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Atmospheric river: NIWA

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Atmospheric rivers are long filaments of moisture that curve poleward. Several are visible in this satellite image. Bin Guan, NASA/JPL-Caltech and UCLA

Zhe Li, University Corporation for Atmospheric Research
This article is republished from The Conversation under a Creative Commons license. Read the original article.

Atmospheric rivers – those long, narrow bands of water vapor in the sky that bring heavy rain and storms to the U.S. West Coast and many other regions – are shifting toward higher latitudes, and that’s changing weather patterns around the world.

The shift is worsening droughts in some regions, intensifying flooding in others, and putting water resources that many communities rely on at risk. When atmospheric rivers reach far northward into the Arctic, they can also melt sea ice, affecting the global climate.

In a new study published in Science Advances, University of California, Santa Barbara, climate scientist Qinghua Ding and I show that atmospheric rivers have shifted about 6 to 10 degrees toward the two poles over the past four decades.

Atmospheric rivers on the move

Atmospheric rivers aren’t just a U.S West Coast thing. They form in many parts of the world and provide over half of the mean annual runoff in these regions, including the U.S. Southeast coasts and West Coast, Southeast Asia, New Zealand, northern Spain, Portugal, the United Kingdom and south-central Chile.

California relies on atmospheric rivers for up to 50% of its yearly rainfall. A series of winter atmospheric rivers there can bring enough rain and snow to end a drought, as parts of the region saw in 2023.

Atmospheric rivers occur all over the world, as this animation of global satellite data from February 2017 shows. NASA/Goddard Space Flight Center Scientific Visualization Studio

While atmospheric rivers share a similar origin – moisture supply from the tropics – atmospheric instability of the jet stream allows them to curve poleward in different ways. No two atmospheric rivers are exactly alike.

What particularly interests climate scientists, including us, is the collective behavior of atmospheric rivers. Atmospheric rivers are commonly seen in the extratropics, a region between the latitudes of 30 and 50 degrees in both hemispheres that includes most of the continental U.S., southern Australia and Chile.

Our study shows that atmospheric rivers have been shifting poleward over the past four decades. In both hemispheres, activity has increased along 50 degrees north and 50 degrees south, while it has decreased along 30 degrees north and 30 degrees south since 1979. In North America, that means more atmospheric rivers drenching British Columbia and Alaska.

A global chain reaction

One main reason for this shift is changes in sea surface temperatures in the eastern tropical Pacific. Since 2000, waters in the eastern tropical Pacific have had a cooling tendency, which affects atmospheric circulation worldwide. This cooling, often associated with La Niña conditions, pushes atmospheric rivers toward the poles.

The poleward movement of atmospheric rivers can be explained as a chain of interconnected processes.

During La Niña conditions, when sea surface temperatures cool in the eastern tropical Pacific, the Walker circulation – giant loops of air that affect precipitation as they rise and fall over different parts of the tropics – strengthens over the western Pacific. This stronger circulation causes the tropical rainfall belt to expand. The expanded tropical rainfall, combined with changes in atmospheric eddy patterns, results in high-pressure anomalies and wind patterns that steer atmospheric rivers farther poleward.

An animation of satellite data shows sea surface temperatures changing over months along the equator in the eastern Pacific Ocean. When they're warmer than normal, that indicates El Niño forming. Cooler than normal indicates La Nina.
La Niña, with cooler water in the eastern Pacific, fades, and El Niño, with warmer water, starts to form in the tropical Pacific Ocean in 2023. NOAA Climate.gov

Conversely, during El Niño conditions, with warmer sea surface temperatures, the mechanism operates in the opposite direction, shifting atmospheric rivers so they don’t travel as far from the equator.

The shifts raise important questions about how climate models predict future changes in atmospheric rivers. Current models might underestimate natural variability, such as changes in the tropical Pacific, which can significantly affect atmospheric rivers. Understanding this connection can help forecasters make better predictions about future rainfall patterns and water availability.

Why does this poleward shift matter?

A shift in atmospheric rivers can have big effects on local climates.

In the subtropics, where atmospheric rivers are becoming less common, the result could be longer droughts and less water. Many areas, such as California and southern Brazil, depend on atmospheric rivers for rainfall to fill reservoirs and support farming. Without this moisture, these areas could face more water shortages, putting stress on communities, farms and ecosystems.

