Cooking up a stratospheric polar vortex disruption

When we last wrote two weeks ago, the forecasts suggested that the stratospheric polar vortex would continue to weaken and warm into January. Though there were a few days around the end of December where forecasts moved closer to predicting a full reversal of the west-to-east winds at 60 degrees North* that comprise a major “sudden stratospheric warming”, the ingredients did not fully come together. Instead, in the last few days there was a minor warming of the vortex. Temperatures in the mid-stratosphere (~19 miles above the surface) rose about 30 degC (55 deg F!) during the 6-day period between December 30 and January 5th, and the winds at 60 degrees North slowed considerably but did not reverse.

Observed and forecasted (NOAA GEFSv12) wind speed (top) and temperature (bottom) in the polar vortex compared to the natural range of variability (faint shading). For the GEFSv12 forecast issued on January 7, only a few model runs predict a reversal of the vortex winds (top, thin magenta lines), and the average of all the runs (thick magenta line) predicts wind speed and direction will be near normal. Individual forecasts for temperature (bottom graph, light red lines) average out to just below normal temperatures (thick pink line). NOAA Climate.gov image, adapted form original by Laura Ciasto.

So what happened? Though we certainly don’t have a definitive answer yet, we can start to understand possible reasons by delving into how the stratosphere and the troposphere come together to bake up these sudden warming events, using a few key ingredients.

Ingredients for a sudden stratospheric warming

A pulse of planetary-size atmospheric waves

Yes, the atmosphere, like the ocean, has waves! footnote 1] And waves are key to how the stratosphere and troposphere communicate. In the troposphere, planetary-scale atmospheric waves arise from air blowing over mountain ranges, temperature differences between the land and the ocean, and even from far away tropical thunderstorms that drive persistent wave patterns called [teleconnections. If these waves sit in just the right place for long enough, they can grow and amplify enough in the vertical direction to reach the stratosphere, where the waves can break. This is analogous to how ocean waves break on a beach, but in the atmosphere, the waves instead break in the stratosphere which can slow the polar vortex winds.

There are two types of persistent weather patterns that typically precede sudden warmings: either unusually low pressure over the Aleutian Islands and high pressure (or blocking) over the North Atlantic, or else simultaneous high-pressure/blocking over both the Aleutian and Ural regions. The location of these patterns are in just the right place to amplify existing non-moving (or “stationary”) wave patterns that form in response to mountain ranges and land-sea temperature contrasts, especially the largest atmospheric waves [footnote 2], helping them to reach the stratosphere.

Atmospheric conditions in the troposphere in late December 2023 (left globe) did not match either of the two patterns (center and right) that usually precede major disruptions of the polar vortex. When planetary waves in the troposphere adopt either of the configurations shown in the middle and right-hand globes, their energy is amplified, enabling them to push upward and break into the stratosphere. The configuration in late December had low pressure over the North Pacific (gray), which is part of the "wave-1" pattern that often precedes the polar vortex getting displaced off the pole. But the pattern lacked the high pressure (red areas) across Greenland and the eastern Arctic. NOAA Climate.gov image, adapted from original by Amy Butler.

A dash of the perfect west-to-east wind speed in the stratosphere (not too fast, not too slow!)

The biggest atmospheric waves [footnote 2] have a particular requirement: they can only travel vertically if the winds through which they are traveling are blowing from west-to-east and also not too fast. In general these wind conditions are met in the Arctic winter months. However, note that this requirement means the waves can’t go into the stratosphere in summer, when the winds blow from east-to-west instead, or right after a major warming, when the winds reverse direction.

An ideal stratospheric “mixing bowl”

The stratospheric polar vortex is often bombarded with waves from the troposphere with little impact. Conversely, sometimes major stratospheric warmings occur with no apparent influence from the troposphere; in fact, probably only about ⅓ of these events are preceded by anomalously strong atmospheric wave pulses from the troposphere. So something else must be needed. We think that, though tropospheric wave forcing can certainly help, the stratosphere may need to be in just the right configuration for a stratospheric warming to occur. For example, we make bells, well, bell-shaped because that shape maximizes the “resonance” of the sound waves when the bell is struck, creating deeper and richer tones as the waves reflect off the inside of the bell. Similarly, if the polar vortex is in the right shape, it can resonate atmospheric waves within itself, leading to wave amplification and ultimately, a sudden stratospheric warming.

If we look back over the last couple weeks, we saw a large amplification of wave activity within the stratosphere itself, suggesting that the second and third ingredients were in place. This allowed the atmosphere to bake up a minor warming, but this time it wasn’t enough to fully rise to a major event (an atmospheric baking fail, evidently). A likely contributing factor is that the first ingredient, a pulse of waves coming up from the troposphere, was largely absent; the tropospheric weather patterns that often precede major stratospheric warmings weren’t in place. If that had been part of the mix, it might have given the atmosphere the extra heat needed to fully disrupt the vortex.

On the menu

What’s the atmosphere cooking up next? Forecasts suggest that the polar vortex will continue to stay weak for the next 10-14 days, with some forecast systems even showing another chance at a major stratospheric warming event. Meanwhile the vortex has changed from being displaced towards Europe to becoming more elongated across North America and Scandinavia, with the possibility of the polar vortex splitting into two lobes in the lowermost stratosphere- an indication of how unstable the vortex has become.

Atmospheric thickness (geopotential height of the 10-millibar pressure level) showing the polar vortex on January 1, 2024 (left), and the forecast for its shape and location on January 14 (right). When the polar vortex is shifted or stretched out off the pole to lower latitudes, it can sometimes help reinforce wintry weather outbreaks. NOAA Climate.gov image, based on Global Forecast System analysis and forecasts.

However, there remains considerable uncertainty in how the polar vortex will evolve after mid-January. Meanwhile, there are indications that even this minor warming may help reinforce the chances of colder weather patterns in some areas over the next couple of weeks- stay tuned!

[*Editor's note. Revised on April 26, 2024. This sentence originally said, "...predicting a full reversal of the west-to-east vortex winds...". It's been revised to prevent confusion between the polar vortex as the average atmospheric flow of the polar stratosphere and as a specific manifestation of that circulation on a given day. For more context, read We're going to stop saying "polar vortex reversal".]

Footnotes

• One of the ways to “see” atmospheric waves is to look at a weather map, where they show up as High and Low pressure systems (the “ridges” and “troughs” of the wave, respectively)
• The biggest waves in the atmosphere span the size of a hemisphere; we describe a wave as being “wavenumber-1” if it has one ridge and one trough that span a circle of latitude. Likewise, a wave is “wavenumber-2” if it has two ridges and two troughs around a circle of latitude. Generally, only waves of wavenumber 1 and 2 are large enough to make it into the stratosphere, though higher wavenumbers are common year round in the troposphere.

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