Meteorologists have a historical penchant for giving names to atmospheric phenomena which, in today’s world, sound rather like phrases made up to fit a dramatic headline than real scientific terminology. Bomb cyclone or sting jet are two leading examples. Another term, which has become increasingly used and misused in the wider media in the past 10–15 years, is polar vortex.
The term ‘polar vortex’ most strictly relates to the large cyclonic circulation which forms in the stratosphere over the winter pole each year – the stratospheric polar vortex, to give it its full name. The polar vortices play important roles in Earth’s weather and climate: the strong polar vortex in the Antarctic drives the highly regionalised formation of the ozone hole, while the much weaker and more variable Arctic vortex prevents ozone hole formation and contributes to subseasonal and seasonal tropospheric variability.
In the broadest sense, when the Arctic vortex is strong, the North Atlantic Oscillation (NAO) is more likely to be positive, while the reverse is true when the vortex is weak. (Equivalent changes in the Northern Annular Mode (NAM) – also known as the Arctic Oscillation (AO) – are due to the close relationship between the NAO and NAM; it is still debated whether they are truly different phenomena). The changes in the NAO are related to a shift in the latitude of the North Atlantic eddy-driven jet stream (i.e., the extratropical storm track): when the vortex is weak, the jet shifts equatorward, and vice versa for a strong vortex. The impact of vortex strength variability can also be considered through changes in the transition and persistence probabilities of different weather regimes: Greenland blocking-type regimes are more likely when the vortex is weak, zonal regimes more likely when the vortex is strong (and some are equally likely either way).
The most extreme manifestation of a weak, disrupted vortex is a major sudden stratospheric warming (SSW), which was first observed over 70 years ago. In the Arctic, these occur around six times per decade, with a strict definition: the daily-mean zonal-mean zonal winds at 10 hPa and 60°N must reverse to easterlies, which is indicative of a deep and substantial reversal of the meridional temperature gradient. Additional criteria exist for ensuring separation of events and distinguishing the springtime dissipation of the vortex in the final warming.
The persistence of the circulation anomalies associated with an SSW, particularly those in the lower stratosphere, make them useful for forecasters and decision makers. The mean changes in the circulation over the North Atlantic favours colder-than-average conditions in the eastern US and northwest Europe – highly populated areas where cold air outbreaks can bring significant impacts.
Yet the term ‘polar vortex’, and exactly what a disrupted polar vortex is, has taken on another largely incorrect meaning outside of the scientific literature.
In early 2014, a severe cold-air outbreak impacted the eastern US and Canada. This was associated with a weather pattern that consists of an anomalous ridge/anticyclone extending up to Alaska, and an anomalous trough/cyclonic anomaly in the east: the Alaskan Ridge weather regime (AKR; have a look at the 2014 time series here). For reasons which remain slightly unclear to me, the extremely cold near-surface air became known in the media and to the public as “the polar vortex”.
Subsequently, in 2017 it was suggested by Waugh et al. that we could refer to the tropospheric circulation poleward of the mid-latitude jet stream as the tropospheric polar vortex (to make matters more confusing, this is distinct from a tropopause polar vortex). As such, episodes like winter 2014 – or any instance of an amplified/persistent AKR regime – could be said to indicate a “disrupted tropospheric polar vortex”.
Naturally, however, the critical adjective “tropospheric” has often been dropped in the media. The result? “Disrupted/weak polar vortex”, which could be referring to the tropospheric circulation or the stratospheric circulation! But how do these two relate?
Over the last six years, a focus of my research has been on the interaction between the stratosphere and North American weather regimes. In papers published in 2019, 2022 and 2025, we showed that the AKR regime is not more likely when the stratospheric polar vortex is in a weakened/disrupted state: only the regime associated with Greenland blocking becomes more likely. The AKR regime is the one which brings the most extreme cold air outbreaks to the eastern US and Canada, such as in 2014.
However, in papers published in 2022 and 2025, we showed that there is a way in which the stratosphere can influence the AKR regime: wave reflection. This is when Rossby waves moving up from the troposphere bounce off the polar vortex and come back down, and it actually often happens when the stratospheric vortex is relatively strong, not weak or disrupted. As a result of the strong temperature gradient between the cold air over North America and the warm Atlantic, these events often then drive a positive NAO and stormy, cyclonic conditions in northwest Europe (as was seen in 2014). Millin et al. also demonstrated this wave reflection–cold air outbreak framework in a 2022 paper. AKR regimes also don’t necessarily need stratospheric involvement and can be driven by both tropospheric internal dynamics and tropical wave forcing.
Somewhat more confusingly, some SSWs can also feature wave reflection and thus be followed by the AKR regime or similar patterns. These SSWs feature a rapid dissipation of the weak vortex anomaly in the stratosphere (due to the divergence of wave activity driving zonal acceleration) following the initial wind reversal. The surface impacts of such SSWs can perhaps best be thought of as arising from the behaviour of transient waves, rather than the impact of anomalies in the vortex itself.
So, how do we simplify all this? To start with, we can all follow Manney et al.‘s recommendation to avoid using ‘polar vortex’ to refer to any tropospheric circulation or cold air outbreak, and use it exclusively to refer to the stratospheric circulation. This BBC News article from January 2025 is a good example of what not to do, in my opinion: this was a cold air outbreak driven by an AKR regime when the polar vortex itself was strong (see also this NOAA Climate.gov post).
So, in summary:
- SSWs are a disruption to the stratospheric polar vortex.
- These events are followed, on average, by Greenland blocking/negative NAO and colder conditions in the eastern US and northwest Europe. Winter 2010 and the “Beast from the East” in 2018 are extreme examples.
- The coldest weather regime for eastern North America is the Alaskan Ridge regime (AKR) and it is not more likely when the vortex is weak, but it is more likely following wave reflection events. Winter 2014 is a good (albeit extreme) example.
- Wave reflection events are not usually associated with a disrupted (i.e., weak) stratospheric polar vortex and their impacts are due to the behaviour of waves, rather than anomalies in the vortex itself.
I gave some thoughts on these topics to The Washington Post today.
TL;DR: Please do not use the winter of 2014, or equivalent AKR-driven cold air episodes, as examples of a disrupted vortex, and please do not describe an AKR regime as a disrupted polar vortex.