In higher latitudes, atmospheric rivers moving poleward could lead to more extreme rainfall, flooding and landslides in places such as the U.S. Pacific Northwest, Europe, and even in polar regions.

A long narrow band of moisture sweeps up toward California, crossing hundreds of miles of Pacific Ocean.
A satellite image on Feb. 20, 2017, shows an atmospheric river stretching from Hawaii to California, where it brought drenching rain. NASA/Earth Observatory/Jesse Allen

In the Arctic, more atmospheric rivers could speed up sea ice melting, adding to global warming and affecting animals that rely on the ice. An earlier study I was involved in found that the trend in summertime atmospheric river activity may contribute 36% of the increasing trend in summer moisture over the entire Arctic since 1979.

What it means for the future

So far, the shifts we have seen still mainly reflect changes due to natural processes, but human-induced global warming also plays a role. Global warming is expected to increase the overall frequency and intensity of atmospheric rivers because a warmer atmosphere can hold more moisture.

How that might change as the planet continues to warm is less clear. Predicting future changes remains uncertain due largely to the difficulty in predicting the natural swings between El Niño and La Niña, which play an important role in atmospheric river shifts.

As the world gets warmer, atmospheric rivers – and the critical rains they bring – will keep changing course. We need to understand and adapt to these changes so communities can keep thriving in a changing climate.The Conversation

Zhe Li, Postdoctoral Researcher in Earth System Science, University Corporation for Atmospheric Research

dave-hoefler-thunderstorm

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Authors: Andrew Dowdy, The University of Melbourne; Conrad Wasko, University of Sydney; Jennifer Catto, University of Exeter, and Seth Westra, University of Adelaide

In media articles about unprecedented flooding, you’ll often come across the statement that for every 1°C of warming, the atmosphere can hold about 7% more moisture.

This figure comes from research undertaken by the French engineer Sadi Carnot and published 200 years ago this year.

We now know there’s more to the story. Yes, a hotter atmosphere has the capacity to hold more moisture. But the condensation of water vapour to make rain droplets releases heat. This, in turn, can fuel stronger convection in thunderstorms, which can then dump substantially more rain.

This means that the intensity of extreme rainfall could increase by much more than 7% per degree of warming. What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming.

Thunderstorms are a major cause of extreme flooding around the world, contributing to Brazil’s disastrous floods, which have submerged hundreds of towns, and Dubai’s flooded airport and roads.

For Australia, we helped develop a comprehensive review of the latest climate science to guide preparedness for future floods. This showed the increase per degree of global warming was about 7–28% for hourly or shorter duration extreme rain, and 2–15% for daily or longer extreme rain. This is much higher than figures in the existing flood planning standards recommending a general increase of 5% per degree of warming.

Why are thunderstorms important for extreme rain?

For thunderstorms to form, you need ingredients such as moisture in the air and a large temperature difference between lower and higher air masses to create instability.

We typically associate thunderstorms with intense localised rain over a short period. What we’re seeing now, though, is a shift towards more intense thunderstorm downpours, particularly for short periods.

Extreme rain events are also more likely when thunderstorms form in combination with other weather systems, such as east coast lows, intense low pressure systems near eastern Australia. The record floods which hit Lismore in February 2022 and claimed the lives of many people came from extreme rain over many days, which came in part from severe thunderstorms in combination with an east coast low.

Climate change pumps up extreme flood risk factors

The latest report from the Intergovernmental Panel on Climate Change (IPCC) states that:

frequency and intensity of heavy precipitation events have increased since the 1950s over most land areas for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is likely the main driver

This increase is particularly clear in short-duration extreme rains, such as those caused by thunderstorms.

Why? In part, it’s because of the 7% figure – warmer air is able to hold more water vapour.

But that doesn’t explain everything. There’s something else going on. Condensation produces heat. So as water vapour turns into droplets, more heat becomes available, and hot air rises by convection. In thunderstorms, more heat fuels stronger convection, where warm, moisture-laden air is driven up high.

This explains why thunderstorms can now drive such extreme rainfall in our warming world. As water vapour condenses to make rain, it also makes heat, supercharging storms.

We are seeing these very rapid rates of rainfall increase in recent decades in Australia.

Daily rainfall associated with thunderstorms has increased much more than the 7% figure would suggest – about 2-3 times more.

Hourly rainfall extremes have also increased in intensity at similar rates.

