What began as a mild winter took a sharp change this month. Photo / NZ Herald
What began as a mild winter took a sharp change this month. Photo / NZ Herald
Feel like winter's only just arrived?
You wouldn't be wrong – and, beyond those big dark storm clouds that have been looming overhead, much of the reason for the cool change lies way out in the central Pacific Ocean.
Niwa meteorologist Ben Noll said winter 2019 began on a relativelytranquil note, with rainfall levels low enough to worry dam operators in Auckland and farmers in the South Island.
"As we've moved into July, we've seen a gradual increase in the amount of westerly winds, and these have dragged in moisture from the Tasman Sea toward New Zealand."
As would be obvious to anyone in Oamaru, which just recorded its wettest August day on record, with a whopping 78mm of rainfall in 24 hours, that trend had ramped up into this month.
"In terms of activeness, maybe we started with a three or four in June, then moved to a five or six on the scale in July, and now we are looking at about seven or eight."
What's been driving it?
Noll pointed to an El Nino that was weakening now, but which had been influencing the climate system here and around the globe since March.
And as far El Ninos go – the ocean-driven climate system is typically characterised by too much rainfall in wetter, western parts of the country and not enough of it in drier, eastern parts of the country – this had been a strange one.
It's what's called an El Nino "Modoki" – Japanese for "similar but different" – where the warmest waters hadn't been in their usual spot near South America, but further west, in the Central Pacific.
El Niño has impacted New Zealand's climate so far in 2019, but it is likely to become less influential and ease to ENSO-neutral over the next few months 📉 pic.twitter.com/t7AIIahOCD
This had signalled weak to moderate El Nino-like effects, albeit ones that had taken some time for the system to bed in and mess with our weather.
"It took a long time for the ocean and atmosphere to link up and begin communicating with each other, but once they did, we started to see things respond to it."
Climatologists had also predicted this winter's activity to be back-loaded by comparing 2019 with previous "analogue" years that had similar climate set-ups.
"Of course, every individual weather event is unique, but the signal for less dryness has certainly been on the cards as we've headed into late winter."
As the change shifted, so too did trends in the Southern Annular Mode, or SAM – a ring of climate variability that encircled the South Pole.
First identified in the 1970s, it involved alternating changes in windiness and storm activity between the middle latitudes, where New Zealand lies, and higher latitudes, over the southern oceans and Antarctic sea ice zone.
In its positive phase, the SAM was associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans.
It was a predominantly positive SAM that had contributed toward our past two unusually balmy summers, and over much of winter, the scales had been tipped more toward this end.
But the past few weeks had seen SAM switch to its negative phase, bringing more westerlies and more unsettled weather, while windiness and storm activity eased over the southern oceans.
"While I think we can point to the SAM having been negative, I think that is more a consequence of other climate drivers – and I prefer to call the SAM more of a climate indicator."
So what lay ahead?
Despite a sharp cold snap in early this month, seasonal temperatures between now and the last stages of spring were forecast to be near average or above average for all regions, largely owing to warmer than average coastal and regional sea surface temperatures.
Source / Niwa
Rainfall, meanwhile, was forecast to near normal or above normal in the west of the North Island and north of the South Island with near normal rainfall forecast for all remaining regions.
All the while, those El Nino conditions would fade away over the next three months, and its influence on our climate system would gradually decline.
Aside from a possible dry patch between early and mid September, Niwa expected September to be potentially unsettled.
"It doesn't look like a particularly warm September at this point, but I wouldn't go as far as to say that it will be below average," Noll said.
A brief spell of higher than normal pressure for Wednesday and Thursday 👍 🌤️
However, the next 1-2 weeks will have several rounds of unsettled conditions. pic.twitter.com/FMlQ4660vL
"We'll have a few more cool spells than we are used to in early spring, and maybe people will be keeping the heaters on a bit longer."
Noll added the chances of below average temperatures across a whole month had grown less likely under climate change – and July marked New Zealand's month of above average temperatures.
This was against average temperatures between 1981 to 2010 – a baseline that would shift to 1991 to 2020 early next decade.
"All of a sudden, there will kind of be a step down – that doesn't mean it will be less warm, but just that the normal has changed, so we'll be less likely to be well above it."
