NIWA’s climate summaries: How they’re produced and the unexpected extremes recorded


Later this month, NIWA will release its latest climate summary for the year that has just been. But looking back over annual summaries since the turn of the century can provide insight into the country’s climate and weather – and confirm some of what we already know about climate change.

Buller High School and surrounds in Westport, flooding, 17 July 2021

Media reports, such as on flooding in Westport in July 2021, help inform NIWA’s descriptions of the impact of extreme events. Photo: Supplied/NZ Defence Force

While the main observations aren’t much of a surprise, there is something that stands out – extreme high temperatures are being recorded, even more so than expected with climate change taken into account.

NIWA (National Institute of Water and Atmospheric Research) climate scientist Gregor Macara has been involved with producing the Crown Research Institute’s annual climate summaries for about eight years – long enough to get a good feel for them and understand the trends – and now leads the project.

If you’ve paid even the slightest attention to news in the past two decades, you’ll know that warmer and warmer temperatures are being recorded, and that’s confirmed by NIWA’s annual summaries, Macara says.

“Since the year 2000, New Zealand’s mean temperature has been higher than average for 16 of those 20 years and only lower than average for four of those remaining years … related to that, six of New Zealand’s top 10 warmest years have occurred since 2012.

“Not only are we seeing warmer than average years, but they’re actually exceptionally warm compared to our full historic record.”

Annual climate summaries

These are a snapshot of a year’s climate conditions – sunshine, temperature, soil moisture, sea surface temperatures, and so on.

The broader climate drivers like El Niño are also examined to help describe what took place.

They’re created using a lot of automated calculations, for example establishing mean and extreme temperatures, but there are also automated and manual quality checks to make sure no inaccurate data slips in.

But it’s not all about data, Macara says.

“We do pull on media reports and public observations of the events that have occurred throughout the year … to include some commentary about the event itself and what it meant for people on the ground in terms of observed flooding or road closures or trees downed or slips that occurred … so people can go back in time and look at these historic summaries online.”

Media reports, for example, help inform descriptions of the extreme events in terms of what impacts they had on factors like state highway closures, deaths or insurance losses.

The information is important for a wider context than just Aotearoa.

“These summaries actually feed into some international publications as well. WMO [World Meteorological Organisation] put out a state of the global climate each year and these summaries that we do I contribute to that, which at the end of the day, helps to contribute to the globe’s understanding of the current state of climate.”

Painting a picture

The annual summary process helps to “paint a picture of just how incredibly varied New Zealand climate and weather is”, Macara says.

“We’ve got weather stations up in the Southern Alps that record every year more than 10,000 millimetres of rain, whereas just a hundred kilometres to the east out in Central Otago they regularly if not every year, receive less than 400mm of rain.

“So there’s incredible diversity in terms of the amount of rainfall, for example, that’s recorded over the country and it speaks to that idea that we really do live in in a pretty dynamic part of the world.”

Aotearoa’s topography – the Southern Alps forming the South Island’s backbone and ranges dotted across the North Island – create what Macara says are “really distinct climate zones”, an element the summaries highlight.

Part of the picture being painted is that extreme events are cropping up every year.

“These extreme events – which we kind of characterise as being in the top four of the historical record, whether it’s the top four highest temperatures or top four lowest, for example … they’re cropping up every year. Every annual summary will have quite a long list in the tables of extreme events that have occurred,” Macara says.

That highlights New Zealand’s changeable and volatile weather.

“It sort of speaks to the idea that the weather and climate is not generally settled in New Zealand, and a feature of our current climate is that it’s very changeable and that will always remain a feature of our future climate as well.

“Climate change will mean warmer temperatures and likely the expectation that extreme rainfall events will become more severe and more frequent.

“But the underlying thought there is that that’s not necessarily anything new – it’s just how extreme they become is going to be the difference.”

Glowing sun on the sky to illustrate heatwave or global warming.

More high temperature extremes are been recorded than would be expected, even accounting for climate change, climate scientist Gregor Macara says Photo: 123RF

Record heat

“Long story short, we’re seeing more high temperature extremes being set than we would expect see, even accounting for climate change.”

That’s Macara’s potted summary of a situation being highlighted by the summaries.

Those extremes are the ones mentioned earlier – the top four results of the historical record.

“What we found by examining our climate summaries, but also taking the analysis back to about the 1950s, was these high temperature extremes are occurring more regularly than would be expected,” Macara says.

“To conceptualise that, let’s say your town has just set up a weather station and it records a certain mean temperature for a year in 2021. When you have that data come through for the following year, you’ve basically got a 50-50 chance that it’s gonna be the highest out of your record, because you’ve only got two years of record.

“But if you had 99 [years of records] and then you moved into your 100th year, your odds are you’ve got a one in 100 chance of setting a record high temperature.”

In short, the longer records exist, the more unlikely it is a record temperature will be set.

But even accounting for the fact that records are getting longer with time, we’re setting more and more higher record temperatures than expected.

And with climate change, we are increasing the odds – even with a longer record – of having a new record temperature.

But we’re getting more extreme high temperature records than we would have expected, even with climate change playing a part, Macara says.

“It’s in the order of three to five times higher than we would have expected.

“That gets to that idea … that while your mean temperatures might not be shifting a massive amount, it’s having a big effect at the tail of the distribution.