What about very sudden, extreme rains? Here, the rate of increase could potentially be even larger. One recent study examined extreme rain for periods shorter than one hour near Sydney, suggesting about a 40% increase or more over the past 20 years.

Rapid trends in extreme rainfall intensity are also clear in other lines of evidence, such as fine-resolution modelling.

To model complex climate systems, we need the grunt of supercomputers. But even so, many of our models for climate projections don’t drill down to grid resolutions smaller than about 100 kilometres.

While this can work well for large-scale climate modelling, it’s not suitable for directly simulating thunderstorms. That’s because the convection processes needed to make thunderstorms form happen on much smaller scales than this.

There’s now a concerted effort underway to perform more model simulations at very fine scales, so we can improve the modelling of convection.

Recent results from these very fine scale models for Europe suggest convection will play a more important role in triggering extreme rainfall including in combined storms, such as thunderstorms mingling with low pressure systems and other combinations.

This matches Australian observations, with a trend towards increased rain from thunderstorms combining with other storm types such as cold fronts and cyclones (including low-pressure systems in southern Australia).


Does this change how we plan for floods?

The evidence for supercharged thunderstorm rainfall has grown in recent years.

Australia’s current flood guidance recommendations, which influence how infrastructure projects have been built, are based on extreme rain increasing by just 5% for each degree of warming.

Our research review has shown the real figure is substantially higher.

This means roads, bridges, tunnels built for the 5% figure may not be ready to deal with extreme rain we are already seeing from supercharged thunderstorms.

While Australia has become more conscious of links between climate change and bushfires, studies show we are less likely to link climate change and more intense storms and floods.

This will have to change. We still face some uncertainties in precisely linking climate change to a single extreme rain event. But the bigger picture is now very clear: a hotter world is likely one with higher risk of extreme floods, often driven by extreme rain from supercharged thunderstorms.

So what should we do? The first step is to take climate change influences on storms and flood risk as seriously as we now do for bushfires.

The next is to embed the best available evidence in how we plan for these future storms and floods.

We have already loaded the dice for more extreme floods, due to existing human-caused climate change and more to come, unless we can quickly reduce our greenhouse gas emissions.The Conversation

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Andrew Dowdy, Principal Research Scientist in Extreme Weather, The University of Melbourne; Conrad Wasko, ARC DECRA Fellow in Hydrology, University of Sydney; Jennifer Catto, Associate Professor of Mathematics and Statistics, University of Exeter, and Seth Westra, Hydrologist, University of Adelaide

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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We can’t just sit around and wait to see what will happen next. We need positive action.

I’ve read a lot of Climate Adaptation Plans and Strategies over the past the last few years, but He Toka Tū Moana Mō Maketū (Maketū Climate Change Adaptation Plan) is hand-down the best. It’s clearly laid out, outlines the community’s priorities, and can readily serve as a template to help every community around Aoteara develop their own Climate Adaptation Plans. Most important of all:

It’s iwi led, community driven, it’s a plan that’s been decided by those who live here. –  Elva Conroy, Kaitohotohu / Facilitator (Video; to listen Watch on Youtube)

Winner of the 2023 Supreme Planning Awards, the Maketū Climate Change Adaptation Plan was developed by Ngāti Pikiao Environmental Society, Te Rūnanga o Ngāti Whakaue ki Maketū , Whakaue Marae Trustees, and Conroy and Donald Consultants.

In the words of the Maketu Iwi Collective, ‘we will be resilient like the anchor stone Takaparore – strong and steadfast against the elements and tides of change and uncertainty. Regardless of what happens as a result of a changing environment, we will remain standing’. – New Zealand Planning Institute, April 2023.

Cop 26 Nature

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October 31: that’s the date for the United Nations Climate Change Conference in Glasgow, COP26. It’s now clear that the Paris target to limit temperatures to 1.5 degrees has failed. Meeting these targets set by governments, including New Zealand, would result in warming well above 3 degrees by 2100.

“We are facing the twin threats of climate change and biodiversity loss. One cannot be solved without addressing the other…. Yet only 3% of global climate finance is spent on nature-based solutions, and only 1% for adaptation.”

One of the three key goals of COP26 is to:

“Protect and restore nature for the benefit of people and climate” and to “call on governments, businesses and civil society to endorse the Leaders’ Pledge for Nature and make ambitious commitments to build nature positive economies and societies.”

