Why winter’s late to party
It’s what’s called an El Nino ‘Modoki’ — which is Japanese for ‘similar but different’
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 began on a relatively tranquil 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.
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 oceandriven 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.
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.”
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.
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 light winds and more settled weather. But the past few weeks had seen SAM switch to its negative phase, bringing more westerlies and more unsettled weather.
So what lies ahead?
Despite a sharp cold snap 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.
Rainfall was forecast to be 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.
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.
“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.”