Aucklanders can worry that plenty of hellishly humid days could be on the way this summer, thanks to the strongest La Nina system in nearly a decade. Photo / Doug Sherring
Aucklanders can worry that plenty of hellishly humid days could be on the way this summer, thanks to the strongest La Nina system in nearly a decade. Photo / Doug Sherring
Plenty of hellishly humid days could be on the way for Auckland this summer, thanks to the strongest La Nina system in nearly a decade - but will the added element of climate change make it feel even worse?
Scientists say that this La Nina will feel much muggier thanwould have been the case without climate change.
Niwa scientist Dr Sam Dean said there should be a difference between this system and the last comparable La Nina that played with our weather - back in 2011-12 - because of the warming that's happened since.
"New Zealand has warmed about a quarter of a degree over the last decade as climate change has rapidly accelerated around the globe, and that warming means that any air that arrives from the sub-tropics is both warmer and carrying more moisture, conditions ripe for muggy weather and extreme flooding."
Niwa has confirmed the arrival to the Pacific of La Nina - a naturally occurring phenonemon that, like its counterpart El Nino, has been a feature of our planet for millions of years.
Caused by a build-up of cooler-than-normal waters in the tropical Pacific, it traditionally brought warmth everywhere in New Zealand over summer - but with stark differences in regional weather patterns.
As more northeasterly winds arrived - as they were predicted to around December and January - rainy weather became a pattern over the North Island's northeast, while drier conditions tended to dominate the south and southeast of the South Island.
What does La Niña mean for New Zealand during summer?
If you have 1️⃣ minute, we'll tell you!
Here's what this climate driver has historically meant to Aotearoa in terms of:
Thanks to the seas surrounding it, and the lack of any mountains, Auckland was already well known for its muggy weather.
Meteorologists typically describe this in two different measures - relative humidity and dew point temperature.
Dew point is preferred for to describing how muggy the weather will feel - though relative humidity is important part of that calculation.
Relative humidity is the ratio of the amount of water vapour present in the air, to the greatest amount possible at the same temperature.
Thanks to the seas surrounding it, and the lack of any mountains, Auckland is already well known for its muggy weather. Photo / Greg Bowker
Dew point, meanwhile, is the temperature to which air must be cooled to become saturated with water vapour.
We really began feeling clammy when the air was both warm enough to make us sweat - but carried enough water vapour to interfere with that process.
The La Nina summer of 2011-12 saw relative humidity average 82 per cent in December, 79 per cent in January and 80 per cent in February - levels slightly higher than normal - amid plenty of rain and so-called "dirty highs" of cloud and heat.
Many Aucklanders would have more unpleasant memories of the mugginess that came with New Zealand's record-hot summer of 2017-18, which was partly powered by a weak La Nina.
That February, Auckland's dew point temperatures failed to drop below 19C for five days, making for 115 hours of high humidity.
Halfway through the month, it reached a sticky 22C - anything higher than 18C is deemed uncomfortable in the city, which typically averaged 15C for dew point over February.
"What happens in La Nina years is basically the gate to the tropics is a little more open - a lot of our heaviest rainfall events happen during La Nina and that extra moisture in the air from climate change means we expect the dew point to increase - making it feel muggier."
However, the effect of climate change on relative humidity wasn't so straight forward.
"Relative humidity doesn't change much, because the moisture saturation point goes up at about the same rate as the temperature, so the ratio between the two stays roughly constant."
Northern spots like Whangārei are likely to see high rainfall events this summer, as La Nina leaves the gate to the subtropics open a little wider. Photo / Michael Cunningham
What happened when we added La Nina to the mix?
"We know that climate change makes places in the north a little more subtropical, and La Ninas are very good at exposing you to the subtropics," he said.
"So you'll be feeling that. There's an increased risk of it being warmer, the temperature should be higher than it was 10 years ago, with more moisture in the air.
"You will should see an increase in the dew point temperature, because of the increased temperature and moisture - and it will make this La Nina feel muggier and more subtropical than it would have 10 years ago. Even though relative humidity should be pretty much the same."
Unfortunately for Auckland's water storage woes, the general climate change pattern for the north of the North Island was predicted to be one of warming and drying, Victoria University climate scientist Professor James Renwick said.
"The northern part of New Zealand, from Auckland northwards let's say, is going to be more influenced by the subtropical high pressure belt, which is a region of very high pressures, very little rain, and temperatures basically like a lot of Australia," he said.
"With high pressure systems moving more poleward, more often in the future, these northern parts of the country will be under this influence.
"That's why, overall, rainfall is expected to decrease in Northland over time. But having said that, when you get a La Nina in future, or some kind of subtropical disturbance heading for the North Island, then it would be wetter and muggier.
"Because, even though that part of the country is expected to dry out on average, there'd still be an increase in the amount of moisture in the air during these events."
What was less clear, he said, was how climate change might influence the natural cycle that produced La Nina and El Nino events.
While scientists believe their effects are becoming more amplified, there's no consensus on whether climate change is shifting their occurrence or frequency.
