New Zealand's unseasonably balmy June owes to warm sea temperatures in the western Pacific. Image / Niwa
On a colour-coded map of sea surface temperatures, the waist of the globe appears as if someone has emptied a pail of orange paint over regions that'd normally be shaded blue.
Meteorologists now say this great blotch of warm water - spanning much of the west Pacific – has helped put New Zealand on track for its balmiest ever start to winter.
Up until Sunday, the nationwide average temperature was 10.97C - well above the long-term average of 8.59C.
More than 80 places across the country – Auckland and Wellington among them - were tracking toward a record or near-record warm month.
With the warmth has come the wet, notably with a slew of low pressure systems packing a tropical punch.
Some examples: the late-May deluge that swamped parts of Canterbury with twice their normal rainfall in the space of days, and wild weather that sent a tornado tearing through South Auckland on Saturday.
More torrential rain is on the cards for the South Island's West Coast on Friday.
The west of the South Island will be living up to its wet reputation over the next several days.
— NIWA Weather (@NiwaWeather) June 23, 2021
Here's a look at our high-resolution two day rainfall forecast.
Impacts include an elevated risk for slips and localised flooding for some areas. pic.twitter.com/tWMu7jRlnG
So where did this extra heat come from, and how does it meddle with our weather thousands of kilometres away?
The biggest reason was the leftover warmth of a now-faded La Nina.
La Nina is an ocean-driven system that's been a feature of our planet's climate for millions of years, and forms part of a cycle of irregular, periodic shift in trade winds, and sea and air temperatures, in the equatorial Pacific.
We mainly know this cycle - called El Nino Southern Oscillation, or ENSO – through its warming phase, El Nino, and its cooling phase, La Nina – both of which have an enormous influence on weather and climate across the globe.
While the last La Nina behaved anything but typically, its stronger tropical trade winds still piled up a vast warm pool of water in the West Pacific.
"We've had this warm pool of water sitting in the western Pacific really since it developed last summer," Noll said.
"When we look at sea surface temperatures, we tend to take the average across the top few hundred metres of ocean, then look at what that tells us.
"Earlier in the year, back in autumn, it peaked at about 2.5C. It's trailed off a little bit now, but we're still talking about ocean temperatures that are above average."
For storm-making potential, that loaded the dice.
Warmer oceans meant more water being evaporated from the sea surface into the atmosphere, where it condensed and formed clouds and rain.
Remnant warm water in the western Pacific, on the back of La Niña, has been a key driver of plumes of tropical moisture down toward New Zealand over the last several weeks 💧
— NIWA Weather (@NiwaWeather) June 21, 2021
There's likely more to come over the next several weeks as the pattern continues. pic.twitter.com/oPAwSRtDYJ
Interestingly, Noll said there'd been a six-to-nine month lag between that heat and moisture building up in the tropics, and being redistributed to lower latitudes.
"Now, finally, when we're in winter, we are seeing some of that being sent down into the southwest Pacific, which is our neck of the woods."
He added that models predicting trade wind patterns – which enabled scientists to forecast whether ocean currents would turn colder or be pushed in a different direction – indicated the warm pool was essentially "locked in" for the time being.
"So we can expect to experience its effects as we go through the next couple of months – and we're talking about a multitude of impacts potentially associated with it."
One of those were low pressure systems, which had been more frequently forming off the east coast of Australia and then wheeling toward New Zealand over the past few months.
"These have been siphoning moisture down from places like Indonesia and Papua New Guinea, before moving into the Tasman Sea and Southwest Pacific," Noll said.
"Not only do they bring that moisture down, they kind of tug that warmth down with it."
Tomorrow and Saturday, for instance, a plume of tropical moisture extending down from southern Indonesia was predicted to bring heavy rain in Fiordland, and could cause flooding and slips.
Normally, these visiting lows wouldn't be anything out of the ordinary.
But when sea temperatures in the southwest Pacific were running anything from 1C to 3C above average, Noll again emphasised, that meant much more ocean heat, and much more moisture.
As lows veered near New Zealand, they also acted as connectors to "atmospheric rivers", which are known to transport more water than the entire Amazon River.
Around 40 of these long slithers of make landfall in New Zealand every year, with around 20 being weak, 10 to 15 being classed as "rank one", or at the lower end of scale, and four or five being strong, predominantly occurring during summer.
But this year, the West Pacific warm pool has had a hand in making these rivers more powerful.
These systems tended to have the greatest impact on the West Coast of the South Island, accounting for much of the heavy rain commonly seen there.
One dramatic case was the March 25, 2019, storm which washed away the Waiho Bridge near Franz Josef on the West Coast.
That event packed the highest ever total of rainfall over a 48-hour period in New Zealand recorded history – reaching 1086mm.
While scientists have known about these giant rivers in the sky for decades, it's only been recently that they've observed shifts in their behaviour.
With a warming climate packing more moisture into the atmosphere, the frequency and magnitude of atmospheric rivers finding their way to New Zealand was expected to increase.
Further, scientists have reported, the locations where they were making landfall appeared to be slowly shifting south.
Under climate change, the planet's oceans have been heating by a rate of 0.06C each decade – but not all of that warmth has been distributed evenly.
In fact, some of the greatest warming over recent decades had occurred in the West Pacific – something that's gradually pulled more cloud-forming winds, moisture, and energy from the Indian Ocean.
The same pattern was also warping the long-term behaviour of a pulse of rain and thunderstorms that circled the globe every 30 to 40 days called the Madden Julian-Oscillation, or MJO.
In certain phases, it could power weather patterns and drive big downpours over New Zealand.
A recent study found the MJO's clouds and rain now spent three to four fewer days over the Indian Ocean - and five to six more days over the Maritime Continent and west Pacific - fuelled by heat and moisture where the warming was greater.
Still, Noll said warmer oceans above us and a fast-heating planet didn't completely explain New Zealand's recent abnormal winter mildness.
Another contributing factor was that our own coastal waters had been running hotter than average – which in turn affected land temperatures and also intensified storms.
Then there was the fact the dominant wind direction hadn't been bringing us the seasonal chill.
"In a winter season, you tend to expect the average flows for New Zealand to be south-westerlies – but the prevailing flow for this month has been north to north-east," he said.
"That's also helped push warm water and air further south, which has contributed to warmer oceans around us."
Finally, Noll pointed to the other major part of the picture: the unusual conditions playing out deep below New Zealand.
Antarctica's polar vortex – that's a large area of low pressure and cold air that sweeps around the frozen continent – was swirling stronger than normal this winter.
If June ended today, it would become NZ's warmest June on record 🌡️
— NIWA Weather (@NiwaWeather) June 23, 2021
Why?
🌊 Warm western Pacific seas fuelling sub-tropical air flows
🌀 Strong polar vortex keeping icy air bottled up near Antarctica
After a warmer weekend, a cold change next week 🥶 — https://t.co/LYbbgvXZy0 pic.twitter.com/jxFHR8YfEG
"It's proven even more effective than it otherwise would have been at keeping all that cold air locked up over the Antarctic region and the Southern Ocean," he said.
"While the polar jet stream can sometimes come up and touch us during the winter, so far, it's been pretty far suppressed in the Southern Ocean.
"Ultimately, the combination of that, and the warmer ocean above us, has proven the kiss of death for a traditional start to winter here in New Zealand."
At least next week, Kiwis pining for their traditional winter might have a taste of it.
A surge of freezing air could approach the country from next Monday or Tuesday, bringing a chance of snow to low levels and icy, strong southerly winds.
But after that: more unseasonably warm days.
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