The Polar Vortex Strikes Again!

Can a Warmer Planet Cause Extremely Cold Weather?

Feb 06, 2023

Polar Vortex - Sounds a bit threatening! If you've seen "The Day After Tomorrow," Polar Vortex might bring to mind the massive arctic vortex portrayed in that film that freezes most of the Northern Hemisphere.

And Polar Vortex was front page news in many outlets this week as the northeastern U.S. braced for a predicted return of this occasional, bitter, mid-winter event. "Northeast faces ‘ferocious Arctic blast’ and wind chills near minus 60," wrote the Washington Post, while the New York Times added "Northeast Braces for the Worst Wind Chill in Decades."

Neither were far from the mark, as wind chills were indeed uncommonly low over much of the region. The New York Post hit the story hardest after the fact with "Mount Washington as cold as Mars with record-breaking wind chills of 110 below." Mt. Washington, at 6,288 feet above sea level, is the tallest mountain in the northeast, and is famous for both lowest temperatures and highest wind velocities!

For more representative numbers, I like to go to the U.S. Weather Service Graphical Weather Forecast site. Their mapped predictions show not only what will happen where you live, but also where that weather is coming from, and how fast the air masses delivering the blow are moving.

Here are maps produced on February 1 predicting morning low temperatures for the northeast for the following four days. It did indeed look like the northeast was in for a quick but severe freeze. The map of actual temperatures for February 4 was nearly indistinguishable from the prediction map from February 1, so kudos to the Weather Service for an accurate forecast!

My home town in southern New Hampshire hit -12^oF early on the 4^th, not breaking the all-time low for this day of -19^oF, but with a wind chill temperature of -37^oF.

This outbreak of Arctic air stands out against the backdrop of an unusually warm and wet winter. Daily temperatures in December were about 3.7^oF above the historical average, and January was even warmer at 9.4^oF above average. Total precipitation across both months was nearly double the average of 7 inches, while snowfall accounted for only about a third of that total.

Is the extreme weather on February 4 due to the dreaded Polar Vortex? What is the Polar Vortex anyway? Has this happened before? Is it related to climate change?

Ironically, it is not the Polar Vortex that causes those outbreaks of cold weather, but the breakdown of that Vortex.

The actors in this play include very cold Arctic air and two high velocity air currents centered over the north pole: the Polar Vortex, at about 10-30 miles above the surface, and the Polar Jet Stream at 5-9 miles up. When the Vortex is stable, the fast moving Polar Jet tends to be linear and stay to the far north. Together these two rivers of air trap Arctic cold over the pole.

Occasionally, warmer air in the lower atmosphere kicks up into the upper Arctic layers, leading to Sudden Stratospheric Warming. This can cause both the Vortex and Jet Stream to become unstable. They can split, migrate, wander, increasing the chance that some of that frigid Arctic air will escape to the south.

The irony here is that it is warmer air infiltrating the cold Arctic air mass that leads to the breakdown in the Jet and the outbreaks of that Arctic air felt so powerfully in the more populous areas to the south. The world has not gotten colder, but the distribution of cold air has changed.

Has this happened before? Yes, many times.

For the northeast, the most extreme Polar Vortex story covers the three winters of 2013-2015. Temperatures for all of New England over those three winters were 5^oC (9^oF) below average for February, and 2^oC (3.6^oF) below average for March. Snowfall records were set throughout the region. Sudden Stratospheric Warming was implicated in the outbreaks. At one point NASA reported a 25^oC (45^oF) increase in temperature in the upper atmosphere (stratosphere) over the Arctic within a single week.

The persistent, recurring extreme cold weather outbreaks in those years resulted from both the breakdown in the Polar Vortex and the development of what is called a blocking pattern that tended to lock the out-flowing cold Arctic air in one region. The result? Not just a cold snap, but record cold winters.

While big news in the northeast, those same winters were unusually warm in the western U.S. and Florida. Miami, Los Angeles, San Francisco, and Seattle recorded above average temperatures and 2014/2015 was one of the warmest winters ever in the northern hemisphere. Globally, those same years were also among the warmest ever recorded (as is true for every year in this century).

As we approach Super Bowl Sunday, here is a highly visible, football-related event that occurred in late December, 1967. The Green Bay Packers beat the Dallas Cowboys 21-17 at Lambeau Field in Green Bay, WI, to win the NFL Championship in what has been reported to be the coldest NFL game ever played. The game has been dubbed the Ice Bowl, and according to one recent recounting of the story, the temperature at kickoff was -13°F, with a wind chill of -36^oF. Referees could not use metal whistles as they would freeze to their lips. Several players were treated for frostbite and a fan in the stands died of exposure to the cold.

We couldn't map temperatures in those days as completely as we can now, but this image of the pattern in air pressure resulting from the escaped Arctic air reflects a weakened Polar Vortex.

The Polar Vortex has been under-performing in the winter of 2022-2023 as well.

