Central and lower parts of both island could be in for big bouts of rain over the next three months - partly thanks to a climate driver called the Indian Ocean Dipole. Photo / NZ Herald
Central and lower parts of both island could be in for big bouts of rain over the next three months - partly thanks to a climate driver called the Indian Ocean Dipole. Photo / NZ Herald
Just as a rare and dramatic event high above Antarctica eases its icy influence on our weather, forecasters are warning another climate phenomenon is taking centre stage.
What's called the Indian Ocean Dipole, or IOD, could mean big bouts of rain in some parts of the country, in a lead-up to summer likely to lean on the cooler side.
New Zealand has only just seen the effects of the strongest stratospheric warming event ever observed in the Southern Hemisphere.
At its peak, this natural phenomenon lifted temperatures in the stratosphere above the South Pole - that's 30km to 50km above ground - to an incredible 70C above average.
The result was a disruption to the polar vortex which swirls around Antarctica, normally keeping its cold effectively locked up.
Niwa forecaster Ben Noll said the vortex was displaced toward South America, bringing bitterly cold conditions to Chile and Argentina - and with knock-on effects for New Zealand.
"In weather circles, we use a term called 'teleconnection' - that's where an event being observed some distance away is also having an influence on the weather patterns where you are," he said.
"So what we saw was that displaced cold being teleconnected to us here. What that meant for us was more southerly winds, frequent cold snaps, several rounds of snow and the fourth coldest September since the turn of the century."
September came in at 0.1C below the 30-year average - breaking a streak of more than 30 months above average temperatures - with parts of the South Island like Nelson and Marlborough seeing the biggest shifts.
While the dissipating event wasn't quite done with our weather, Noll expected that any further cold blasts it created would come within the first half of this month.
As it was fading, New Zealand would begin to feel a greater influence from what's being described as one of the strongest IOD events ever.
An oceanic seesaw
The IOD typically had three phases: neutral, positive and negative.
Events usually kicked off around May or June, peaked between August and October and then rapidly decayed when the monsoon arrived in the Southern Hemisphere around the end of spring.
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Learn about one of New Zealand's primary climate drivers for the upcoming season, the Indian Ocean Dipole 👩🔬 👨🔬 pic.twitter.com/6HhaBV0NTB
In a positive phase, westerly winds weakened along the equator, allowing warm water to shift towards Africa.
Changes in the winds also allowed cool water to rise up from the deep ocean in the east.
This ultimately set up a temperature difference across the tropical Indian Ocean, with cooler than normal water in the east and warmer than normal water in the west.
Auckland's skyline, blanketed in spring rain this month. Photo / Michael Craig
Noll described it as a "seesaw" of sea surface temperatures.
"That seesaw creates a dipole that brings high pressure over the cool waters in the east - and low pressure over those warm seas in the west."
In Australia, this had already contributed to the unusual dryness much of the country had been experiencing.
The Indian Ocean Dipole is up to +1.76C above normal. This is the 16th strongest IOD weekly value since the archive began in 1981. In terms of years, 2019 ranks now as the third strongest IOD event in the archive. The strongest +IOD was Oct 1994 (+2.57C), second Nov. 1997 (+2.5C) pic.twitter.com/XdOMYtqHZ2
"And here in New Zealand, it's been linked to what was still quite a dry September for the South Island specifically. We've seen storage in the hydro lakes there decline quite dramatically."
As spring marched on, Noll said we could expect to see the IOD play a more noticeable role, by driving increase in westerly winds.
"What that means is those western areas of the South Island which have been quite dry will likely turn wet as the season unfolds. Obviously, that's not ideal for those folks who are caravaning around the south."
A strongly positive Indian Ocean Dipole will suppress convection across the Maritime Continent over the next few months.
In November wet weather was likely for the central and lower parts of both islands, including Raglan, Hamilton, Rotorua down to Ohakune and northern Taranaki.
The wet trend would continue into December with more rainfall than average in the South Island and the west and upper north of the North Island.
Noll pointed out that a positive IOD observed last year - and weaker than the current system - was linked with an extreme rainfall event that hit Otago.
"That gives you an idea that we may need to be vigilant or on high alert about the strong driver that we have in the climate system now, and how it might influence rainfall and westerlies as we go through the next three months."
The wet didn't necessarily come with warmth: when the IOD was strongly positive, temperatures historically hedged in the cooler direction.
Cooler trend
Added to that was the fact New Zealand's coastal sea temperatures were all sitting at below average - another departure from the past two years' balmy trend.
"It doesn't mean we won't see warm weather at times, but when you have warm seas, often the air temperature tends to paddle in the same direction."
Forecasters say the coming months could be on the wetter side - and there's little in the mix to make things warmer. Photo / Paul Taylor
Several models also indicate the polar region below New Zealand will be characterised by higher than normal pressure over the next three months, allowing for a ring of low pressure to develop around the mid-latitudes.
That meant that, although seasonal temperatures would still climb from October through December, the door to the Southern Ocean might remain ajar, allowing air masses from the south to continually influence our weather.
And out in the equatorial Pacific, oceans were sitting in an ENSO-neutral state - or in neither La Nina or El Nino conditions.
Noll said a subtle hint of the latter over the final months of 2019 - but not the conventional type of El Nino that brought hotter conditions to eastern areas.
Overall, Niwa's latest climate outlook predicted little chance of above-average temperatures anywhere between now and December.
It remained to be seen what the would mean for 2019's hitherto hot run in the record books.
The January to August period had come in at 0.96C above average – second only to the corresponding stretch in 2016, which was 1.10C above average.
