There's been some severe winter weather over the past few weeks, thanks to the polar vortex. Is this just a typical weather pattern, or another sign of climate change. In this week's "Issues of the Environment," WEMU's David Fair looks into this phemonenon with Dr. Richard Rood, professor of Climate and Space Sciences and Engineering at the University of Michigan.
- Winter weather in southeast Michigan is predictably cold, but it is also highly variable which can lead to the perception that it is unpredictable. Factors that get tossed around are La Nina/El Nino, Arctic Oscillation (related to the polar vortex), and climate change. These elements each contribute to weather patterns in our area, but with varying importance.
- It seems that the record cold temperatures in Washtenaw County that result from the Polar Vortex are becoming the norm, but are they? Dr. Rood points out that the circling mass of polar air that has dipped down into Michigan several times in the past decade is becoming smaller and more easily displaced. He also recognizes that NOAA predicts that the 2018-19 winter may well be the warmest on record due to the interplay of El Nino and climate change, although Michigan tends to be insulated from some of these global changes due to the lake effect.
- Part of the reason weather is difficult to predict in Michigan has to do with the uncertainty of focusing on a relatively small region surrounded by massive lakes. As the area gets smaller, the uncertainty gets higher.
- The largest variability of winter temperature in Michigan due to the Arctic Oscillation. Arctic Oscillation is connected to the polar vortex, the predominant force shaping winter weather in southeast Michigan for the past several years.
The Polar Vortex and Climate Change
This, written by Richard Rood and published in the Washington Post this month, highlights the most important connections between the Polar Vortex and climate change.
February 8, 2019 - The author, Richard Rood, is a professor of atmospheric science at the University of Michigan.
A few years ago, the “polar vortex” surged into the public language of weather and climate change. Now, in 2019, when I ask my students to describe a cold wave in Michigan, they immediately talk about the vortex coming down from the pole. The story continues: The polar vortex is surrounded by the jet stream; the jet stream is getting more wavy; it is more wavy because of climate change, and likely, the loss of Arctic sea ice.
This is a concise narrative, reasonably extracted from news reports and knowledgeable scientists. As with most concise narratives of complex situations, there is a core of truths, surrounded by a quagmire of confused details.
There is, in fact, an official definition of cold wave, namely, a drop of temperature within a 24-hour period that requires increased protection of our enterprise. The cold wave of late January 2019 in the central eastern United States and Canada was characterized by temperatures dropping rapidly to values that are dangerous to human life and damaging to buildings and infrastructure.
In Ann Arbor, Mich., where I teach, the low temperature on Jan. 31 was minus-17. Farther to the west, in places where the temperature is less influenced by the Great Lakes, it was much colder. F or example, in Lacrosse, Wis., it was minus-31 on Jan. 30. The record or near-record cold temperatures came after record or near-record warm temperatures earlier in the month.
The cold wave was predicted days in advance, and people and institutions took steps to protect themselves. As predictable as the weather was the emergence of the political messaging that the reality of a cold wave showed the falseness of global warming. This political messaging is so predictable that scientists, like myself, reposted their explainers and prepared for calls from reporters.
Potential record-cold temperatures and threats to life and property are compelling events, as are the messages of such events. Plus, on its surface, it is, perhaps, paradoxical that we see record-cold temperatures on a planet that is, on average, definitively warming. It is, therefore, worth disentangling the confusion that surrounds the core of truths in my students’ ready explanation of the polar vortex coming to Michigan.
Because Earth’s rotation influences atmospheric motion, many weather systems can be described as either a vortex or a wave. A vortex in the atmosphere is, simply, air in a circular flow. Most are familiar with the circular motion of hurricanes or tornadoes; both are vortices.
The vortex of our wintertime cold waves is often called the polar vortex. It is, approximately, a circular flow that forms around the wintertime pole. This flow has a narrow high-speed wind field, a jet stream, at its edge. The jet stream isolates the air inside the polar vortex. That air is cold — very cold, because it is at the wintertime pole, and the sun is not present at the winter pole. Because the darkness at the wintertime pole is related to the tilt of Earth, relative to the sun, it gets cold at the pole whether or not Earth’s carbon dioxide is increasing and the planet is warming.
