Noboru Nakamura
Big Brains podcst

Extreme Heat Waves: Why Are They Surging? with Noboru Nakamura (Ep. 96)

Research examines—and could help predict—extreme weather events around the world 

Noboru Nakamura
Big Brains podcst

Show Notes

It’s not your imagination, summers have been getting hotter and hotter with extreme heatwaves occurring earlier and more frequently. But why is this happening and can we better predict heatwaves in advance to give people time to prepare?

In June of 2021, an unprecedented heatwave shocked the Pacific Northwest and Canada. It ended up being one of the most deadly extreme weather events in the region. But no one could figure out how it occurred, until one Professor of Geophysical Science at the University of Chicago, Noboru Nakamura, saw it as an opportunity to test a new theoretical framework he had developed for understanding atmospheric phenomena.

On this episode, we discuss the science of heatwaves, the mystery behind the Pacific Northwest heatwave, and how Nakamura’s framework can be used to better understand and predict climate change and other extreme weather events.

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(Episode published July 7, 2022)



Paul Rand: Last summer, an extreme heat wave hit the Pacific Northwest and Western Canada.


The Pacific Northwest is bracing for scorching heat from Northern California to Washington state.

Paul Rand: In a region known for its cool temperatures and moderate climate, it was a deadly shock.

Tape: Air conditioners, fans, bottled water all going fast inside this hardware store in Renton. Not a lot of time to think on it.

Paul Rand: Tragically, more than 1000 people died from heat exposure, making it one of the deadliest extreme weather events in the region.

Tape: How did it go so tragically wrong? That’s what the province and some cities are being asked as the death toll from the heat wave continues to rise.

Paul Rand: But equally terrifying is that no one could piece together exactly how this heat wave happened. It was a mystery until.

Noboru Nakamura: I thought, well, this is something that’s worth the investigation.

Paul Rand: That’s Noboru Nakamura, a professor in the Department of the Geophysical Sciences at the University of Chicago. Nakamura is an expert on the fluid dynamics of the atmosphere.

Noboru Nakamura: I am an atmospheric dynamitist. I don’t do weather forecasts.

Paul Rand: You might picture weather forecasters tracking storm patterns or recording wind directions to try and explain weather events, but you’ll find Nakamura crafting precise statistical equations and mathematical frameworks to make scientific sense of the chaos in the clouds.

Noboru Nakamura: I have certain ways of looking at the data and trying to tell a story of what physically is happening to call it certain events in the atmosphere.

Paul Rand: Nakamura uncovered the story behind the historic Pacific heat wave, and in the process proved the viability of a new framework he designed that will help us predict and understand all sorts of extreme weather events.

Noboru Nakamura: Now, we can’t really predict weather 20 years ahead of time, but the statistics of weather can be predicted reasonably well.

Paul Rand: Which is important because it’s not just your imagination. This summer has also been off to an incredibly hot start.

Tape: Turn now to the dangerous heat on this first official day of summer. Temperatures reaching near the triple digits from the South to the Midwest posing a potential health threat to millions.

Paul Rand: And these temperatures aren’t an anomaly. They’re part of a trend. In just the first few weeks of June, many US cities saw temperatures hit record highs in the triple digits.

Tape: Las Vegas, Phoenix, Denver, and St. Louis are among some of the cities that have reported record setting temperatures this month. And more than 50 million Americans are expected to endure triple digit temperatures in the days to come.

Paul Rand: And it’s not just here in the United States.

Noboru Nakamura: People in different regions will be affected by heat differently.

Paul Rand: The ability to better predict future heat waves will be incredibly important, especially as extreme heat is now the leading cause of weather related deaths in the United States.

Noboru Nakamura: Greater than the fatalities associated with tornadoes, hurricanes or floods. It’s usually the most vulnerable populations that get the brunt of heat, and we really have to prepare ourselves.

Paul Rand: From the University of Chicago podcast network, this is Big Brains, a podcast about the pioneering research and the pivotal breakthroughs that are reshaping our world. On this episode, the science of heat waves and how we could get better at predicting them.

Paul Rand: Most of us hate extremely hot days. We can tolerate them every now and then, but what exactly differentiates a heat wave from the occasional really hot day?

Noboru Nakamura: According to National Weather Service, heat wave is when you have two or more consecutive days of abnormally high temperatures. So here in Chicago for example, if you had 90 degrees three days in a row in the month of May, that will probably qualify as heat wave. But of course the longer a heat wave, the more problematic it becomes.

Paul Rand: But scientifically speaking, what exactly is going on up there in the sky?

Noboru Nakamura: A region experiencing a heat wave is typically under the influence of a high pressure system that sits there. Air is subsiding and compresses on the weight of the atmosphere inside high pressure system. So this sinking air is compressed, and therefore it raises the air temperature, and this allows clouds to dissipate or evaporate. So inside high pressure, the sky is clear and this allows ample sunlight to reach the ground and heat the ground up. Since the warm air is subsiding in high pressure, the heat is trapped near the ground and it cannot escape to the higher altitude. This raises surface air temperature to an abnormal level.

Paul Rand: Some areas are becoming frequent targets or hotspots for this kind of heat.

Noboru Nakamura: According to a recent study, heat wave has increased its frequency all over the world since 1950s, but there are certain hotspots, like the Middle East, North Africa and parts of South America that experienced a uniquely large increase in the frequency of heat waves.

Paul Rand: The places that are experiencing severe droughts and historically low rainfall are also more prone.

Noboru Nakamura: Physical properties of the ground are one of the main factors that control the heat waves. Usually dry ground is more conducive to extreme heat. If you have mountains and if you have persistent winds going over the mountains, the downwind side of the mountain is more conducive to high temperatures because the air flow that goes down the slope of the mountain gets compressed by the weight of the atmosphere and heats up. And then it’s conducive to high temperatures in the downwind side.

Paul Rand: But looking at the typically rainy region of the Pacific Northwest, no one would’ve expected one of the deadliest heat waves to happen there, and yet.

Noboru Nakamura: The heat wave was really unprecedented. Great manifestations of atmospheric circulation anomalous weather that exemplifies the complexity of the behavior of our atmosphere.

Paul Rand: During this heat wave, the city of Portland, Oregon recorded an all time record high of 116 degrees. And in Seattle, Washington, the temperature broke a record of 108 degrees,

Noboru Nakamura: A small town of Lytton, Canada, which is just outside Vancouver, registered 121 degrees Fahrenheit, and that was the highest temperature ever recorded in a country. So those numbers were really unheard of for this normally cool region, Pacific Northwest.

Paul Rand: Meanwhile, Nakamura was.

Noboru Nakamura: I was in my office in Chicago.

Paul Rand: The heat wave wasn’t yet a scientific interest, but a personal one.

Noboru Nakamura: One of my daughters live in Seattle. She evacuated. I also had a friend who lived in Portland, Oregon. Her house didn’t have air conditioning because normally she doesn’t need one. So, she also evacuated.

Paul Rand: Following the news and updates from his family, Nakamura quickly realized that he had been developing something that could maybe solve the mystery of what happened.

Noboru Nakamura: I had developed a certain theoretical framework to look at data and back out what transpired to make this event so unusual.

Paul Rand: If Nakamura’s team could explain the story of the heat wave using his new framework, it would prove its ability to explain the mechanisms behind all sorts of large scale atmospheric weather.

Noboru Nakamura: We decided to look at the atmospheric condition that preceded the heat wave, and sure enough, we identified very strong atmospheric blocking event, very large meandering of the jet stream.

Paul Rand: Atmospheric blocking events are what happens when there’s interference with the jet stream.

Noboru Nakamura: Blocking is a disruption to the jet stream.

Paul Rand: Jet streams encircle the globe, moving weather systems from west to east. But when there’s blocking, it’s like a traffic jam that stops them from flowing naturally.

Noboru Nakamura: So we really wanted to understand this phenomena. It turns out that using a simplified model and mathematical theory of [inaudible 00:08:56] dynamics, we found that atmospheric blocking event is mathematically equivalent to the traffic jam on a highway. So we can think of jet stream as a highway of weather. A strong jet stream makes the atmosphere less chaotic and more predictable.

Paul Rand: Now things might move slowly under normal traffic conditions, but every once in a while you get stuck in congested traffic that feels like it will never move. That’s what a weak jet stream looks like.

Noboru Nakamura: When this happens, weather systems lose steering because jet stream is weak and it becomes stagnant.

Paul Rand: That’s exactly what was happening near the Pacific Northwest during late June 2021.

