Westerly flows have returned to New Zealand – and are now at their most prominent since last June. Photo / NZME
Westerly flows have returned to New Zealand – and are now at their most prominent since last June. Photo / NZME
Noticed a sharp change in the weather lately? Westerly flows have returned to New Zealand – and are now at their most prominent since last June. Science reporter Jamie Morton explains.
What are westerlies and how do we experience them?
As you’d expect with an archipelago divided by mountain rangesand lying between the subtropics and Antarctica, New Zealand’s climate is immensely complex.
Yet, for much of the last few years, we’ve been missing the prevailing pattern that shapes a great deal of our day-to-day weather.
Now, our familiar old westerly winds are back, and blowing at a frequency not seen for more than a year.
“If you were to look at average wind for New Zealand over a whole year, you’d find it’s slightly south of west,” Niwa meteorologist Ben Noll explained.
“So, a westerly wind is actually pretty typical – or what we might consider average here.”
Being in the mid-latitude zone of these winds regularly puts us in the path of anticyclones, which generally track across the North Island and follow more northerly paths in spring and southerly paths in autumn.
Between these lie troughs of low pressure, often packing cold fronts oriented northwest to southeast.
Over what’s been an often-showery school holidays, many Kiwis would have experienced a classic effect of this weather set-up.
That’s fronts coming from the west, bringing stronger wind and more cloud, followed by hours-long periods of rain, before a change to cold, showery, southwesterly winds.
“Wind direction ultimately influences the types of weather patterns that we experience, whether it’s warm, cool, wet, dry or humid. Over the past year and before, we’ve grown accustomed to northerly and easterly winds,” Noll said.
“That’s meant that our weather has been routinely influenced by the tropics and subtropics, and hence, many regions have been warm and moist.”
Northerly and easterly winds happened to be major features of New Zealand’s last three record-warm winters – and also helped to ferry down rain-maker after rain-maker over our extreme summer.
The big culprit behind this three-year run of atypical Kiwi weather was, of course, La Niña - and it’s now being replaced by its climate driver counterpart El Niño.
“During El Niño, we get high pressure north of New Zealand and low pressure south of the country, and we can think of these two things like cogs,” Noll said.
“We have an anti-clockwise flow around the high pressure and a clockwise flow around the low pressure – and these two cogs work together to create the westerly winds that sweep over us.”
Do more westerlies spell a miserable few months ahead?
“That depends on where you live in the country – and it depends on what kind of weather you like,” Noll said.
“I guess it just brings us back to a climate state that people would associate with New Zealand: think the West Coast of the South Island getting frequent fronts and rain bands.
Niwa meteorologist Ben Noll. Photo / Michael Craig
“We can also expect dryness to become more of a theme in the east, in regions like Canterbury, eastern Marlborough, Hawke’s Bay and Gisborne, as the air parcels within those westerly winds dry out as they move across the Southern Alps and the Central Plateau.”
For people in the upper and eastern North Island, especially, more westerlies would spell a welcome change from relentless subtropical lows and atmospheric rivers, Noll said.
“That’s certainly been the pattern over the last year.”
In losing La Niña’s wetness, we could also expect to lose its winter warmth.
While last month proved New Zealand’s fifth-hottest June on the books – something partly down to a La Niña lag effect and stubbornly warm local sea temperatures - El Niño was already bringing us back closer to winter’s traditional chill.
Niwa’s outlook for the next three months picked average or above-average temperatures as being most likely – again, warmer seas were taking much of the bite out of coming cold fronts - but Noll said the anomalies wouldn’t be so far from baseline as the last three seasons.
Most of New Zealand could expect normal or below rainfall through to September, while the South Island’s West Coast could see normal or above normal levels.
Will these westerlies only get stronger?
“If El Niño continues to strengthen, which we think it will as we approach summer, then the westerly pattern will continue to be enhanced,” Noll said.
“And when you combine the increase not just in the speed and frequency of winds coming from the west, but a seasonal cycle that brings more sunshine, hotter temperatures and longer days, that might not be such a good thing over a long period.
“For holidaymakers on the east coast this summer, it might bring long stretches of good weather – but for farmers and growers who rely on regular rainfall, it can become a problem.”
Some of New Zealand’s biggest droughts have played out under El Niño - including a horror event in 1997-98 that cost hundreds of millions of dollars, and major dry spells in 1972/73 and 1982/83.
Some of New Zealand’s biggest droughts have played out under El Niño . Photo / Alan Gibson
While our most recent strong summer El Niño in 2015-16, also brought southwesterly winds, a switch to more northerly-quarter winds later that season eased farmers’ fears of a disastrous repeat.
We also might experience stronger-than-normal westerly winds in late winter, spring, and summer, which could dry out soils, increase the risk for wildfire development and bring occasional hot air masses from Australia.
Yet, with the finer details of the El Niño still taking shape, Noll said it was still too early to say exactly what might play out six months from now.
“We know that El Nino can take on different forms, particularly in the context of a changing climate with warming seas, so what exactly next summer will look like, the climate system has yet to decide for us.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