Therefore, even in a warming world, we have the physical processes that form cold air. There are important details about how the cold air is changing. For example, the area of very cold air should get smaller from year to year — a phenomenon that is already observed. The coldness, that is the absolute minimum temperatures that we might expect, would likely stay about the same as in the past, but air getting to those temperatures would be less common. Indeed, we would expect very cold air to become more rare.
In our warming world, there is more heat to transport and more energy to displace the cold polar air. We have the situation where a smaller vortex gets shoved around more than in the past. If that air gets shoved to different places than in the past, as it touches those places, we set records. New cold records are, however, increasingly rare.
The increased heat at the poles is going to melt sea ice and permafrost. The decrease of sea ice and Arctic warming might feed back to make the displacement of the wintertime polar vortex more common and its wanderings more ranging. However, it remains a matter of nuance, controversy and research.
There are other characteristics of the late-January weather pattern that are more telling than the small area of cold air transiting the eastern United States. At the same time, it was warmer in parts of Norway than in North Florida. Anchorage was above freezing for several days. Warm air is moving toward the poles, and from decade to decade that air is getting warmer, and its areal extent is increasing.
And, back in Michigan, less than a week after that minus-17, the temperature shot up to 51 degrees.
We seem to want to amplify our experience of coldness — to declare that we are cold, but we know the planet is warming up. What we are seeing is that the Arctic is relentlessly warming, and the little Arctic cold that remains is being unceremoniously shoved out. We are in a time when the climate is changing, and we should both psychologically and actually expect the unexpected.
Climate scientists warn that 2019 may be the warmest year on record largely as the result of a possible El Niño event exacerbated by man-made global warming.
There is a 90 percent chance that El Niño will form and continue through the Northern Hemisphere winter of 2018-19 and a 60 percent chance that it will continue into the spring of 2019, according to the Climate Prediction Center at the National Oceanic and Atmospheric Administration (NOAA).
El Niño is a part of a routine climate pattern that occurs when sea-surface temperatures in the tropical Pacific Ocean rise to above-normal levels for an extended period of time.
It can last anywhere from 4 to 16 months and it typically has a warming influence on the global temperature. The opposite of El Niño, La Niña, is when sea-surface temperatures in the central Pacific drop to lower-than-normal levels.
These warm and cool phases are part of a recurring climate pattern that occurs across this section of the Pacific, known as the El Nino-Southern Oscillation (ENSO), according to the National Oceanic and Atmospheric Administration (NOAA). The strong El Niño of late 2015 to early 2016 helped boost global temperatures to their warmest on record in 2016, according to AccuWeather Senior Meteorologist Brett Anderson. "However, if there was no El Niño during that period, I still suspect that 2016 would have still ranked as the second warmest year on record globally due to the steady increase in greenhouse gases into the atmosphere, which trap heat closer to the surface," Anderson said.
So far, 2018 is on pace to likely be the third warmest year on record, behind 2016 and 2017. "What's interesting is that 2018 started out under La Niña conditions, which usually has a cooling influence on global temperatures, but it was not nearly enough to cancel out the warming from the release of man-made greenhouse gases," Anderson said. However, since late April 2018, sea-surface temperatures across much of the east-central tropical Pacific returned to neutral levels following the La Niña of 2017-18, meaning neither La Niña or El Niño present.
Players in our Winter Weather
Predicting winter weather in our area is far from an exact science, and there are several reasons why. Dr. Rood pointed out that the size of an area is inversely correlated to the likelihood of an accurate prediction. From a weather prediction perspective, southeast Michigan is a speck on the globe, so it is hard to know with certainty if winter will be wetter, snowier, warmer, etc.. than average. Still, we try.
The past couple of winters have been deeply influenced by the polar vortex, and even if the La Nina pattern comes to pass, that could still be the case this winter. The polar vortex in heavily influenced by Arctic oscillation. Never before measured variability has been found in the Arctic over that past decade, most likely due to climate change. So, even if it was possible to absolutely predict the historically expected weather based on models, climate change could alter everything.
