tip of the South Island is currently registering on the NZ Drought Index. Image / Niwa
How severe could the dryness get?
"We're seeing soil moisture deficits now of as much as 30mm in parts of the South Island – it's certainly not looking all that wet for eastern Otago and Canterbury – and we can expect those values to further decline in the next week or so with forecast warm temperatures."
That said, it was still too early to start talking drought.
"Yet, if mid-November comes around and those particular areas are showing even stronger moisture deficits – and keep in mind a La Niña-like high is returning for the second week of November – then there could definitely be some farmers starting to get concerned," he said.
"At this time of year, it only takes one or two weeks of dry conditions and you're off to the races."
Federated Farmers adverse events spokesperson Wayne Langford said he expected Canterbury farmers would be making supplementary feed after what was a healthy winter season or rain.
"The likes of the West Coast and Nelson also had some significant rain and flooding in August but are now running a bit drier than normal, so they'll be hoping for a bit of rain over the next while," said Langford, who runs a Golden Bay dairy farm.
"It's not normally an issue for us going dry at this time of year, but we'll be keeping an eye on it for sure."
Most of last summer's dryness played out across January, when a plethora of locations were forced to introduce water restrictions – and a meteorological drought was confirmed in western Northland and small pockets of Waikato.
The summer before it was even more uncharacteristic of La Niña - with eastern areas like Hawke's Bay, Bay of Plenty and Wairarapa all recording well-below rainfall – but nonetheless warm.
Source / Niwa
"This time around, I think it's more likely than not that we'll see similar extended periods of heat throughout the country, as we did last summer," Noll said.
That probability was also increasing with sea temperatures around New Zealand likely to be running unusually warm over summer – with widespread marine heatwave conditions on the cards.
What does this mean for fire danger?
Noll said plenty of growth over spring followed by a big dry-out and warm temperatures posed an obvious risk for wildfire danger – but fewer northwesterly winds might help lower that threat.
Fire and Emergency NZ wildfire manager Tim Mitchell said it was important for Kiwis to plan ahead as we headed toward the hotter, drier months.
"As a result of a very wet winter, and the recent return of warming conditions, grass is growing at a faster rate."
Property owners were encouraged to mow and graze grass as much as possible before it dried and became a fire hazard later in the season.
"It helps to defend your home and property from wildfires by keeping grass short and clearing dead leaves and debris from gutters and around decks."
The agency's outlook predicted "increasing availability" of medium, heavy and subsurface fuels as they dried out from November onward.
"The moisture of these fuels has less of an impact on fire spread rates, but as they dry the fuel availability increases resulting in greater fire intensity, making suppression more difficult."
The outlook indicated the risk could be potentially higher in western parts of the South Island – including Westland, Fiordland and Southland – but more humidity, easterly winds and rain would likely limit the danger around Marlborough and Canterbury.
"There will however still be periods when the westerly flows return and, especially if they are strong and not accompanied by precipitation, they are likely to result in spikes in the fire danger, including in the east," the outlook said.
“It should also be noted that although eastern parts are not expected to have higher than normal fire danger, their usual summer fire dangers are still relatively high.”
