Polar Vortex Intensifications: the overlooked influencer

The phrase “polar vortex” is often associated with words and phrases like “cold”, “sweater”, and “please, please, please don’t let my furnace break down now.” The disruption and weakening of the polar vortex winds do increase the odds of a cold air outbreak across the eastern US and northern Europe/Asia (but they don’t guarantee it). However, the polar vortex is a multi-faceted phenomenon. Read on to learn about the less-hyped side of the polar vortex.

There’s more to the polar vortex than just disruptions

We spent much of last season talking about the repeated disruptions in the polar vortex. This is when the stratospheric winds at 60 degrees North weaken substantially (or reverse direction in the case of a major sudden stratospheric warming) as the vortex is often pushed entirely off the pole or stretched like taffy. 

This year, however, the polar vortex is showing a different side of itself. The strong, stable version of itself. For much of the winter season since November, the stratospheric winds at 60N have been well above the average speeds for this time of year. When these stratospheric winds become strong enough, it is referred to as a polar vortex intensification event. During these events, the polar vortex tends to sit more closely over the pole [footnote #1] isolating the cold stratospheric air within it.

From a chilly polar vortex to a toasty troposphere

Similar to polar vortex disruptions, polar vortex intensifications in the stratosphere can communicate themselves down through the troposphere to the surface and affect our weather patterns. To get an idea of what they might look like, we first need to identify the polar vortex intensification events. Unlike stratospheric warming events, which are classified as events when the winds reverse direction, there’s really no clear threshold that marks a polar vortex intensification event. For this analysis, we’ll define an intensification event when the west-to-east winds exceed the 80th percentile, or strongest 20 percent, of daily winds [footnote #2] for at least 10 days between November and March.

By taking the average of the surface temperature and atmospheric thickness for the 30 days after all the polar vortex intensification events in the observational record, we can average out day-to-day variations in the weather and see more clearly what persistent weather patterns look like after the polar vortex intensifies substantially.

How atmospheric thickness (left) and surface temperature (right) compare to the 1958-2023 average for the 30 days after the 48 observed polar vortex intensification events from 1958-2023. On average, intensification events are followed by lower-than-average thickness (purple) at the 500 hectoPascal pressure level, or about 3.5 miles (5.5 km) in altitude, and cooler-than-average surface temperatures (blue) across much of the Arctic. Across the mid-latitudes, it’s the opposite. NOAA Climate.gov image based on ERA5 reanalysis data provided by Amy Butler.

During these intensifications, the atmospheric thickness decreases over the pole, shown in these maps as a negative anomaly in tropospheric geopotential height over the Arctic, particularly Iceland and northern Norway. Around the Pacific, North Atlantic, and Europe, the atmospheric thickness is greater than normal [footnote #3].

An intensified polar vortex tends to coincide with a more northward, less wavy polar jet stream that keeps Arctic air from spilling into lower latitudes. (Remember, though, the polar vortex is not the jet stream. They are separated vertically by many miles, but they can become coupled and move in sync on occasion.) Because of this pattern, some parts of the Northern Hemisphere mid-latitudes can warm up in the 30 days after a polar vortex intensification. This is especially true for much of northern Asia and parts of Europe. For North America, the effects are less clear. Much of Canada and Alaska tend to lean slightly cooler than average, but the rest of the US, particularly the eastern half, don’t experience noticeable differences from the average temperatures.

The point of these maps is to provide a sense of where changes in temperatures and pressures are most likely to occur in the month after an intensification. As we’ve said before, maps of "typical" surface impacts are not a forecast and do not represent the expected day-by-day weather when one of these events occurs. These typical patterns associated with intensification events don’t always emerge after a specific event because either there’s no downward coupling, or if other factors are playing a larger role (ENSO, day-to-day weather).

The stratosphere and troposphere are mostly indifferent to each other. Differences from average atmospheric thickness (“standardized geopotential height anomalies”) in the column of air over the Arctic for the stratosphere and troposphere. Over the last two months, the stratosphere and troposphere have been largely uncoupled. The main exception was in mid-to-late December and late January when the low thickness anomalies (indicative of a stronger than average polar vortex) extended from mid-stratosphere to the surface. The latest forecast from January 29 2025 indicates that this weak interaction will continue for at least the next two weeks. Standardized anomalies are based on departures from the 1991-2020 Climate Forecast System Reanalysis climatologies and have been divided by the standard deviation. Data are from the Global Forecast System observational analysis and forecast. NOAA Climate.gov image.

So far, this year has been a good example of how the polar vortex doesn’t always impact weather at the surface and why we need to remember these typical impacts are not guaranteed. The polar vortex winds have been much stronger than normal, but other than some brief interactions in late December and late January, the stratosphere and troposphere have largely ignored each other. We did note a couple of weeks ago that the stretching of the polar vortex seemed to be in line with the extension of the jet stream, but we think the cold air outbreak that led to historic snow in the South was more likely influenced by other factors.

Looking ahead: polar vortex likely to remain intensified

Observed and forecasted (NOAA GEFSv12) wind speed in the polar vortex compared to the natural range of variability (faint blue shading). Since mid-November, the winds at 60 degrees North (the mean location of the polar vortex) have been stronger than normal. According to the GEFSv12 forecast issued on January 29 2025, those winds are forecast to remain stronger than normal for at least the next couple of weeks (bold red line). By the end of February, there is considerably more uncertainty whether the polar vortex winds will remain strong or weaken. NOAA Climate.gov image.

Current forecasts aren’t showing much of a change in the strength of the polar vortex. A few of the individual forecasts predict a strong weakening or reversal of the stratospheric winds, but the average of all individual forecasts indicates the polar vortex will remain stronger than normal for at least a couple of weeks. Furthermore, the latest forecasts do not indicate any significant interaction between the stratosphere and the troposphere. So for at least the next two weeks, the polar vortex seems content to stay strong but silent.

Footnotes

[1] In practice, a strong polar vortex is often slightly off the pole or elongated. This is partly because the polar vortex varies a bit on its own. As we described in a previous post, the strong west-to-east winds can be a great conduit for tropospheric waves to nudge the polar vortex. But when the winds become too strong, those waves are less likely to break. Sometimes that means instead of those waves nudging the polar vortex, they bounce right off and are reflected back to the troposphere.  We hope to talk more about that in a future post.

[2] We know this sounds overly technical so let’s explain a little more. For each day between November and March, we collect all the daily-mean wind values in the 1958-2023 ERA5 reanalysis record. From that collection, we find the value in which 80% of the winds are weaker and 20% are stronger. This is the 80th percentile.

[3] For those of you familiar with the terminology of the Arctic Oscillation, the tropospheric response to polar vortex intensifications is often associated with the positive phase of the Arctic Oscillation.