Though the term was only popularized in recent years, polar vortices aren’t anything new. The National Weather Service explains that a polar vortex — a large area of low pressure and cold air surrounding both of the Earth’s poles — always exists but weakens in the summers and strengthens in the winter.
“The term ‘vortex’ refers to the counter-clockwise flow of air that helps keep the colder air near the Pole,” the Weather Service explained. “Many times during winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream. This occurs fairly regularly during wintertime and is often associated with large outbreaks of Arctic air in the United States. Similar outbreaks of extreme cold were also reported in 1977, 1982, 1985 and 1989.
Here is a nice explanation from Dr. Rood that explains how the pattern could very well be tied in to climate change:
I will write from the point of view of the gardener or someone who likes to be outdoors and pays attention to the season and the weather. In the winter, the Sun becomes low in the sky because of the tilt of the Earth’s orbit. At polar latitudes, the Sun is below the horizon. There is no solar heating. It is dark at the pole.
During winter at the pole, the Earth continues to emit energy to space. This energy is emitted as infrared radiation. It gets cold. It is worth remembering that if there is no solar energy to heat the Earth, the Earth will get very, very cold. It would start to approach the background temperature of outer space. At the pole, in the winter, it gets cold, say, – 40 degrees below zero. (The cool thing about 40 degrees below zero is that this is where Fahrenheit and Celsius are equal.)
Here in the U.S., it is intuitive to the gardener that the winter is cold, and dark, and it gets colder and darker the farther north you go. It’s right there on the back of the seed packet.
The atmosphere responds to this cooling at the pole; whenever and wherever there is a hot-cold contrast, a temperature gradient, there is motion. The wind blows.
A fact of the Earth is that it rotates. That rotation strongly determines the winds; the motion of the air aligns with the rotation of the Earth. Something of a river of air, the polar jet stream, forms around the pole. Most outdoor people have gotten pretty familiar with the jet stream, and that the jet stream is sometimes wavier than at other times and that that influences the weather – a lot.
Now here is something that is important, that is not quite as intuitive. The jet stream that forms around the pole largely isolates the air in the polar region from air outside the polar region. Here is how I would develop some intuition, imagine you are next to a rapidly flowing stream and you put a leaf in the stream. Does it flow across the stream to the other side, or is it rapidly carried downstream? It is carried downstream, and therefore, one side of the stream is effectively isolated from the other. The jet stream around the pole, this river of air, effectively isolates the pole. Therefore, not a whole lot of heat is carried to the pole; the sun is down; it gets cold at the pole.
This isolation of the pole during the winter occurs, whether or not there is global warming. The Sun goes down for a long period of time. Without transport of heat to the pole, the pole can get as cold now as it did 50 years. It might take a few days longer, but if it is isolated long enough then it gets just as cold. So we have a store of cold air at high latitudes.
Here is another, perhaps less intuitive fact. For the rotating atmosphere of the Earth, the hot-cold contrast, the temperature gradient, represents a source of energy for atmospheric motion. The atmosphere does not like these gradients. It wants to mix them up. If it as cold at the pole as it used to be, and warmer outside of the pole, then there is MORE energy for that mixing. So when the mixing occurs it is, likely, more vigorous, more energetic.
With this more energetic mixing, then it is possible that when the jet stream is wavy, it is very wavy compared to history. It is possible that the cold polar air goes farther south than it used to go. And more warm middle latitude air finds itself at the pole. Previously isolated polar air is pushed off the pole. It sits over Asia, Europe – North America. For a time in the middle of the winter, it can stay cold for a long time. And up at the pole it is warm. And if that cold polar air is pushed just a little bit farther south than historical, it can be damaging record cold.
Dr. Rood explained that the Arctic Oscillation is often more important to Michigan than El Nino.
