A weak jet stream and powerful weather

A special article on the Earth Day

In late February this year, the weather in Texas was… abnormal, to say the least. Temperatures dropped close to zero Fahrenheit in some places, leading to widespread power outages that have grabbed headlines in recent days. Conservatives point to it as proof that global warming is a hoax while liberals point to it, claiming it as proof that climate change is real. How common are these extreme events around the world, however, and why do they occur? And do they have anything at all to do with climate change in the first place?

Weather or climate change?

In short, abnormal weather can come in 4 varieties. Unusually cold or hot, unusually dry or wet. And most of us are well aware that extreme weather events have always occurred. There were always typhoons and hurricanes, well before global climate change became a worry. Monsoons have caused floods, droughts have parched regions and destroyed crops since antiquity. There is a tendency among many liberals to blame every extreme weather event that occurs these days on global climate change, but that really doesn’t make sense. Extreme weather didn’t begin in the last 50 years- it’s always been with us. When climate scientists are asked whether a particular weather event is the result of climate change, they typically hem and haw and give a vague answer along the lines of “well, no single weather event can be definitively linked to the growing climate crisis…”

Marshal Shepherd wrote in an article for Forbes, “I co-authored a 2016 National Academy of Science report on extreme weather attribution to climate change. In that report, we concluded that contemporary extreme events likely have climate change “DNA” in them… to me, it is like asking if the 358th home run hit by a Major League Baseball player using steroids was caused by the performance-enhancing drug. That player could likely hit a home run naturally without steroids. However, it is likely that a detectable influence of the steroids can be seen in his overall home run statistics (number and length).”

Climate scientists find it very difficult to ascribe any one particular weather event to climate change because these types of climate events have occurred throughout history apart from global warming, but there is clearly a link to climate change in terms of frequency and severity. Whether or not a particular event is caused by climate change is moot. What is critically important is whether or not climate change is making these events more common. Are they becoming more common, though? More on that in a minute, however, because addressing that question without fully understanding the physical processes creating these extreme events would be meaningless.

A jet stream: what is it? And why?

The jet streams are air currents that circle the globe and when we talk about it, we’re normally referring to the Polar Jets- in particular, the one circling the arctic circle. These air currents are strongest at 30,000 to 39,000 feet elevation and travel an average of 110 mph, though there is a lot of variability in the speed; the Polar Jet stream flows from west to east. We haven’t always known about the jet stream. In fact, it was only first truly understood during World War II when pilots over the Atlantic, Pacific, and Europe discovered incredibly strong winds that either helped or hindered them in their long distance bombing runs. In fact, American pilots tasked with dropping bombs over Japan sometimes ran out of fuel and crashed because they’d burnt it all fighting a 200mph jet stream headwind they hadn’t known existed!

Today, we know that the jet stream plays a major role in our day-to-day weather, driving storm systems predominantly from the west toward the east at the Northern Hemisphere’s mid-latitudes. What actually causes the jet stream in the first place, however, is a very simple yet fascinating process.

The globe is covered in what are called atmospheric circulation cells that operate in the troposphere, from the ground surface to a height of approximately 15 km. These circulation cells caused by the fact that the Earth is heated unevenly by the sun and the fact that warm air is less dense and rises compared to cold air. An idealized version of these air currents is shown below.


A circulating atmosphere

The Earth receives the most direct sunlight (and thus thermal solar energy) at the Equator. There, the warm air rises. Further north, the air cools and becomes more dense, dropping back to the surface. Now, the simplest way to envision this is for air to rise at the equator and then drop at the poles, but the distance is simply too great for a single circulation current to cover. Thus, the current divides into several circulation cells.

This first circulation cell that rises from the equator is the Hadley Cell and is responsible for the trade winds that allowed global trade to cross the oceans for hundreds of years. This cell drops air to the surface just outside of the tropics. Where the Hadley Cell drops its cool air, it meets the Ferrel Cell or Mid-latitude Cell, whose motion is mostly controlled by the circulation cells on either side of it and which is dropping its own cool air. This air has traveled south from nearer the edge of the Arctic Circle where the current is rising and meeting the Polar Cell. The Polar Cell rises from just outside of the Arctic and Antarctic Circles and collapses at the poles.

Even though you may never have heard of these convective atmospheric cells, it is difficult to overstate their importance to the global climate and our ability to live in a particular region. Many of the world’s wettest and most verdant regions are found where these cells are rising with warm moist air. Meanwhile, many of the world’s deserts spanning Africa, Asia, Australia, and the Americas are found where colder dry air is descending from the cells.


In the Northern Hemisphere, we are most interested in where the Polar Cell and Ferrel Cell meet, because this is what creates the Polar Jet Stream that so dominates our weather. The meeting of these cells and the resulting downdraft creates the Polar Jet via the Coriolis Effect.

