Why extreme weather is the new norm
Monday, 8 January 2018
ANALYSIS: On an otherwise ordinary winter's day in the North Otago of Oamaru, a powerful storm dropped four months of rain in one day.
The storm had already been through Dunedin, where it caused near-record rainfall before heading north.
When it reached Oamaru on July 21, it dropped 161mm of rain - about one third of the town's average yearly rainfall - before barrelling north to cause similar chaos in Canterbury.
It was the wettest day in Oamaru since records began in 1941. Last year was one of the wettest years Oamaru's ever had, entirely due to that storm.
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In the five months afterwards, the town had 160mm of rain - almost exactly the same as what fell on that one chaotic day in July.
Oamaru serves as a good example of last year's wild weather, as detailed in Niwa's annual climate summary. It was, in many respects, the year of the storm: Just a handful of events were enough to skew rainfall records for much of the country.
It's easy to forget about the storms that hit New Zealand last year, and again in the new year, if you weren't among those directly affected.
The past few months have seen some of the most unusually warm and settled weather in New Zealand's history, which has broken records of its own.
August, September and October of 2017 comprised the warmest equivalent three month period in New Zealand since at least 1909; December was 2.4C warmer than usual, a feat only achieved in a given month three times this century.
In Milford Sound, it did not rain for three weeks straight. At Mt Cook, the average air temperature in November was on par with Kerikeri in the far north. Snapper were caught in Fiordland last month, which has never happened in December.
Cromwell had 23 consecutive days warmer than 25C; it didn't rain in Christchurch for 47 days in a row; Ninety locations nationwide had record or near record mean temperatures in December alone.
These are all clear and obvious examples of a warming climate, in line with projections for how New Zealand will be affected by climate change.
And yet, almost all of the country had a wetter than usual year, some parts significantly so.
The major cities had all reached their usual annual rainfall by the beginning of October; Christchurch reached its average yearly rainfall halfway through August, and would likely have broken rainfall records if not for the unprecedented dry spell late in the piece.
The graphs at the top of this story show how rainfall was distributed through the year. In most cases, the rain fell in a handful of spikes - or, in some cases, just one spike - which can, in almost all cases, be attributed to one of four storms.
The most notable storms were the Tasman Tempest in March, followed by Ex-Tropical Cyclone Debbie a few weeks later, which devastated Edgecumbe.
It was followed by Ex-Tropical Cyclone Cook a week later, then finally, the storm that hit Oamaru in July.
Combined, the four storms caused damage worth around $201m, according to the Insurance Council of New Zealand. In April alone, 46 locations notched up record or near record rainfall totals due to the two successive storms.
That is the flipside of a warming world: As Dr Jim Salinger at the University of Otago notes, 'New Zealand is now ranked as 'high hazard' for these events by the Global Facility for Disaster Reduction and Recovery.'
The extent to which climate change influences the power of storms may not be entirely understood, but as with last week's storm, there is a clear connection between warm seas and powerful storms, and there is no doubt a warming climate is influencing the weather already.
'Climate change is a part of every weather event, whether it's a warm year or an exceptional heavy rainfall event or dryness; our earth is warmer than what it was 10 years ago, 30 years ago and 100 years ago,' says Niwa scientist Chris Brandolino.
'So there is a component of climate change [in the extremes witnessed in 2017], but the question is is how big is that? And that is something I cannot answer, unfortunately.'
Early indications show 2018 is likely to be warmer than 2017, with the next three months set to be warmer than usual for most of New Zealand.
We're likely to enjoy be a great summer - but it's important to remember the power of the storm, and that there is a flipside to the gentle summer heat in a world that is steadily warming.