In support of this goal, every day from 01 September until October 31, we’ll be posting an extract from this website on Facebook.

Ashley Rakahuri River during flood 2021

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Left: 4.00pm 29 May. Water flow 4 cumecs.

Right: 4.00pm 30 May. Water flow 966 cumecs.

Fig.1 (Photos: Nick Ledgard)

Ashley River Rakahuri

By definition, braided rivers are made up of multiple channels or ‘braids’. Systematically forcing them into the solitary confinement of single channels is turning them into what University of Waikato’s Professor James Brasington calls “zombie” rivers, waterways locked into position between stopbanks and their headwaters. Planting willows and poplars as a ‘natural’ way to hold them in place has just exacerbated the problem. Confining braided rivers not only creates problems for freshwater ecosystems including habitat for braided river birds, it also makes these rivers prone to catastrophic flooding.

“If we put our rivers into straight-jackets, they lose the diversity of form and process that are fundamental to the creation of thriving ecosystems. Instead, we should make space for rivers to erode their corridors, flood naturally in areas that are of less value which will in turn, reduce risks in more sensitive areas. We must work with natural processes to reduce the flood risk and support healthy river ecosystems.” – James Brassington

Last weekend, several zombie rivers in Canterbury broke free and invaded towns and properties.

It’s not like we weren’t warned. In 2019, NIWA published this extraordinarily well-ignored report outlining the current NZ$40 billion flood risks to Canterbury (no, that’s no a misprint; it’s billion, not million) (Fig. 2) .

Fig. 2: From 2019 NIWA's 2019 report, 'New Zealand Fluvial and Pluvial Flood Exposure' (page 8). Exposure to flood risk does not mean all of the areas on the map (click the image) will flood. However, the risks are increasing as the climate changes as warmer air carries more moisture.
Fig. 2: From 2019 NIWA’s 2019 report, ‘New Zealand Fluvial and Pluvial Flood Exposure’ (page 8). Exposure to flood risk does not mean all of the areas on the map (click the image) will flood. However, the risks are increasing as the climate changes as warmer air carries more moisture.

I’ve yet to meet anyone who read their entire report, much less acted on it. A small extract made headline news in a North Canterbury paper and was promptly forgotten.

Two weeks ago, I gave a presentation to the Waimakariri drainage group on the risks of flooding from pluvial and fluvial events and rising sea levels. It caused some discomfort, but then again, I was talking about climate change. A concept that probably seemed too remote to lose much sleep over.

Last weekend’s floods will no doubt make headline news again in our local papers. And in a few weeks it will also probably be forgotten by most people, especially those who weren’t directly affected. Part of the reason is that the phrase ‘1-in-100 event’ is often misunderstood, leading to a false sense of security that these zombies and their big brother rivers are unlikely to escape again in our lifetimes. (The term ‘1-in-100 years’ is in fact a statistical annual exceedance probability. It’s arguably moot in any case as many hydrologists consider stationarity to be dead.)

And then there’s the false perception that ECan is obligated to shove every metre of rivers back into solitary confinement. The situation on the Okuku River is a case in point. The poor little Okuku, like many zombie rivers, is so glutted with weeds that it’s unrecognisable as a braided river. It’s not even mentioned on the braided rivers website. I mention it here now, because this section of the Okuku is outside the river ratings area. Councils are under no obligation to protect those who  live and farm in its riverbed.  At some point, properties in riverbeds are likely to be deemed too risky to insure, which raises the question of who will pay for more extreme weather events? And how can we tell if climate change is the cause?

In January this year, nine New Zealand river experts wrote an article in The Conversation, titled ‘Why we should release New Zealand’s strangled rivers to lessen the impact of future floods’.

“We shouldn’t be surprised when our rivers break their banks — that’s just a river being a river. Current management practices in Aotearoa treat rivers as static, in the hope of making them more predictable.

“But this can lead to disasters.

“Unless we change management practices to work with a river, giving it space to move and allowing channels to adjust, we will continue to put people and rivers on a collision course.

“When flood risk is managed poorly, disadvantaged groups of the population are often disproportionately impacted. Given climate change predictions of more extreme floods and drought, the problem will only get worse.”

One of the authors of this article, Dr Jo Hoyle from NIWA, will be presenting a paper at the next Braided Rivers seminar at Lincoln University Wednesday 14 July. If you are reading this after the event, a PDF of her presentation will be available here.

For more information see the pages on this website: