February 20, 2024
Dr. Judah Cohen from Atmospheric and Environmental Research (AER) embarked on an experimental process of regular research, review, and analysis of the Arctic Oscillation (AO) and Polar Vortex (PV). This analysis is intended to provide researchers and practitioners real-time insights on one of North America’s and Europe’s leading drivers for extreme and persistent temperature patterns.
During the winter schedule the blog is updated once every week. Snow accumulation forecasts replace precipitation forecasts. Also, there is renewed emphasis on ice and snow boundary conditions and their influence on hemispheric weather. In late Spring, we transition to a spring/summer schedule, which is once every two weeks. Snow accumulation forecasts will be replaced by precipitation forecasts. Also, there will be less emphasis on ice and snow boundary conditions and their influence on hemispheric weather
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The AO/PV blog is partially supported by NSF grant AGS: 1657748.
- The Arctic Oscillation (AO) is currently positive and is predicted to remain positive this week but trend towards neutral as pressure/geopotential height anomalies across the Arctic are currently mostly negative and are predicted to become remain negative to mixed over the next two weeks. The North Atlantic Oscillation (NAO) is currently positive with mostly negative pressure/geopotential height anomalies across Greenland and the NAO is predicted to be positive to neutral the next two weeks as pressure/geopotential height anomalies turn more mixed across Greenland.
- The general pattern across Europe this week is troughing/negative geopotential height anomalies across Northern Europe with ridging/positive geopotential height anomalies across Southern Europe. The quasi-zonal pattern will support normal to above normal temperatures across much of Europe including the United Kingdom (UK) this week. However, starting next week increasing ridging/positive geopotential height anomalies in the North Atlantic will support deepening troughing and colder temperatures slowly spreading south across Western Europe including the UK while temperatures remain mild across Central and Eastern Europe.
- The general predicted pattern across Asia the next two weeks is strengthening ridging/positive geopotential height anomalies centered over the Urals forcing troughing/negative geopotential height anomalies across Northern Asia this week and then East Asia next week. This pattern favors widespread normal to above normal temperatures across Western and Southern Asia with normal to below normal temperatures across Siberia and Central Asia this week but will sink south across East Asia the next two weeks.
- The predicted pattern across North America this week is ridging/positive geopotential height anomalies across Alaska and Western Canada forcing troughing/negative geopotential height anomalies across Eastern Canada. This pattern favors normal to above normal temperatures across Alaska, Western Canada and much of the United States (US) with normal to below normal temperatures across Eastern Canada. However next week troughing/negative geopotential height anomalies will deepen across western North America forcing strengthening ridging/positive geopotential height anomalies across eastern North America. Therefore, next week cold temperatures are predicted to become more widespread across Alaska, Western Canada and the Western US as temperatures turn much milder across Eastern Canada and the Eastern US.
- In the Impacts section I discuss a complex weakening polar vortex (PV) and the impacts to Northern Hemisphere (NH) weather as winter winds down.
Plain Language Summary
Now that we have made it to the third week of February the temperature pattern for winter 2023/24 is pretty much fully baked. Widespread warmth rules but also relatively cold in Alaska and adjacent Canada, Scandinavia, Northwest Russia, widespread across Siberia and possibly Eastern China when is said and done (see Figure).
Figure. Estimate of the observed surface temperatures (°C; shading) from 1 December 2023 – 20 February 2024 based on GFS initializations and the GFS forecast from the 20 February 2024 forecast.
Some cold for the Eastern US and Scandinavia but main action is looking to be in East Asia (see Figure 9). Could eventually turn colder for Northern Europe and then the Eastern US but still lots of certainty due to second large polar vortex disruption in two months.
I will start off with something less than profound, winter is running out of runway. So even though all the models are predicting an ongoing large polar vortex (PV) disruption the impacts to our weather are questionable. Given the strengthening sun will it result in cold and white or just cool and wet? The late SSWs of last winter and March 2016 did not produce much snow, instead just made for less than stellar springs. A disappointing prospect snow lovers and haters can both agree on.
Even though the zonal mean zonal wind at 60°N and 10hpa did reverse from westerly to easterly/from positive to negative (the definition of a major sudden stratospheric warming or SSW) for at least part of the day yesterday, as far as I can tell it did not average below zero for a whole twenty-four-hour period. Instead, the PV disruption will likely achieve major warming status in early March with the ECMWF predicting a date around March 4th. Oh well, it would have been nice to achieve SSWs in both January and February but then again given the lack of winter weather, maybe it is more appropriate for it not to happen.
As a quick tangent the SSW predicted for early March could be defined as a final warming and not as a major warming if the winds don’t return to westerly before next fall. But I think that this is a technical issue and the impact to our weather is identical whether it is a major or final warming because it is clearly dynamically driven thanks to predicted strong Ural ridging (see Figure 5).
In Figure i, I show again the seasonal polar cap geopotential height anomalies (PCHs) with the upward and downward influences in large PV disruptions shown in solid arrows and also the downward propagation of the major SSWs. Ironically in January the impact on our weather seems to have been stronger from the minor SSW (or if you prefer the SSW that came up short) rather than from the major SSW about two weeks later. A lesson for everyone out there aspiring to do great things, don’t let arbitrary labels define or limit your potential!
