The phrase “polar vortex” is usually related to phrases and phrases like “chilly”, “sweater”, and “please, please, please don’t let my furnace break down now.” The disruption and weakening of the polar vortex winds do enhance the chances of a chilly air outbreak throughout the jap US and northern Europe/Asia (however they don’t assure it). Nonetheless, the polar vortex is a multi-faceted phenomenon. Learn on to study in regards to the less-hyped aspect of the polar vortex.
There’s extra to the polar vortex than simply disruptions
We spent a lot of final season speaking in regards to the repeated disruptions within the polar vortex. That is when the stratospheric winds at 60 levels North weaken considerably (or reverse path within the case of a serious sudden stratospheric warming) because the vortex is usually pushed completely off the pole or stretched like taffy.
This yr, nevertheless, the polar vortex is exhibiting a special aspect of itself. The robust, secure model of itself. For a lot of the winter season since November, the stratospheric winds at 60N have been properly above the typical speeds for this time of yr. When these stratospheric winds grow to be robust sufficient, it’s known as a polar vortex intensification occasion. Throughout these occasions, the polar vortex tends to take a seat extra intently over the pole [footnote #1] isolating the chilly stratospheric air inside it.
From a cold polar vortex to a toasty troposphere
Much like polar vortex disruptions, polar vortex intensifications within the stratosphere can talk themselves down by means of the troposphere to the floor and have an effect on our climate patterns. To get an thought of what they could seem like, we first must determine the polar vortex intensification occasions. Not like stratospheric warming occasions, that are labeled as occasions when the winds reverse path, there’s actually no clear threshold that marks a polar vortex intensification occasion. For this evaluation, we’ll outline an intensification occasion when the west-to-east winds exceed the eightieth percentile, or strongest 20 %, of every day winds [footnote #2] for a minimum of 10 days between November and March.
By taking the typical of the floor temperature and atmospheric thickness for the 30 days after all of the polar vortex intensification occasions within the observational document, we will common out day-to-day variations within the climate and see extra clearly what persistent climate patterns seem like after the polar vortex intensifies considerably.
How atmospheric thickness (left) and floor temperature (proper) evaluate to the 1958-2023 common for the 30 days after the 48 noticed polar vortex intensification occasions from 1958-2023. On common, intensification occasions are adopted by lower-than-average thickness (purple) on the 500 hectoPascal stress stage, or about 3.5 miles (5.5 km) in altitude, and cooler-than-average floor temperatures (blue) throughout a lot of the Arctic. Throughout the mid-latitudes, it’s the alternative. NOAA Local weather.gov picture primarily based on ERA5 reanalysis knowledge offered by Amy Butler.
Throughout these intensifications, the atmospheric thickness decreases over the pole, proven in these maps as a unfavourable anomaly in tropospheric geopotential top over the Arctic, significantly Iceland and northern Norway. Across the Pacific, North Atlantic, and Europe, the atmospheric thickness is larger than regular [footnote #3].
An intensified polar vortex tends to coincide with a extra northward, much less wavy polar jet stream that retains Arctic air from spilling into decrease latitudes. (Keep in mind, although, the polar vortex is not the jet stream. They’re separated vertically by many miles, however they will grow to be coupled and transfer in sync occasionally.) Due to this sample, some elements of the Northern Hemisphere mid-latitudes can heat up within the 30 days after a polar vortex intensification. That is very true for a lot of northern Asia and elements of Europe. For North America, the results are much less clear. A lot of Canada and Alaska are likely to lean barely cooler than common, however the remainder of the US, significantly the jap half, don’t expertise noticeable variations from the typical temperatures.
The purpose of those maps is to supply a way of the place modifications in temperatures and pressures are most probably to happen within the month after an intensification. As we’ve mentioned earlier than, maps of “typical” floor impacts are not a forecast and don’t symbolize the anticipated day-by-day climate when one in every of these occasions happens. These typical patterns related to intensification occasions don’t all the time emerge after a particular occasion as a result of both there’s no downward coupling, or if different components are enjoying a bigger position (ENSO, day-to-day climate).
The stratosphere and troposphere are largely detached to one another. Variations from common atmospheric thickness (“standardized geopotential top anomalies”) within the column of air over the Arctic for the stratosphere and troposphere. Over the past two months, the stratosphere and troposphere have been largely uncoupled. The principle exception was in mid-to-late December and late January when the low thickness anomalies (indicative of a stronger than common polar vortex) prolonged from mid-stratosphere to the floor. The newest forecast from January 29 2025 signifies that this weak interplay will proceed for a minimum of the following two weeks. Standardized anomalies are primarily based on departures from the 1991-2020 Local weather Forecast System Reanalysis climatologies and have been divided by the usual deviation. Information are from the International Forecast System observational evaluation and forecast. NOAA Local weather.gov picture.
Thus far, this yr has been instance of how the polar vortex doesn’t all the time affect climate on the floor and why we have to bear in mind these typical impacts will not be assured. The polar vortex winds have been a lot stronger than regular, however aside from some temporary interactions in late December and late January, the stratosphere and troposphere have largely ignored one another. We did notice a few weeks in the past that the stretching of the polar vortex appeared to be in keeping with the extension of the jet stream, however we predict the chilly air outbreak that led to historic snow within the South was extra seemingly influenced by different components.
Wanting forward: polar vortex prone to stay intensified
Noticed and forecasted (NOAA GEFSv12) wind velocity within the polar vortex in comparison with the pure vary of variability (faint blue shading). Since mid-November, the winds at 60 levels North (the imply location of the polar vortex) have been stronger than regular. In line with the GEFSv12 forecast issued on January 29 2025, these winds are forecast to stay stronger than regular for a minimum of the following couple of weeks (daring pink line). By the top of February, there’s significantly extra uncertainty whether or not the polar vortex winds will stay robust or weaken. NOAA Local weather.gov picture.
Present forecasts aren’t exhibiting a lot of a change within the power of the polar vortex. A couple of of the person forecasts predict a robust weakening or reversal of the stratospheric winds, however the common of all particular person forecasts signifies the polar vortex will stay stronger than regular for a minimum of a few weeks. Moreover, the newest forecasts don’t point out any important interplay between the stratosphere and the troposphere. So for a minimum of the following two weeks, the polar vortex appears content material to remain robust however silent.
Footnotes
[1] In observe, a robust polar vortex is usually barely off the pole or elongated. That is partly as a result of the polar vortex varies a bit by itself. As we described in a earlier put up, the robust west-to-east winds is usually a nice conduit for tropospheric waves to nudge the polar vortex. However when the winds grow to be too robust, these waves are much less prone to break. Generally meaning as an alternative of these waves nudging the polar vortex, they bounce proper off and are mirrored again to the troposphere. We hope to speak extra about that in a future put up.
[2] We all know this sounds overly technical so let’s clarify slightly extra. For every day between November and March, we accumulate all of the daily-mean wind values within the 1958-2023 ERA5 reanalysis document. From that assortment, we discover the worth through which 80% of the winds are weaker and 20% are stronger. That is the eightieth percentile.
[3] For these of you conversant in the terminology of the Arctic Oscillation, the tropospheric response to polar vortex intensifications is usually related to the constructive section of the Arctic Oscillation.