When I started my Milankovitch cycles video with a reference to Game of Thrones,
I didn’t realise exactly how apt my analogy was. In that TV show, the armies of the Night
King in the north are trapped behind a giant wall. Just like in the TV show, it turns out
that on Earth there exists a powerful, icy force to the North that is seeking to overflow
its bounds and rush southward. That powerful winter is also kept at bay by a mighty wall,
one that allows the nations to the South to enjoy relatively tranquil conditions.
And just like in the TV-show, that wall eventually gets breached, in a wave of ice
that sweeps down and threatens the lives of all those in its way. You might not recognise what
I’m talking about. Where is this wall? And what is the icy winter it protects us from?
The answer to the first question is a name that I find particularly cool; the Polar Night Jet. And
it turns out that the Polar Vortex it protects us from is a biting chill not to be underestimated.
What is the Polar Vortex? And how does the Polar Night Jet protect us from it? I’m Alex McColgan,
and you’re watching Astrum. And today we’ll learn more about this climate-shattering phenomenon,
and what happens whenever the Polar Night Jet breaks down and the polar vortex is unleashed.
There are actually two polar vortexes: one at each pole of the planet. And even there,
each vortex comes in two parts: a tropospheric polar vortex, spinning in the section of the
atmosphere known as the troposphere, from ground level up to about 10 or 15 km. This is where 75%
of the total mass of the atmosphere resides. Above that lies the stratospheric polar vortex,
a technically separate weather phenomenon that has its own size, seasonal cycle,
and influence on the global climate. Each of these massive cyclones sits over the pole,
spinning with the planet’s rotation, with wind speeds that can reach up to 240 km/h.
Why do these winds happen? The first piece of the puzzle is temperature difference.
At locations like the equator where sunlight is most concentrated, the air is warmed and starts
to rise. As it does so it creates an area of low pressure beneath it that draws air into it from
its surroundings like a giant vacuum cleaner. Meanwhile, at places like the poles where it’s
much colder, air contracts and falls, creating zones of high pressure where air molecules want to
spread out like a crowd of school children being released into an open field. So naturally, with
these two forces at play, there is a tendency for air to rush from the poles towards the equator.
Freezing cold winds are constantly trying to escape from the North and South Poles.
This model is a little simplified, though, as wind does not travel in one continuous
line from the pole to the equator. Instead, because air from the equator cools and falls
much sooner than the pole (at around latitude 30°), and air from the poles warms and rises
much sooner than the equator (at latitude 60°), there are 3 cells of air on each hemisphere that
air circulates in. The Polar cells (the air masses above the poles) and the Hadley cells
(the air masses above the equator) both cycle in the temperature-driven way I described.
However, the middle cell – known as the Ferrel cell – is not temperature driven. Like a gear,
it is dragged by the rotation of the other two cells and rotates opposite to their motion.
In terms of our northern polar vortex, this means that once the air from the pole heads south,
it’s met by warmer wind travelling in the opposite direction. And when two air fronts of different
temperatures meet, they clash rather than mix. So, the Polar vortex is trapped, bounded,
clashing against winds coming in from all sides. There’s more at play here, though. If this was on
its own, the cold air from the North could just slide underneath the warm air from the south,
not really being trapped at all. There is a second force at play that redirects those winds, spinning
them up into a vortex that keeps them circling the poles rather than coming down towards the equator.
Where does the spin come from? It’s due to something known as the Coriolis force.
In a simple, flat world, cold wind from the poles would travel towards the equator, while
warm wind from the equator would float over it towards the poles. But the world isn’t a simple,
flat sheet – it’s a rotating sphere. You are travelling right now at somewhere between 0
km/h if you’re at a pole (and decided to watch an Astrum video while you were there ), and 1600km/h
at the equator, from the west to the east. You might not notice this fact because everything
next to you is on average travelling at the same speed, in the same direction.
But what happens if you were to travel from the equator towards the pole?
Conservation of momentum states that you would still be travelling eastward at the
same speed as previously, but suddenly the Earth beneath you is not travelling quite so fast.
Remember, at the pole, you’d have 0 eastward speed, but would simply be rotated slowly.
If you keep all your eastward momentum from the equator and travel towards the pole,
suddenly it will appear compared to everything else like you are travelling east really fast.
In practice, this means that air that travels up towards a pole from the equator (whether towards
the north pole or the south) will not go straight up, but over large distances will start to curve
towards the east. This rapidly eastward-travelling air is why you get jet streams. There are at least
4 of these, straddling the gaps between the Hadley cells, the Ferrel cells and the Polar cells. The
Subtropical Jet stream lies between the first two, and is a little weaker, but the jet streams we are
interested in are the polar jet streams, ringing the frigid air off that develops in the North and
South poles. These ribbons of air circle the globe in an almost continuous path, a little underneath
the boundary between the troposphere and the stratosphere. They are only a few kilometres deep,
but can be hundreds of kilometres wide, and in their hearts, the wind can travel at 400 km/h.
As a reminder, over 120 km/h is getting into strength level of hurricanes.
The jet stream around the south pole is fairly stable. Its powerful winds
overrule the polar winds trying to leave the polar air mass, whipping them along
with it and dragging the entire polar cell into a massive, antarctica-spanning vortex.
At this point a keen-eyed observer might have noticed a flaw in this model. If conservation
of momentum means that air going from the equator towards the pole veers towards the east, why is
it that air travelling from the pole towards the equator doesn’t do the exact opposite?