Figure i. Seasonal and observed and predicted daily polar cap height (i.e., area-averaged geopotential heights poleward of 60°N) standardized anomalies. Blue Arrows indicate troposphere-stratosphere-troposphere coupling. Numbers explained in 4 December 2023 blog. The forecast is from the 00Z 2024 GFS ensemble.
We have another upcoming major SSW. Like in January the models were predicting the event to peak in February and then for the PV strengthen just for what is more likely to occur – the weakening in February is a precursor and the main weakening of the PV is delayed until early March. Not to get too distracted but I raised this on Twitter/X because it may be common for SSWs to peak in the upper stratosphere first and then peak in the mid-stratosphere later on despite the weather model forecasts. This has now happened twice this winter. It is important because it could very well delay the impact to our weather. Rather than starting the two-week countdown from the upper stratosphere peak it is delayed until the mid-stratosphere peak in the PV disruption. But please keep in mind that it is only recently that I have looked at PCHs as high as 1 hPa so I don’t have much experience or data points to draw from.
One theme from the blog this winter is that the PV has been so active quickly transitioning between different PV disruptions so much so that the influence of one PV state or phase is overlapping, intersecting and interfering with other PV states in close proximity in time.
Certainly, seems to me that the weather models have really struggled in February, and at least for me symbolized by the failed snowstorm in Boston last week. The models were predicting as recently as a week ago, strong high latitude blocking including simultaneous Greenland and Alaskan blocking for it just to vanish into thin air. My explanation would be that the models were predicting strong high latitude blocking in association with downward propagation of the major warming at the end of January that “dripped” or descended down from the stratosphere to the surface shown in the first dashed arrow. But maybe why that fizzled is because of the strong upward Wave Activity Flux (WAF) in the vertical direction last week and this week that dominated the high latitude circulation. Strong upward WAF contributes to warm, high pressure in the polar stratosphere but cold, low pressure in the tropospheric Arctic and the upward WAF ruled the day and dominated the downward propagation from the major SSW two weeks earlier.
But what comes next. The latest PCH forecast suggest the downward propagation from the minor SSW over the weekend will reach the surface the end of February and we can see an increase in high latitude blocking at the very end of February and early March (see Figure 8). The blocking predicted is not terribly impressive and not strong enough to support cold in the Eastern US and/or Northern Europe. But just like the model over predicted the high latitude blocking for mid-February the models can equally be underpredicting the high latitude blocking for late February and into early March. The models are equal opportunity when it comes to poor forecasts.
And then regardless, there is the potential downward propagation from the major SSW in early March shown by the second dashed arrow. Given that at least I am so uncertain about the possible impact of the minor SSW, hard to interpret the impact of the ensuing major SSW.
Seems like everyone is on the warm train (and I really can’t make a strong argument against ghat sentiment) but let me play devil’s advocate and make some arguments for a colder solution first in the Eastern US than current thinking. All models are predicting cold temperatures to pool over Alaska and Western Canada. It is within striking distance of the Eastern US and it would only take a slight shift in the position of the North American wave eastward to bring the colder air east of the Rockies. SSWs are supportive of wave reflection and stretched PVs that favors colder weather east of the Rockies (though for now no signs of one in the weather models so only hypothetical). Finally with the predicted upcoming SSW, some of the coldest temperature anomalies in the stratosphere extend from the UK to the Northeastern US. This could be suggestive of a cold pool in the troposphere as well over the Northeastern US.
For Europe the argument for cold is harder. First as far as I know there no analogous wave reflection over Europe as there is for North America and if there is, it must be much less common. Also, we just had a large buildup of cold air in Siberia, but it unloaded into East Asia where once again I am seeing reports of record cold and snowstorms. So, the best reservoir for cold air for Europe to tap into, is now depleted. From what I can tell it is mostly a GFS forecast but some model runs are forecasting easterly flow over Northern Europe (see for example PV animation tweeted out with announcement of the blog being available). Or alternatively for the PV center to be displaced over Europe (for an example see Figure ii) If this same flow can translate to the mid-troposphere, it would turn much colder in Europe. But this is a big if!
Figure ii. Forecasted average 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 6 March 2024. The forecasts are from the 12z 20 February 2024 GFS ensemble. Plot taken from https://www.tropicaltidbits.com/analysis/models/.
I had to also include the latest ECMWF forecast as well (see Figure iii) as it illustrates the potential of both my ideas, the PV center over Europe and favorable for easterly flow and the cold pool over the Northeastern US.
Figure iii. Forecasted average 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 6 March 2024. The forecasts are from the 12z 20 February 2024 EPS ensemble. Plot taken from https://charts.ecmwf.int/.
I think that the biggest update for today is the latest PCH which does not show any downward propagation or “drip” of warm/positive PCHs over the next two weeks (see Figure iv) in contrast to yesterday’s forecast which did in early March (see Figure 11).
Figure iv. Observed and predicted daily polar cap height (i.e., area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecast is from the 00Z 2024 GFS ensemble.
The PCH forecast tends to be quite volatile, so too early to declare one forecast more accurate than the other, however, the predicted Ural ridging/blocking is very impressive (yet another volatile or low confidence forecast). With Ural ridging this strong, I would expect the upward WAF to remain strong as seen in Figure v, and hard for me to see how we get any downward propagation of warm/positive PCHs with upward WAF that strong.