Kielder. I'm Chelsea Daniels and this is a bonus episode of The Front Page, a daily podcast presented by the New Zealand Herald. Over the last year, the term atmospheric river has entered our daily vocabulary as New Zealand sees more intense rainfall hit our shores. From the Auckland Anniversary floods to two months worth of rain heading the East coast in just one day. The whole country seems to be susceptible to increase rainfall. But is this really the
case and if so, what's the cause. Today on the Front Page, doctor Daniel Kingston from Otago University joins us to discuss their latest research into the cause and effect of rainfall in this country. Daniel, we've heard this word a lot in recent years, or this phrase rather, but can you explain what our atmospheric rivers?
Well, the areas in the atmosphere where we see really strong, really intense transport of moisture vapor through the atmosphere in a long, thin section. So it looks a bit like a river when you look at a weather map of this, although it's not actually a river like we think of on the land surface.
Right so literally, if we were looking at it from above, it would be kind of like a river above us.
Yeah, it would certainly look like that in the sense that it's long and it's thin, and it's carrying an awful lot of water, but it's water vapor rather than liquid water itself.
They seem to bring a lot of rain to New Zealand. But how much do these rivers of the sky contribute to our rainfall?
It varies a little bit around the country. So in places like the west coast of the South Island, then we might be looking up up to eighty percent of our annual rainfall comes through these atmospheric river events. Places on the eastern side of the country maybe like Dunidan or christ Church a bit less, so maybe you know, we're talking less than fifty percent here. So it varies a bit around the country, but they're really important in
some parts. And then particularly when we start to look at our heaviest rainfall events, then these do generally come from atmospheric river events.
Meanwhile, heavy rain and strong winds have seen a state of emergency declared for part of Hastings after three breaches of the Home wan Sea Wall. Police are knocking on doors of affected residences to evacuate. The town of de Kataka, thirty kilometers from Gisbon, is cut off in both directions in State Highway thirty five between Gisbon and Tolika Bay is closed due to surface flooding.
So this one's a new one for me. IVT or vertically integrated water vapor transport. Not quite as catchy as atmospheric river, is it? What does that mean?
No? Not quite. Well, that's basically described how we measure how much water is being transported through the atmosphere. So when we're looking to say whether an atmospheric river is occurring or not, we look at this IVT variable vertically integrated water vapor transport. And so if that IVT is above a certain threshold and the kind the spatial pattern of IVT kind of matches what we expect of an atmospheric river, then we can say, well, this is an
atmospheric river. And so if it's too low or if the shape isn't quite right, then it's not.
Now I'm assuming that atmospheric rivers aren't anything new. Why do you reckon it's come into the public's consciousness.
Now, Yeah, people first started researching them in the nineteen nineties and it didn't get picked up a huge amount initially, but kind of from the two thousands onwards, people started paying increased attention to it. Partly, it's just a catchy name, like you said, so it's something that people latch onto and gets a bit more recognition.
It's visceral, isn't it.
Yes, it is that everyone knows what a river's like. So yeah, it kind of really gives you a clear idea of what's going on. And then I think probably as people have become more and more aware of climate change and we've seen the effects that climate change has on some of our more extreme weather events, then you know, as we start getting more and more floods and damages from floods, then people naturally want to know what's causing it.
And so there's kind of two things allided to give us this increased attention on atmospheric rivers.
Well, you've absolutely segued perfectly into my next question. So you've recently worked on a study, hey, looking into the relationship between rainfall and climate change in New Zealand. What did your team investigate there?
Ah? Yeah, So this is work done by a PhD student here at a tigo niftin Krishna and he's well, what we've been doing to together is looking for the trends in IVT over New Zealand and then trying to unpick what's caused those trends. So is it just that there's more water in the atmosphere and that's leading to a higher IVT or is it changes in the wind so it's wind speed getting stronger or weaker. You know,
where's it coming from. So that's kind of been the key things that we've been asking and what we seem to have found is that certainly over the southern part of the country we are seeing higher IVT, so more moisture being transported over the country.