On December 22, the Denver airport reported a low temperature of -24^oF, the coldest since 1990, and wind chills approached -40^oF. Many commercial flights were cancelled and offices and schools were closed. This NASA image captures the Arctic air escape path on that day. A video of changes from December 13 to December 22 is here.

In January, record cold occurred across a wide swath of Asia, including Korea, China and especially Afghanistan.

And in February it was our turn in the northeastern U.S.

Isolated stories like these grab the headlines, but is there a pattern to the frequency of Polar Vortex breakdowns, and is that frequency changing with time?

Since 2003, NASA has been able to monitor global temperatures in real time by satellite. This snapshot is for February 27 of each year. The cold outbreaks over the northeast in 2014 and 2015 can be seen in comparison to, say, 2010. The full video for 2003 to 2020 is here.

Reaching farther back in time, the Wikipedia site for Sudden Stratospheric Warming lists a total of 44 major events since 1958, with no apparent trend in frequency. Possible impacts of a warming planet on Polar Vortex dynamics remains an active area of research.

Can we track and predict these outbreaks? Introducing the Arctic Oscillation Index.

NOAA's Climate Prediction Center publishes data and predictions for several indexes that represent the state of what are called oscillations in the climate system. Oscillations, as the name implies, are processes that change cyclically, and are represented by one or a few standard measures, or indexes. The most familiar of these is the El Niño/La Niña system, the index for which is variation in sea surface temperature in the central Pacific Ocean.

For the Polar Vortex, the Arctic Oscillation index tracks changes in air pressure over the north pole. When the index is positive, low air pressure keeps the Jet Stream tightly circling the pole. A negative index means higher pressure and increased likelihood of more wobble (to use the technical term), and more cold weather outbreaks.

The Climate Prediction Center site is unique in many ways, but one is that it presents both predictions for the near future and how well previous predictions performed. In the lower panel here, the darker black line is the measured value and the lighter blue line is the average of predictions from several models. The yellow area and red lines express the amount of variation among those model predictions. The average of model predictions for this index 7 days into the future matches later measurements fairly well.

The black line in the upper panel presents measured values of the index and the red lines capture future predictions from those different models. The extreme negative values in mid-to-late December coincide with the cold outbreak in Colorado captured in the NASA image above. The outbreak over Asia in January also coincides with a long period of low index numbers. The predictions for the timing of the recent outbreak in the northeast accurately suggest a relatively temporary event. Following near-record lows on the night of the 3^rd to 4^th, temperatures were well-above normal by the afternoon of the 5^th.

The U.S. Weather Service Climate Prediction Center is well worth a visit.

So the Polar Vortex is a real thing. Changes in that rapidly circulating, high elevation air stream can impact the lower elevation Jet Stream, and the two together can either trap Arctic air over the pole or allow it to break out to the south, spreading extreme cold to more populous regions and generating major headlines.

While the Arctic Oscillation Index can track and to some extent predict when conditions are favorable for these outbreaks, that alone cannot predict where the outbreak will occur. The complex computer models used to generate your local weather forecast are required to see how the next outbreak will play out (Colorado? Asia? Northeastern U.S.?). If you run the full video from which that snapshot for February 27, 2003-2020 above is captured, you can get a sense of the complexity of the atmospheric dynamics involved.

Disruption of the Vortex is not an uncommon event, and while it seems that those events are not necessarily increasing with time, there is irony in that these outbreaks of extremely cold weather are caused in part by a warming of the upper atmosphere over the pole. That fact suggests that the frequency of disruptions of the Vortex might change over time.

We will keep hearing about the Polar Vortex from time to time as outbreaks occur. Where they occur will determine our perception. If the super-cold weather is happening where you are, it is probably not happening elsewhere. In fact, temperatures may be warmer than average in the next region over. If it is not happening where you are, you may not hear much about it, so our perceptions of the frequency and intensity of Polar Vortex breakdowns may be a bit provincial.

These locally extreme cold events can pose serious threats to health and property, so they bear watching when predicted for your area, but they do not portend a chilling of the global climate. The upward trend in global temperatures will not be affected by these short-term, regional outbreaks. Whether that warming will affect the frequency and intensity of breakdowns in the Polar Vortex seems to be an open question.

Sources

The Washington Post quote is from:

https://www.washingtonpost.com/weather/2023/01/31/northeast-arctic-blast-polar-vortex/

The New York Times quote is from:

https://www.nytimes.com/2023/02/02/us/northeast-frigid-weather.html

but has been edited since first posting

The New York Post quote is from:

https://nypost.com/2023/02/04/mount-washington-as-cold-as-mars-as-polar-vortex-brings-record-breaking-windchills-of-110/

Record low temperature for Durham, NH is from:

https://www.extremeweatherwatch.com/cities/durham-nh/record-low-by-day

and the wind chill number is from:

https://www.wunderground.com/forecast/us/nh/durham