Moving to the right

The Coriolis Effect is a simple phenomenon that can be pretty difficult to envision. The effect is this: on a large scale, due to the rotation of the planet, the motion of objects is deflected either to the left or to the right. In the Southern Hemisphere, moving objects are deflected to the left; in the north, they are deflected to the right. We won’t delve too far into the explanations for this effect because we’re trying to understand extreme weather events, not teach an entire university course on atmospheric physics.

So where the Polar and Ferrel Circulation Cells meet, the air is rising and deflecting to the right from either direction. By far, the dominant motion is from the west to the east, however. And that Coriolis deflection of air is what we call the Polar Jet Stream. To the north of that jet stream, you have the cold polar air of the Polar Circulation Cell, also known as the Polar Vortex. To the south of the jet stream, you have the warmer temperate air of the Ferrel Cell.

Rossby Waves: wiggles in the jet stream

For most of recorded history, these massive atmospheric circulation cells have been quite stable, as has been the boundary between them. There have always been fluctuations and shifts in the jet stream, harmonic variations known as Rossby Waves, that have resulted in pulses of cold arctic air making their way south. In recent years, however, this boundary has begun to weaken and the Rossby Waves have grown larger with extreme consequences.

On an almost annual basis during the last decade, the Polar Vortex- the swirling mass of arctic air contained within the Polar Circulation Cell and penned in by the Polar Jet Stream- has been wandering into the lower latitudes during extreme Rossby Wave events, resulting in deep freezes to Europe, Northern Asia, and North America. At the same time that the Polar Vortex makes its way south during these extreme events, that air is replaced in the north with warm southern air. Extreme Rossby Wave events have resulted in the 2013 European floods, the 2012 China floods, the 2010 Russian heat wave, the 2010 Pakistan floods, and the 2003 European heat wave. Deep freezes in the continental US have become much more commonplace during the last decade, with the most recent events in Texas being a dramatic example.

Why have these Rossby Waves grown larger and more frequent, causing meteorological devastation in normally temperate areas? We now have a firmer understanding.

A warmer Arctic and a weaker wobbly jet stream

The recent rise in global temperature is a well-established and thoroughly documented fact.


Moreover over, this global warming has been extremely uneven. Not all locations have gotten warmer and a few places, in particular, have gotten much much warmer than others. In particular, the Arctic has warmed far more than most of the planet, in large part due to reductions in snow cover and sea ice. That reduction in reflective ice and snow means more solar energy is captured and turned to heat, leading to further melting and the release of methane- a major greenhouse gas- into the atmosphere from thawing permafrost.


The addition of so much heat into the Polar Vortex weakens the Polar Circulation Cell- remember that this cell is driven by the extreme cold of the Arctic in comparison to the air in the south. As the Arctic warms, that temperature difference that drives it has become less extreme and the Polar Circulation Cell has weakened. The weakening of the descending air currents between Polar and Ferrel Circulation Cells has led to a weaker Polar Jet Stream. This weaker jet stream results in more pronounced Rossby Waves, which reach more dramatically to the north and south.

The result: for a few days or weeks, residents of Alaska go out sunbathing while Arizona is struck by blizzards. Texas gets weather it’s never seen before. California burns with wildfires while Colorado is buried under feet of snow just a few hundred miles away. Similar out-of-whack occurrences have been popping up sporadically across Europe and Asia- just a few weeks ago, Siberia was recording record-breaking cold weather of its own. Never before seen temperatures- both record cold and record heat- crop up more frequently across the world, driven by our now weakened jet stream. As global warming grows worse and as the Arctic continues to warm faster than the rest of the planet, the polar vortex will continue to weaken, as will the Polar Jet Stream. Stronger and more frequently severe Rossby Waves will push extreme cold southward and extreme heat northward on an ever more regular basis.

Experts are now warning that this is no longer theory, but an observed fact. The statistics are now clear: extreme weather and economic losses are growing year by year across the globe. Please have a look at the citations below to learn more about those studies!

In a nutshell…

So was the extreme weather in Texas the result of global warming? Was any one particular event caused by climate change? It’s impossible to say. But the climate is changing. A warming Arctic has weakened the atmospheric convection systems that create the jet stream and our weaker jet stream allows for more frequent extreme weather events. We understand how it works and we’ve seen the devastating results.

As our planet continues to warm, experts are warning that we need to be ready for the change. Abnormal weather will become much more normal.





















Attribution of Extreme Weather Events in the Context of Climate Change

By National Academies of Sciences, Engineering, and Medicine, Division on Earth and Life Studies, Board on Atmospheric Sciences and Climate, Committee on Extreme Weather Events and Climate Change Attribution https://books.google.com/books?hl=en&lr=&id=WWEpDQAAQBAJ&oi=fnd&pg=PR1&dq=weather+extreme+climate+change&ots=DfDCOPtjn9&sig=MMDZJaFQ3YUMqouKsi4JbyLmlq0#v=onepage&q=weather%20extreme%20climate%20change&f=false





Geologist, a lover of all science, father of a young child, published writer on Forbes and Mental Floss

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