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A disastrous drought and a ferocious flood side by side.
A disastrous drought and a ferocious flood side by side. (Images: left RNZ, right Hawkes Bay Civil Defence)

ScienceFebruary 17, 2023

The wet-dry flip-flop

A disastrous drought and a ferocious flood side by side.
A disastrous drought and a ferocious flood side by side. (Images: left RNZ, right Hawkes Bay Civil Defence)

Three summers ago the north of New Zealand was in drought while Southland suffered floods. This summer, we’ve got the inverse. What’s going on?

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With the north drenched to its core by so much rain, it’s easy to forget that just a couple of summers ago we were in the midst of a drought. Aucklanders were being urged to conserve water as dam supplies dwindled and Northland farmers were forced to cull stock. Meanwhile, Southland was ravaged by floods and landslides.

This summer, the situation is inverted, with dry heat in the south and way too much wet in the north. Data journalist Chris McDowall, who looked into which areas of New Zealand are likely to become wetter and drier as part of The Spinoff’s 2020 series 100 Year Forecast, wondered what was behind this wet-dry flip-flop. So I asked Sam Dean, Niwa climate scientist.

A dried up Auckland dam during the 2020 drought.
A dried up Auckland dam during the 2020 drought. (Photo: Watercare)

‘Blocking high’ behind north-south pattern

The “north-south dipole” is a pattern that’s often seen in New Zealand, says Dean. “When you analyse rainfall, it’s one of the primary patterns that comes out — but it’s not the only pattern. Just because it’s wet in the north doesn’t guarantee it’s dry in the south.”

Dean explains that behind this flip-flop, a bunch of large scale systems align to create “blocking highs” — including the depletion of the ozone layer, which has shifted regular storms poleward, encouraging highs to form above New Zealand. Exactly where these high pressure systems sit determines who gets wet, and who stays dry. This year, the forces of La Niña and other regional climate cycles have led to a blocking high off the east, which has opened the door for rainfall-bearing systems like the ex-tropical cyclone to bear down on the north. “The cyclone is being affected by this high pressure east of the country: it’s got nowhere to go, and has become very slow moving. This increases the damage because it goes on for longer, rather than nipping by,” says Dean.

Will we see more cyclones as the climate changes?

On average, we see one ex-tropical cyclone pass close to New Zealand every summer, and Dean says we don’t have any evidence that this number will increase. But as the climate warms, “there’s more fuel for them when they do happen — more heat, more moisture — so they can have stronger winds and more intense rainfall.”

People wait for a rooftop rescue (Photo: Royal New Zealand Air Force)

Averages mask the extremes

Looking at 100 Year Forecast’s seasonal maps, you can see which regions will get wetter or drier on average. But what these seasonal averages mask are the extreme events that will define our climate future; the flip-flops between drought and downpours. “Models suggest that short duration flash flooding — like we saw in Auckland in January — is going to increase the most under climate change,” says Dean. “Also, Northland and Auckland will see the biggest increases. That part of the country is becoming distinctly more variable.”

While the models predict that severe rainfall will become more intense and more frequent, the “off-the-charts” events like Auckland’s floods pose a challenge for scientists when it comes to understanding how they form, and how they’re affected by climate change. “We are embarking on a crazy experiment, fundamentally terraforming the planet into a different kind of place. And there are massive risks associated with that,” says Dean. “We don’t necessarily have a good understanding of how some of these most extreme events are going to behave when you give them that much more energy to work with.”

January flooding in downtown Auckland. (Photo: Sarah Brady)

While such extremes are scary, Dean emphasises that New Zealand will always be relatively habitable compared to many places around the world. “As long as things aren’t changing too fast, then people are really adaptive,” he says. “The more we can slow the changing climate, the better chance we have to adapt.”

And for those impacted by the current devastation: “I just hope for those people that they can rebuild their lives and find a way to be safe in a changing climate.”

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