Noboru Nakamura: We looked at the strength of the blocking event from meteorological data, and the strength of the blocking itself was really off the chart. It’s one of the strongest, probably the strongest blocking event ever recorded in this region over the last 40 years or so. It felt that there must have been a good reason why this was such a strong blocking. Our research showed that it was actually additional heat released by a cyclone that occurred in the upstream region of the blocking, namely over the Gulf of Alaska, a few days prior to the blocking event.

Paul Rand: A cyclone, which is another word for a low pressure system.

Noboru Nakamura: Cyclones are the big swirling winds in the Northern Hemisphere. They spin counterclockwise and they tend to create bad weather. Air not only spins around cyclones, but it converges toward the center of the cyclone and conversing air is pushed up. The rising air is conducive to expansion and cooling and condensation and water vapor that is conducive to cloud formation. So, cyclones are the weather system that brings about bad weather, rain, cloud cover, and stormy weather.

Paul Rand: This cyclone started releasing additional heat into the environment, making it the ripe condition for something even stranger to happen. The cyclone spurred an anticyclone.

Noboru Nakamura: Then this released heat is blown away by the jet stream and ended up becoming the core of the anticyclone.

Paul Rand: If you’re wondering what an anticyclone is, well, it’s exactly what it sounds like.

Noboru Nakamura: Cyclones and anticyclones are the flip sides of the coin of the same weather systems. When we talk about weather systems, whereas anticyclone is the opposite, it spins clockwise in the Northern Hemisphere and air is subsiding and spreads out at the base, and the subsiding air is conducive to the dissipation, evaporation in clouds. So, the sky tends to be cloud free. The succession of weather from fair weather to stormy weather is associated with the migration of anticyclones and cyclones from west toward east riding on the jet street.

Paul Rand: Now, my understanding is that there was a two to three day lag between the blocking incident we talked about, and then the whole idea of peak temperatures. And if that’s accurate, why is that particularly interesting?

Noboru Nakamura: Essentially this demonstrates that it was a top down phenomenon. First, anticyclone developed in the upper part of the atmosphere, and it drove this down draft and created a favorable condition for the extreme heat. So, the fact that the blocking occurred a couple days prior to the extreme heat really demonstrates the control of large scale circulation at the atmosphere in the upper part of the atmosphere over what’s happening near the ground.

Paul Rand: It also happens to be very close to the summer solstice. It was truly a perfect storm.

Noboru Nakamura: That really created enormous blanket effect that dropped heat near the ground and raised surface temperature.

Paul Rand: When heat waves form, we can think of the heat acting like a massive blanket. It’s trapping all that heat beneath it. Picture it like a heat dome.

Noboru Nakamura: So the ground temperature was already pretty warm, but this additional blanketing effect from blocking anticyclone really caused the surface temperature to soar.

Paul Rand: Nakamura’s framework was able to answer just how the Pacific heat wave came to be, but that framework can be used for predicting and understanding all sorts of other extreme weather events. With the climate crisis worsening every day, it couldn’t have come at a better time. More on that after the break.

Paul Rand: Hello, Big Brains listeners. The University of Chicago podcast network is excited to announce the launch of a new show. It’s called Entitled, and it’s about human rights. Co-hosted by lawyers and new Chicago Law School professors, Claudia Flores and Tom Ginsburg, Entitled explores the stories around why rights matter and what’s the matter with rights.

Paul Rand: The unprecedented 2021 Pacific heat wave is unfortunately not an isolated event. If you live in the US, you’ve probably already noticed that these extreme weather events are becoming more and more prevalent. The question is why.

Noboru Nakamura: So yeah, the increase in heat wave frequency is therefore a very tangible evidence that global warming is real and ongoing. Even without the human influence on climate, the heat waves always occur, but by adding more greenhouse gases to the atmosphere, the atmospheric temperature generally increases. It is very well understood and it has been well predicted that if you generally increase the atmospheric temperature, it’s not just the mean temperature that changes, but the most visible impact of global warming is the change in the frequency of extreme heat. What used to be once in every 200 years extreme heat may be happening now every 10 years. It’s still a rare event, but the frequency increased quite dramatically.

Paul Rand: So in this case, it’s because there’s an increase in greenhouse gases, primarily that is affecting the frequency of heat waves, the frequency and the intensity of heat waves. Is that right?