From Dr. Rood: “The way carbon dioxide changes the Earth’s climate is by changing the heating and cooling. A common comparison is to compare additional carbon dioxide to a blanket which holds the Sun’s heat closer to the Earth’s surface. This blanket causes the Earth to heat up more at the pole than at the Equator. The poles are also special because the Sun goes down for the winter and it cools off. In fact, it gets very cold, and as discussed in the previous blogs, the stream of air that gets spun up isolates the pole enough to let the cooling really get going. With these changes to heating and cooling, if we add a lot of carbon dioxide to the atmosphere, then it is reasonable to expect that the Arctic Oscillation might change.
The studies prior to, say, 2008, suggested that the effect of carbon dioxide being added to the atmosphere would be to cause the Arctic Oscillation Index to become more positive. This would be the pattern of the Arctic Oscillation where the cold air is confined to the pole; that is, the less wavy pattern (scientific references: for example, Kuzmina et al. 2005 and the 2007 IPCC AR-4). The studies prior to 2008 support the idea that the additional carbon dioxide is a leading suspect in the changes after 1960 noted in Figure 1. That is, without carbon dioxide increasing in the simulation, the models cannot reproduce the statistical characteristics of the observations and with it increasing, they can.
Those pre-2008 studies, effectively, only considered increasing carbon dioxide. They did not represent the huge changes in the surface of the Arctic that have been observed. Notably, sea ice and snow cover have declined. These surface changes also cause changes in heating and cooling. The decline of sea-ice, for example, changes the surface of the Arctic Ocean from white to dark. This changes the surface from a reflector of energy to an absorber of energy. Sea ice is also a temperature insulator; hence, without the ice the ocean and atmosphere exchange heat more easily. There are many other changes as well, but all I want to do here is establish the plausibility that large changes at the surface are also likely to change the behavior of the Arctic Oscillation. Why? Changes in the patterns of heating and cooling, leading to changes in high and low pressure systems, which then with the influence of the Earth’s rotation, change the waviness of the stream of air around the Arctic.
There have been a series of papers in the past couple of years that suggest that the changes in sea ice and snow cover are having large effects on the weather in the U.S. If you look across these papers, then there is growing evidence that the meanders (or waviness) of the Arctic Oscillation are getting larger and that storms over the U.S. are moving more slowly. Here is a list of quotes from these papers.
There is some controversy about the work connecting the changes in the sea ice and snow cover to changes in the Arctic Oscillation and to changes in extreme weather in the U.S. (Barnes (2013):Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes, Francis response, and Freedman @ Climate Central ).
I think there is significant merit in the work that connects changes in the Arctic Oscillation to increases in carbon dioxide and related changes to the surface of the Earth. Part of my intuition comes from a career of working with atmosphere models. If a model is radiatively dominated, then the vortex over the pole is very strong. In this case, there is little waviness in the jet stream. This is analogous to the case of increasing carbon dioxide and the Arctic Oscillation becoming more common in its positive phase. If a model is less driven by radiative forcing, then it is easier for the waves that are initiated by the flow over the mountains to grow and distort the edge of the jet stream – more waviness. This is like the negative phase of the Arctic Oscillation. Though in the end it will require a careful calculation of the energy budget, the removal of sea ice from the surface of the Arctic Ocean allows more heat into the polar atmosphere, which means the radiative cooling will be less intense. Hence, the vortex will be weaker or the Arctic Oscillation will more commonly be in its negative phase. If there are changes in the Arctic Oscillation, which are realized as changes in the waviness and speed of the jet stream around the Arctic, then there will certainly be consequences to the weather in the U.S.
Potential changes in the character of the Arctic Oscillation are an important issue for those thinking about how to respond to climate change. The loss of sea ice is a large change, which will undoubtedly have important impacts in the Arctic. It is reasonable to expect large impacts on weather at lower latitudes, in the U.S., Europe and Asia. The change in the Arctic sea ice has happened very rapidly. This challenges the assumption often made in planning that climate change is a slow, incremental process. The weather of the here and now and/or the next fifty years, a common length of time for planning, is likely to be quite different from the past fifty years. Since we rely on our past experience to plan for the future, this is a direct challenge to our innate planning strategies. If we are cognizant of the possibility of significant changes to weather patterns on decadal lengths of time, then we can develop new planning strategies that will improve our resilience and make our adaptation decisions more effective.
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