Is the polar vortex causing the cold air epidemic in the United States?

Parts of the Midwest and East Coast were blanketed by the first major snow of 2025, with temperatures dropping as fast as my New Year's resolutions. Have we got the stratosphere, the polar vortex in particular [footnote #1]to blame (or thank) for this recent cold air outbreak? We don't think so. This seems like a case where the polar vortex is accused of being a troublemaker, when in reality it is minding its own business. Well, tell you why below.

Snowy day on the East Coast. Photo credit: Laura Ciasto.

Our usual suspect has an alibi: the polar vortex was strong

The common contribution of the stratospheric polar vortex to cold air outbreaks over the eastern United States generally involves two key ingredients. First, the westerly (west to east) winds about 19 miles above us at 60 degrees north weaken significantly, or even reverse direction (east winds = sudden and major stratospheric warming).

Second, these changes in the polar vortex must communicate through the troposphere to the surface (stratosphere-troposphere coupling). If this communication and coupling took place, we would expect to see a wavier than normal tropospheric jet stream, with many dips and troughs. [footnote #2]. If one of these troughs extends southward over North America, cold air from the Arctic accompanies it.

Now, yes, that's what the jet stream is doing right now. But no, it's probably not the fault of the polar vortices.

On the one hand, the winds from the polar vortex are currently strong, much stronger than normal. In fact, in late December, westerly winds were stronger than they have been since at least 1990. You can see this in the graph below.

Observed and predicted wind speed (NOAA GEFSv12) in the polar vortex relative to the natural range of variability (light blue shading). Since mid-November, winds at 60 degrees North (average location of the polar vortex) have been stronger than normal. According to the GEFSv12 forecast released on January 5, 2025, these winds are expected to remain stronger than normal for at least the next few weeks (bold red line). NOAA Climate.gov image, adapted from the original by Laura Ciasto.

In addition, the troposphere seems little interested in the strong winds of the polar vortex. If the powerful polar vortex caused the troposphere to follow its behavior, we would expect to see thin anomalies in the stratosphere drain into the troposphere, leading to a much less wavy jet stream, which would keep the air cold confined to the Arctic.

Not much coupling! Differences from mean atmospheric thickness (standardized geopotential height anomalies) in the air column over the Arctic for the stratosphere and troposphere. For much of the recent period, until early November, the stratosphere and troposphere were largely decoupled. The main exception occurred in mid-to-late December, when thin anomalies (indicative of a stronger than average polar vortex) extended from the mid-stratosphere to the surface. The latest forecasts predict that this weak interaction will continue for at least the next two weeks. Standardized anomalies are based on deviations from the Climate Prediction System reanalysis climatologies from 1991 to 2020 and were divided by the standard deviation. Data comes from observational analysis and forecasts from the Global Forecast System.

Could the shape of the polar vortex be the cause?

Is the case closed? As shown in the graphs above, the polar vortex winds are very strong with very little interaction between the stratosphere and troposphere. So the polar vortex isn't the reason I'm shoveling my driveway?

Although we think it's unlikely, we can't rule it out entirely because the polar vortex and the tropospheric jet stream agree on one thing: they both extend over eastern America from the North.

Atmospheric conditions in the stratosphere and upper troposphere on January 5, 2025. The shape of the polar vortex (left) has been slightly elongated with lower atmospheric thickness (geopotential heights) extending over North America, mainly in the is from Canada. In the upper troposphere (right), where the jet stream resides, there is also a region of thinner anomalies over eastern Canada. The question is: are these thin regions in the stratosphere and troposphere connected or is it a coincidence? NOAA Climate.gov image, based on Laura Ciasto's Global Forecasting System (GFS) analysis data.

As we mentioned in a previous article, interesting research suggests that the shape of the jet stream that can cause bursts of cold air is associated with an elongated polar vortex. But it is not clear whether this will actually happen. Associated with this simply means that they seem to happen at the same time, without them being connected. In fact, if we consider the vertical structure of the polar vortex, this elongation over North America does not extend as far as the lower stratosphere. There, it extends further towards northern Europe.

Shape and structure of the polar vortex throughout the stratospheric column on January 5, 2025. The top of the polar vortex is elongated, extending toward North America, but the bottom of the vortex is more confined to the Arctic and even shifted toward Northern Europe. Asia. NOAA Climate.gov image based on chart provided by Zac Lawrence (stratobserve.com).

The short answer after a long explanation is that the stratosphere does not impact the surface as normally expected, through a weakened polar vortex. The stratosphere and troposphere move in similar ways, but these similarities do not carry over from the surface to the stratosphere. It's also important to remember that cold, snowy weather doesn't need a stratospheric culprit, sometimes it's just the weather.

Is the polar vortex preparing for future trouble?

Current forecasts for the polar vortex do not indicate a significant weakening of winds over the coming months. Interestingly, however, the strong polar vortex winds provide an excellent channel for upward propagation of tropospheric wave activity. This is one of the ingredients needed to disrupt the vortex, but additional steps would need to be taken to make these waves break and slow the vortex. For now, the winds are so strong that any breaking waves would only weaken the vortex to its normal speed for this time of year. If we imagine the polar vortex as a spinning top, it might only take one or two good nudges to send it into an irreversible spin. If the troposphere continues its wavy weather vagaries, this could be just the nudge needed for the polar vortex to respond.

Footnotes

[1] There is a lot of talk in the media about the polar vortex and its impact on the current cold air epidemic. The polar vortex is found in the stratosphere, although some may call the tropospheric jet stream the polar vortex. But it's important to remember that the polar vortex is not the same thing as a blast of cold air.

[2] We haven't talked about it much yet, but changes in the tropospheric jet stream are characterized by a phenomenon called the North Atlantic Oscillation. Well, focus more on that in a future article, but for those of you who are interested now, check out this article.

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