It has zero momentum, moving to zones that have considerably more momentum – comparatively,
it should be quickly left behind, appearing to start spinning to the west. This is true, but
cold wind has much more friction to contend with as it starts to slide underneath the warm front,
and then drags along the ground. This seems to slow it down enough that the countervailing
jet stream overrules it. It’s important to note this tension at play, though, as it becomes much
more important in the Polar Night Jet. The Polar Night Jet is the Jetstream that bounds the vortex
at the North pole. Specifically, it bounds the stratospheric polar vortex, keeping it in check
during the coldest part of the year for the North – the polar night. Here during the winter months,
the sun is absent from the sky entirely, creating even more freezing temperatures.
Interestingly, this colder climate creates a deeper pressure difference between the air around
the pole and the air further south, which actually strengthens the forces that create the Polar Night
Jet, meaning that during the coldest part of the year, this freezing air is usually well contained.
However, this does not always hold true. There are things that can disrupt a Jetstream.
There are certain zones, such as the boundary between sea and land, or the presence of a
large mountain, that can cause disruption to wind. Coastal environments create their own winds that
can suck in jetstreams, while mountains force an air current to move around it.
Even other weather phenomenon such as El Nino, which you might recognise from one
of my recent videos, can have an impact on the path the Jetstream takes. As the
Jetstream is not fixed down, but is a balancing point between a range of opposing forces,
hitting such obstacles causes it to deviate from its course. And once it starts deviating,
it will rock back and forth like a string that has begun bouncing. It shifts, no-longer is in
balance, overcorrects itself, and is no longer in balance again, and overcorrects itself again,
in massive planetary waves that cause the polar night jet to meander around the Earth, rather than
travel in a straight line. And these oscillations can reach a point where there is a breach.
The first sign of this comes in the form of a Sudden Stratospheric Warming,
most common in late winter. An SSW even can represent a time where temperatures rise in
the polar region by as much as 50°C over the course of just a few days. Something within
the system of the Jetstream can be so thrown by this that it leads to the southern-moving
westerly winds overpowering the Jetstream, partially or completely reversing its flow.
No longer contained, arctic wind moves south and meets warmer and warmer air, and pushes faster and
faster south to attempt to balance the gradient. The entire jet stream buckles, and suddenly,
it pivots. It massively reorients itself, travelling down the planet so that regions like
Europe and America – usually safely on the warm, temperate side of the polar night jet – suddenly
find themselves in the domain of the polar vortex. The forces of winter have arrived.
In fairness, not all of these events are devastating. With sufficient preparation,
you can simply put on some warm clothes, or try to avoid going outside for the month or so that
the polar winds are overhead. As long as you’re ready for cold, it’s not the end of the world.
However, sometimes the outcome is serious. In the UK in 2009-2010, the Big Freeze saw parts
of Scotland reaching temperatures as low as -22°C, the coldest in nearly 40 years,
with widespread transport disruption, event closures, and power failures.
Sadly, this in turn lead to the death of 25 people. In the US, the 2019 January-February north
American Cold wave saw a polar vortex move down across much of the country, with similar outcomes.
Some areas saw temperatures as low as -50°C if you also take into account windchill factor from the
blustery, freezing winds. Snow storms raged. You could get frostbite from being outside in
just 10 minutes. Sadly, another 22 people died, with hundreds more needing frostbite treatment.
Responsible was the ranging polar vortex. In time, the imbalances in the global temperatures
restore themselves, and the jetstream returns to its previous position. However, it’s worth
noting that some level of Jetstream breakdown occurs in the north 6 times every decade.
If you live in the Northern Hemisphere, you will likely see many more of these events over the
course of your lifetime, although hopefully not all as powerful as those two examples.
In the South, you are likely safer – there have only ever been 2 instances
in recorded history of the Southern Polar Jet Stream breaking down in the same way as the
Northern one. It has happened, though, and the mechanisms behind it are not fully understood.
It’s difficult to say, as global temperatures gradually rise, what influence this might have on
the Jetstreams. Some evidence indicates that they are travelling further poleward on average, year
on year, although this is apparently not unheard of in the planet’s history. There is some more
evidence that the jetstreams have strengthened since 2002. If so, we should be grateful. Although
unpleasant, the biting cold of the polar vortex usually is only a passing weather phenomenon.
We see it return north within a month. But if the Jetstream were to go, the polar vortex would come
down from the North to stay, and we’d truly know what it is like to live in arctic conditions.
The more I’ve learned about the subject, the more I’ve discovered that the winds of our planet are
this fascinating weave of interplaying forces and effects that tug and pull on each other,
finding perfect balances and yet constantly shifting in rhythms and patterns. And yet,
it does so practically invisibly. So much is going on that we simply do not see,
down here on the ground, just because air is, well, air. And yet, I’ve learned its
importance. The Polar Night Jet is not just wind with a cool name. It is a bastion of protection,
a wall against the frozen wind. It really makes you think of the incredible majesty
of the world around us – how much is going on that protects us, that we simply do not see. If you’ve
enjoyed learning about the jet stream and the polar vortex, don’t worry; there’s more
to discover. Jet streams as a phenomenon were not always understood, and there’s an excellent
article on them called “Searching for the River of Wind” on the website of today’s sponsor, Nautilus.
Nautilus is a fantastic science publication that I’ve become really impressed with lately,
for its accessibility, strong narrative sense, and fascinating insights. Its award-winning authors
are scientists from many different disciplines, so you get cutting-edge research on all sorts
of topics, including one near to the heart of any Astrum viewer; Space! Don’t just take my word for
it, though – in a rare event, Nautilus is offering a 15% discount on their yearly membership,
so now’s the perfect time to go see for yourself. Click on my link in the description below!
Thanks for watching! If you liked this video, you’ll love these other videos on the various
cycles of Earth. A big thanks to my patrons and members. If you want your name added to this list,
check the links below. All the best, and see you next time.