From drought scare to deluge despair: The science of the storm
Friday, 5 January 2018
ANALYSIS: After a period of calm, dry weather for much of the country, in which century old records for dryness were toppled, the furious storm from the north seemed to come out of the blue.
What may at first seem like atmospheric whiplash was actually a case of cause and effect – and may be a taste of things to come.
The sub-tropical low roaming down the country, which formed earlier in the week near Queensland in Australia, is the most significant storm to hit New Zealand in many months.
Within 24 hours of hitting land on Thursday, it had rained more in Auckland than it had through November and December combined; Rotorua had surpassed its usual total January rainfall; and coastal and riverside areas were about to be inundated by an unfortunately timed king tide.
Part of the storm's intensity, however, can be traced back much further, to the settled days of late November when much of the country was cloaked in sunshine and worrying about drought.
**READ MORE:
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A weather pattern consistent with La Niña caused arm temperatures and widespread dryness, particularly in the south. It didn't rain at all in Christchurch for more than 40 days, an effect which spread like a halo to much of Canterbury where rainfall totals for the month were in single digits. In Milford Sound, the wettest part of the country, it didn't rain for 23 straight days.
Those warm, dry, and settled conditions contributed to an unusual phenomena: a marine heat wave, in which sea temperatures around New Zealand were about 2 degrees Celsius warmer than average.
Off the west coast, in the Tasman Sea, temperatures were as much as 6C above normal – at the time, it was the largest sea temperature anomaly in the world.
When weather conditions are settled – effectively meaning a lack of wind – there is no mechanism for deeper, colder water to come to the ocean's surface, which keeps the seas warm, says Niwa scientist Chris Brandolino.
For a storm, warm seas are like fuel: the water stores the heat as potential energy, similar to a rubber band ring stretched back, about to be flung.
'Warm ocean temperatures release a lot of heat and a lot of energy into the atmosphere, and if you have a storm or a low pressure passing over that, it can provide the necessary energy to really ramp up the intensity of the storm,' Brandolino says.
'In other words, if the same storm were to pass over waters that were cooler than average, as opposed to warmer than average, I'd be shocked if we got a similar result.'
That effect is a major reason why climate scientists say rising temperatures will increase the intensity of extreme weather events: warmer oceans can empower storms, potentially increasing rainfall amounts and wind speeds.
While the unusual warmth of the sea around New Zealand has ebbed and flowed since November, it is still much warmer than usual, and will likely remain so for the coming months.
The water is not only warmer on the ocean's surface, but much deeper down, to depths of about 100 metres. Even if winds churn the water, it may not have a significant cooling effect, Brandolino says, which raises the prospect of more empowered storms.
'For the next three months, the main things that will drive the weather is La Niña and these very warm ocean temperatures,' he says.
'There will be days where that might not be the case, but we're talking about the overall theme and flavour.
'Because of these warm ocean temperatures persisting, we still have to be on guard for some future rain events, low pressure systems, that could produce significant rainfall. I'm not saying that's a slam dunk but there's probably an elevated risk for additional storms that will move over the Tasman sea and be stronger than usual because of these warmer than average ocean temperatures.'
While heavy rain and fierce wind gusts have caused havoc, it is another unfortunate spin-off of the storm doing damage to coastal areas.
The storm reached its peak on Friday morning, roughly aligned with a king tide which would have happened with or without the storm.
The consequential effect is called a storm surge, which is like a tsunami in miniature: the storm increases the height of the water beyond what would typically be expected in normal conditions, which is already high in a king tide.
Storm surges are typically the most damaging component of a major storm, particularly in a coastal country like New Zealand. In Thames, for example, Friday's rainfall was far from the heaviest in New Zealand, but storm surges caused significant flooding and damage to infrastructure.
With rising sea-levels, as expected under a warming climate, storm surges will get higher and reach further inland – issues already evident in pockets around the country, where homes and infrastructure have been damaged.