Insights & News

Stratosphere Warming Events

14th December 2018

As detailed in my piece on the stratosphere and the polar vortex, the polar vortex is a vast atmospheric circulation that strongly influences mid-latitude weather patterns between mid-autumn and mid-spring.

It doesn’t have an easy existence, mind. It’s frequently pushed, jabbed at or even violently squeezed by waves of energy surging up from the troposphere into the stratosphere.

Yes… it’s an almighty battle, routinely enacted for half the year at a time.

 

Normal Service

Without strong enough assaults from the troposphere, the polar vortex dominates the polar stratosphere.

At its strongest, satellites have observed a vast donut of sustained wind speeds, peaking in the 200s mph in the mid-to-upper levels (60-90 km altitude). You can see an excellent depiction of this below, courtesy of earth.nullschool.net (thanks to creator Cameron Beccario).

Figure 1: A strong polar vortex as observed via satellite measurements of air motion in the upper stratosphere. Source: https://earth.nullschool.net/#current/wind/isobaric/10hPa/orthographic=-28.36,83.70,337/loc=-166.806,70.144

Figure 1: A strong polar vortex as observed via satellite measurements of air motion in the upper stratosphere. Source: https://earth.nullschool.net/#current/wind/isobaric/10hPa/orthographic=-28.36,83.70,337/loc=-166.806,70.144

What we’re interested in right now, however, is what happens when energy waves from the troposphere distort or even break down this (sadly metaphorical) donut.

Messing with the Temperatures

Those energy waves excite the atmosphere within the stratosphere. Constituent atoms vibrate more vigorously than before. We observe this as an increase in temperature.

Now, a couple of key points:

  • The polar vortex owes its existence to temperatures being a lot lower above the Arctic region compared to the mid-latitudes (explained in the stratosphere and the polar vortex).
  • Energy waves are associated with big movements of air from warm to cold regions; typically from outside the Arctic to within it.

Yes – energy waves mess with the temperature pattern that the polar vortex depends on.

The consequences of this depend on the total strength of the associated energy transfer and whether it’s occurring in one region (known as wave 1 activity) or two (wave 2 activity).

Let’s look at the more straightforward case of wave 1 activity first.

Distortions & Displacements

Adding a lot of energy to one region of the stratosphere causes that air to expand, for that’s what gases do when they’re becoming excited. It’s fair to imagine that the atmosphere ‘balloons’ upward and outward.

This ‘ballooning’ can be observed by a measure of the atmosphere called geopotential height* (GPH). The excitement also results in a measurable increase in temperature (stratospheric warming).

Within the core of the polar vortex, atmospheric pressure typically reaches 10 hPa at 28 to 29 km altitude. Around it, heights of 30 to 31 km are typical, but strong wave activity can raise it by another km or so.

A big dome of increased heights effectively ‘pushes’ on the polar vortex. This causes it to elongate and become displaced away from the geographical pole.

Knocked off its perch like this, the vortex is no longer fully aligned with the Coriolis Effect imparted by the Earth.

In the example shown below (Figure 2), the wave-1 stratosphere warming of 21st Feb 2008 (right) can be seen to have displaced the polar vortex a good way toward Europe.

Figure 2: Maps showing geopotential height of the 10 hPa level, for an example of a strong polar vortex with no stratospheric warming impacts (left) and an example of a polar vortex displaced by wave-1 warming (right).

Figure 2: Maps showing geopotential height of the 10 hPa level, for an example of a strong polar vortex with no stratospheric warming impacts (left) and an example of a polar vortex displaced by wave-1 warming (right).

The poleward half of the vortex now has the Coriolis Effect working against it. This leads to gradual weakening, as does warming of the air within the vortex due to increased exposure to the sun’s rays.

The polar vortex doesn’t tend to dissipate, though, which is not true of the stratospheric warming. The processes driving wave-1 activity just aren’t sustainable enough. Before long, they give way and the stratospheric warming subsides. The polar vortex can then return to centre itself near or over the pole.

Squeezes & Splits

Wave-1 activity is all very well, but it’s not nothing on wave-2.

Put simply, wave-2 pushes against the polar vortex from opposite sides simultaneously, in a pincer movement. It’s as damaging as it sounds – the vortex isn’t just displaced, its divided.

As you can see in Figure 3, the resulting ‘sibling’ vortex circulations (right) are much weaker than that of a strong, untroubled vortex (left).

Figure 3: Maps showing geopotential height of the 10 hPa level, for an example of a strong polar vortex with no stratospheric warming impacts (left) and an example of a polar vortex split by wave-2 warming (right).

Figure 3: Maps showing geopotential height of the 10 hPa level, for an example of a strong polar vortex with no stratospheric warming impacts (left) and an example of a polar vortex split by wave-2 warming (right).