“So despite climate change just happening steadily, we’re tending to see these extreme events with regard to temperature happening more regularly than would be expected even under climate change or even accounting for climate change.”

New Zealand isn’t unique in this – the same thing is happening around the world, Macara says.

NIWA is keeping track of how many more records we’re seeing compared to what we would expect.

“We will continue to monitor that in future and we are hoping to add that as almost a tracker that we can put into our summaries saying ‘This month we had this many extremes and it is this percentage more than we would have expected even given climate change’,” Macara says.

“So we’re hoping to add that line of explanation or detail to our summaries in the future. We’re not there yet but we’re hoping to roll it out perhaps in the next year.”

One ‘particularly unusual’ event

The annual summaries have been “fitting that narrative that we expect where under a warming climate we’re going to see higher temperatures, we’re going to see more high temperature extremes and so on”, Macara says.

While the general theme has been one of nothing unexpected or unusual, there is one event that has stood out for Macara over the past eight years.

This particular event happened back in June 2015 – when temperatures dropped so low they were almost records.

“We had Pukaki Airport, which is just near Twizel. That recorded a temperature of -19.8 on the 23rd of June 2015. The following day, a weather station at Tara Hills, which is just further to the south near Omarama, that dropped to -21 degrees on the 24th of June 2015.”

That -21C is the fourth lowest temperature ever recorded in New Zealand.

It’s all the more unusual given the trend of consistently high temperatures and extremes.

“It really helps to demonstrate too that of course the climate is changing and will continue to change and become warmer, but it doesn’t mean that we won’t necessarily see record cold events or low temperatures.

“All the possibilities – they can occur if the right set of circumstances line up … the fact this event occurred demonstrates that.”

But in the future they’re going to be a lot less common than the record hot events.

Why did it get that cold?

For those chillingly cold days in June 2015, three key factors had to line up, Macara says.

He’s got a special knowledge of the conditions required after several years ago studying New Zealand’s coldest ever recorded temperature – 25.6C recorded at Ranfurly, Central Otago on 17 July, 1903.

“I think what happened in the Mackenzie Basin in 2015 was essentially the same set of circumstances.

“The first key aspect was a snowfall, so we had a decent snowfall in the Mackenzie Basin, and it was what they call a warm advection snowfall – this warm moist air arrives in the northwest, and it sort of spills over the main divide of the Southern Alps, but it falls into these basins which have this cooler air trapped in there because often you know you get a southerly blowing up through Central Otago and so on and this cold air sits in the basins and then this warm moist air rolls over top of the Southern Alps and it drops the rain or precipitation through that cold layer and creates snowfall.”

The second element came after the snowfall – a really cold southerly front that blew through and delivered even colder air into the Mackenzie Basin.

The third was a big high pressure system that came and sat over top of of the South Island.

“So you had snow on the ground. You had really cold air sitting in the [Mackenzie] Basin and this high pressure system rolled over top and you got these beautiful blue clear skies that enabled calm weather to persist,” Macara says.

“What happens is you get a huge amount of loss of radiation or heat from the surface of the land under those clear skies, particularly when there’s snow on the ground because the sun during the day can’t heat up the dark substance of soil, for instance – it’s hitting that white reflective surface of the snow and just kind of bounces back out into the atmosphere so you don’t get daytime heating.”

It means things are already very cold – and overnight get even colder than they otherwise would.

“To put it technically, the amount of heat loss from the surface was enhanced because there was snow on the ground.

“Because of that persistent high pressure system, day after day this temperature profile setup and it just got colder and colder until we hit as I say, minus 21 on the 24th, which is remarkable really. It’s pretty pretty incredible to see that.”

“As I say, it shouldn’t be unexpected because we do have that variable weather and climate in New Zealand, so we’re still going to have cold events in the future, but they’re just going to be less likely than the hot events.

New norms

The current average temperature for the country is based on the 1981-2010 period.

A team from NIWA are currently working on the average for the 1991-2020 period – although it’s not as simple as just taking temperature records and doing the maths.

“It’s taking longer than we had anticipated or hoped. So the new baseline would be 1991 to 2020. And of course those years now exist in our records, so there’s no reason why we can’t calculate the new normals as they are called – the climate normals, which is the 30 year average,” Macara says.

“But the issue here is around the quality assurance of the data. You actually have to go through a really rigorous and thorough process according to World Meteorological Organisation standards to assess the data and make sure there’s not a certain proportion of the data missing, for example, or if there is missing data, it can’t be for a certain number of consecutive days.”

The new normals should be ready some time this year.

Results will be interesting, Macara says.

“The reality is, you know, we’ve seen an increase in temperature of around about 1 degree per century. So per hundred years in our historic records, so really shifting the normal period by what is effectively 10 years may only see those normal temperatures go up by about 0.1 degree for example … which doesn’t necessarily sound like a lot.

“But you know, it is just that ongoing sort of insidious increase in temperature which is just ticking away in the background and it’s shifting the norms as we know, and you know when you shift the normal or an average you’re also shifting the distribution of temperatures, meaning that your extreme high temperatures are going to become more extreme, for example.

“And your extreme low temperatures, on average, are likely to become you know, less lower than they were historically as well, so whilst the shift in the average might be subtle it means the whole distribution of temperatures is shifting as well and that means that we tend to observe more extremes.”

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