Noboru Nakamura: I often use this analogy or illustration to my class, which I think of a baseball player on steroid. A slugger can hit home runs without enhancement, but the drug improves his performance, increases his home run rate. But you can’t really tell whether a particular home run he hit is due to the enhancement. You can only discuss this in a realm of probability or statistics.

Paul Rand: Climate change is making extreme heat even more common worldwide, but scientists are still trying to determine how these patterns might change as the planet warms further.

Noboru Nakamura: Generally speaking, our atmosphere is chaotic, so we have only a limited forecast skill to begin with. What’s really difficult to foresee is how the circulation of the atmosphere changes in response to a greenhouse forcing. That is because the circulation of the atmosphere, like I said, it’s a chaotic system, and therefore there are many complex interplay, and these physical processes have competing effect on the behavior of the jet stream, for example. Therefore it is extremely difficult to tease out how climate change will affect weather patterns one way or the other. So, that’s a difficult part.

Paul Rand: But the framework Nakamura validated with the Pacific heat wave could also be used by climate scientists.

Noboru Nakamura: The framework we worked on is really to help climate scientists to quantify the rate at which the statistics change in response to climate forcing.

Paul Rand: This is how it works.

Noboru Nakamura: Climate and weather operate on very different time scales. So in order to talk about frequency of extreme weather, you need many, many events to make up for a robust statistics. Extreme events are relatively rare. So, in order to build credible statistics, you have to have very long record of data and very long climate simulations by computer models, which are very resource intensive and expensive. We came up with a theoretical framework to streamline that process and looking at some simple metric that characterizes the waviness of the jet streams. Like I said, the jet stream’s behavior affects weather in the mid latitude in many ways, but if we were to pick one simple metric of weather variability in the mid latitude, that would be the degree of meandering of the jet stream.

Noboru Nakamura: We use fluid dynamical principles to quantify this metric. We also have the equation that governs this evolution of this meandering of the jet stream. So that’s a relatively simple equation. You can call it the equation of motion for the jet stream, if you will. We can play a game, by forcing this simple framework with some hypothetical statistical properties of forcing and into the fluid dynamical forcing associated with weather variabilities or things like that, how the jet stream will respond to a slightly changed forcing magnitude or if the mean speed of the jet stream gets faster or slower, how the meandering will change and so forth. So this is a relatively inexpensive framework that allows climate scientists to explore relatively efficiently, inexpensively to build credible statistics and understand how the statistics will respond to perturbations to the climate.

Noboru Nakamura: We consider this as a tool to connect dynamics and statistics and build a confidence in our understanding. It doesn’t necessarily improve day to day predictability of extreme events. For that, you still need very sophisticated, complicated model. We have to keep improving the level of sophistication of climate models to do that. But in order to really understand the long term change in the statistics in response to the climate forcing, we need a different tool, and this is what our contributions are.

Paul Rand: We might not be able to predict what extreme weather will look like 20 years from now, but using statistics, we can at least predict how climate change might influence it.

Noboru Nakamura: You really can’t predict weather two weeks ahead of time, but how come can you predict climate 20 years ahead? That’s a good question. Climate is what you expect, weather is what you get. So the connection between the two is not necessarily causal. Climate is an average of weather. The dynamics of day to day weather variability is very complicated. We have only a limited predictability of weather that way.

Paul Rand: If we can use these indicators combined by Nakamura’s framework, we might be able to spot future deadly heat waves before they hit.

Noboru Nakamura: There is no unified theory about the cause of heat waves, but then the heat waves are affected by multiple factors. The good news is that heat wave is probably more predictable than other storm related hazards that tend to catch people by surprise. So at least we have a few days of lead time to prepare ourselves typically. So, find the nearby shelter or cooling stations, and if you have to transport your relatives or elderly relatives to cooling station, you have to coordinate your effort. Then this is also very important for the energy industry to have some lead time to adjust the energy output accordingly. Sometimes it may be necessary to schedule intentional blackouts or things like that in extreme cases. So it’s very important to pay attention to weather predictions because weather predictions are still a very powerful tool when it comes to natural hazards, particularly heat waves that are more predictable than other forms of natural hazards.

Matthew Hodapp: Big Brains is a production of the University of Chicago podcast network. If you like what you heard, please leave us a rating and review. The show is hosted by Paul M. Rand and produced by me, Matt Hodapp and Lea Ceasrine. Thanks for listening.

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