They’re also markedly weaker than tends to be observed of a displaced polar vortex. The cold air that the vortex depends on has first been divided up, then shoved even further away from the North Pole than is typical of a displacement event, leading to even more warming by the sun’s rays.

This makes it more difficult for the polar vortex to recover. Compared to a displacement event, it can take several weeks longer. Sometimes, if a split happens in late winter or early spring, it never does, before the inevitable mid-late spring dissipation.

 

Wacky Weather Patterns

By weakening the polar vortex, both displacement and split events have significant effects on the weather patterns we experience in the troposphere.

Essentially, large swathes of the hemisphere experience a reduction in the driving force that propels weather systems eastward. This permits more in the way of northward and southward movements, transporting more cold air equatorward and warm air poleward.

Not only that, but weather events, moving more slowly, effect locations for longer. This equates to increased extremity of severe weather – whatever may occur, from heavy snow to drought.

Splits Hit Harder Than Displacements

Broadly speaking, a displacement event over the Arctic significantly affects Northern Hemisphere (NH)** weather patterns for 3-6 weeks. There is a tendency for 1-2 weeks of unusually cold, often snowy weather to impact North America and western or eastern parts of Asia. Occasionally, Europe is also affected.

A split event has significant impacts for much longer: 6-8 weeks. As if that wasn’t enough, most places in the NH typically experience up to 4 weeks of cold, sometimes snowy weather. Different locations see it at different times – and before or afterward, it can be extraordinarily mild, wet, dry, or some combination of those.

To best illustrate this, for each event type, I’ve looked at the coldest fortnights within the typical impact periods, for events in the NH (Figure 4).

Figure 4: Maps illustrating the coldest fortnights typically seen during the impact periods following displacement (top) and split (bottom) sudden stratospheric warming events. This has been achieved by identifying the lowest 14-day mean temperature for each event, taking a mean across all events, then subtracting the 1981-2010 long-term average from it.

Figure 4: Maps illustrating the coldest fortnights typically seen during the impact periods following displacement (top) and split (bottom) sudden stratospheric warming events. This has been achieved by identifying the lowest 14-day mean temperature for each event, taking a mean across all events, then subtracting the 1981-2010 long-term average from it.

The cold temperatures are far more widespread and intense for the split type event. In the modern climate, it’s the most reliable precursor to widespread severe winter weather in the mid-latitudes of the NH.

That’s why I’ve written nearly 1,500 words detailing stratospheric warming events and the difference between the displacement and split varieties!

 

James Peacock MSc

Head Meteorologist at MetSwift

* The geopotential height (GPH) is the altitude at which the air pressure falls to a reference value. That value is usually given before the measure e.g. ‘500 hPa geopotential height’ – altitude at which air pressure is equal to 500 hectopascals. GPH changes depending on both the temperature and the vertical motion of the air.

GPH reduces with temperature. The colder air is, the denser it is, meaning more weight per unit volume. This causes it to become more concentrated toward Earth’s surface. This means for an observer gaining altitude within it, the amount of air pressing down on them from above is reducing more rapidly than would be the case in warmer air. So, air pressure reduces to the reference value more quickly; the GPH is lower.

For similar reasons, downward movement of air also reduces GPH, due to the air being compressed, making it denser. Rising motion has the opposite effect. This is due to complex reasons involving the Coriolis Effect and how it affects vertical and horizonal air motions differently.

GPH does not fluctuate with changes in vertical motion as much as sea level pressure (SLP) does. This tends to produce a clearer picture of how weather patterns are evolving over time, or what patterns are dominating over a week or a month. This is why you often see GPH used instead of SLP for such activities.

** Some of you may be wondering, why did I switch focus to the NH?

Well, it comes down to the number of historical cases and how much land area is affected. In the Southern Hemisphere (SH), only a few displacement events have been observed and just one split event. Contrast that with dozens of both types in the NH!

The reason for this rarity takes us back to those all-important energy waves.

They arise from weather systems interacting with areas of high terrain, especially mountain ranges. Those are aplenty in the NH, but the SH only has one of any significance (the Andes). In fact, most of it is water – as flat as you can get!

So, the SH polar vortex is rarely interfered with much. It’s typically much larger and stronger than its NH counterpart, and takes 1-2 months longer to dissipate during the SH spring-into-summer.

The one split event, in Sep 2002, resulted in exceptionally stormy weather across the southern halves of Peru and Argentina. Effects on the southern reaches of Africa and Australia were relatively minor. Most of the wacky weather occurred over the Southern Ocean, which fully encircles Antarctica.

While I could analyse the few known SH events like I did for the NH ones, the results would be unreliable; too much risk that weather patterns are actually responding as much, or more, to other driving forces.

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