If I whip out my year seven science knowledge, if the world is getting warmer, wouldn't that then mean that there is more water in the atmosphere because it gets evaporated.
Yeah, that's right, No, you're exactly right. You know, we've got a warming planet, so that means that there's more energy at the surface that could be used to evaporate water. So as it gets hotter, we have more evaporation. But also as the air gets hotter, it can hold more moisture. So you know, if you keep evaporating, eventually the atmosphere will saturate and the water will have to fall out.
But what we're doing to the atmosphere is increasing its capacity to hold that moisture, so the air can hold more moisture, so when a rain event does happen, there's more moisture in there that can be rained out. And that's why we expect to see and why we are seeing heavier rain events as climate change progresses.
From your research, which areas of the country are most at risk from bearing the brunt of increased rainfall, it would be.
The southern half of the country, particularly the western side of it. So in a general sense, our weather comes from the west. You know, it might be a bit more southwesterly or northwesterly, but generally it's from the west. So that means that the west coast of the South Island and the North Island experience those weather systems first, and so that's where the rain falls initially. That's why we know that the West Coast, as you know, wetter locations.
Remaining Westport residents may be forced to evacuate this evening with serious flooding expectant to peak overnight. The rain delivered is predicted two months worth falling in just the last three days, with more on the way.
So I'm here to say that global heating and climate change is really having an impact on rainfall.
Yes, would be a simple answer. It gets a bit more complicated rainfall because it's so episodic in nature, it varies a lot from place to place. It makes it a lot harder to pick out a signal compared to temperature. Let's say there's always a temperature, we can always measure it, and it doesn't vary quite so much from place to place. It's much easier to pick out trends in temperature than it is in rainfall. But still we expect to see
those extremes increase with climate change. But at the same time, we might expect our dry periods to increase as well, because if we've got higher temperatures, then that water will evaporate from the surface, will dry out more quickly, and then if the atmosphere can hold more moisture, there might be a longer time period between rain events, even if those rain events actually end up being heavier.
I suppose dozens of atmospheric rivers make landfall here every year. At what point should we be concerned?
I guess you've really just got to listen to what Met Service are saying, what the weather forecast is. And a lot of these atmospheric rivers are actually beneficial, right. They provide the water that we use to generate electricity. They stock up our snowfield during winter so people can go skiing. A lot of what they give us is beneficial when it gets potentially dangerous. That when Met Service will give us our weather watches or our orange or red warnings.
A year ago, record rainfall hit the Upper North Island, delivering devastating flash floods at the start of the Anniversary weekend holiday. Amid the flooding and the slips, four people lost their lives, thousands abandoned their homes, resulting in about two billion dollars of insurance claims.
The Auckland Anniversary floods seemed to catch everyone of God, hey, are we becoming better at predicting rainfall at all?
Yeah? I mean weather forecasting in general, it improves over time. It improves as kind of people do more research, and it improves as we get bigger and better computers. There were various kind of unique characteristics. I think of that anniversary weekend blood that led to its rapid intensification, and that did catch forecasters out a little bit in terms of not being able to provide the lead in time that we might have hoped. But the contrast would be
as psychoon Gabriel, which is also incredibly damaging. But we had a good idea that that was coming from more than a week out and was forecast really well by comparison. So you know, some events get really well forecast, others not quite so much. But the gradual track over time is yet that they get forecasts better and better.
Thanks for joining us, Daniel. That's it for this episode of the Front Page. You can read more about today's stories and extensive news coverage at enzed Herald dot co dot nz. The Front Page is produced by Ethan Siles with sound engineer Paddy Fox. I'm Chelsea Daniels. Subscribe to The Front Page on iHeartRadio or wherever you get your podcasts, and tune in tomorrow for another look behind the headlines.