September 23, 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.
Subscribe to our email list or follow me on Twitter (@judah47) for notification of updates.
The AO/PV blog is partially supported by NSF grant AGS: 1657748
Summary
- The Arctic Oscillation (AO) is currently negative and is predicted to mostly trend positive the next two weeks as pressure/geopotential height anomalies across the Arctic are currently mostly positive and are predicted to become increasingly mixed next week. The North Atlantic Oscillation (NAO) is currently negative with strongly positive pressure/geopotential height anomalies across Greenland and the NAO is predicted to persist negative the next two weeks as pressure/geopotential height anomalies are predicted to remain positive to neutral across Greenland.
- Over the next two weeks ridging/positive geopotential height anomalies across Greenland will support troughing/negative eopotential height anomalies centered on Western Europe. This pattern will support the next two weeks normal to below normal temperatures across Northern and Western Europe including the United Kingdom (UK) while induced southwesterly flow will support normal to above normal temperatures across Southeastern Europe.
- The general pattern predicted for Asia this week is ridging/positive geopotential height anomalies centered over Western Asia forcing troughing/negative geopotential height anomalies across Central Asia with more ridging centered across East Asia. Then next week the troughing in Central Asia will expand into Eastern Asia. This pattern favors this week widespread normal to above normal temperatures across much of Asia with normal to below normal temperatures limited to Western Siberia and Eastern Kazakhstan however next week the colder temperatures are predicted to spread into Central and Eastern Siberia and much of East Asia.
- This general predicted pattern across North America is troughing/negative geopotential height anomalies in western North America forcing ridging/positive geopotential height anomalies across central and eastern North America. This pattern favors normal to below normal temperatures limited to parts of Alaska, and the West Coasts of Canda and the United States (US) with normal to above normal temperatures across most of Canada and the US. One exception this week is troughing supporting normal to below normal temperatures in the US Southern Plains.
- In the Impacts section I discuss the unusual weak polar vortex so early in the season and its potential influence on the weather in the Northern Hemisphere (NH).
Plain Language Summary
Greenland blocking/high pressure) is bringing cooler weather to Western and Northern Europe (see Figure 6). In contrast low pressure troughing in the Gulf of Alaska is persisting the mild pattern across much of Canada and the US (see Figure 6). But the polar vortex has been unusually weak for so early in the season (see Figure iv). This is a real wild card and could certainly bring colder weather in October but will it have any long lasting impact is an open question.
Impacts
As I have noted and others the Northern Hemisphere (NH) polar vortex (PV) has been unusually weak, near or at record weak, all of September so far, exemplified by the atrongly negative AO shown in Figure i. In general I don’t consider what happens in September either with boundary conditions or with the atmospheric circulation informtaive for the upcoming winter season, with October being the first month that provides clues for the upcoming winter. Therefore my instinct is to conclude that we shouldn’t read too much into the anomalously weak PV and to be fair the difference between a weak and strong PV is very little in Spetember. There is one caveat though, the weakened PV in September is following a summer where low- to mid-sratospheric polar cap geopotential height anomalies (PCHs) have been strangely and continuously warm/positive PCHs (see Figure 11). So if September is a continuation of an ongoing trend that is far from over, then yes this could very well be a precursor to an overall weak PV this upcoming winter. But for now I am taking a “I will believe it, when I see it” attitude on this.
Figure i. The predicted daily-mean AO at 10 hPa from the 00Z 23
September 2024 GFS ensemble. Gray lines indicate the AO index from each individual
ensemble member, with the ensemble-mean AO index given by the red line with
squares.
Why the unusual high PCHs and warm temperatures all summer is mostly a mystery but could be related to the infusion of a tremendous amount of water vapor into the stratosphere with the Honga Tonga vocanic eruption. But it is important to keep in mind that water vapr is a greenhouse gas. In summer when there is abundant sunlight, water vapor can absorb solar radiation and heat the atmosphere locally. However in winter when there is perpetual darkness, water vapor emits longwave radiation causing the local atmosphere to cool. So water vapor could be a net source of heating in summer and a net source of cooling in winter. This could result in warm/postive stratospheric PCHs in summer but cold/negative PCHs in winter, i.e., a strong winter PV.
But I could argue for a weak PV without evoking radiative arguments. During the third week of September, just a beast of a block/high pressure existed over the Urals and Scandianavia coupled with downstream low pressure troughing in Northeast Asia and the northern North Pacific (see Figure ii). A wave pattern of this amplitude and length is highly unusual in Spetember and more common in winter and is nearly ideal to trigger upward propagating wave energy from the troposphere into the stratosphere that weakens the PV.
Figure ii. Observed 500 mb geopotential heights (dam; contours) and
geopotential height anomalies (m; shading) across the Northern Hemisphere from
16 September 2024.
And we can see on the heels of this Ural block the upward wave energy has become very active and should continue through the end of the month (see Figure iii). This clearly would weaken the PV. Confidence in the wave energy forecast beyond a week is low but the pattern of upward immediately followed by downward (both in the absolute or the anomalous sense) wave energy is characteristic of wave reflection or a stretched PV. I don’t see much sign of this yet but something I will be watching.
Figure iii. Observed and predicted daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N. The forecast is from the 00Z 23 September 2024 GFS ensemble.
Today’s PV animation (see Figure iv) clearly shows warming predicted in the Central Arctic in the stratosphere. This is charactersitic of a “sudden stratospehric warming or SSW” event. I put SSW in quotes because they just don’t happen in Septembe,r so to use the term SSW is a stretch. But when SSWs occur they are the midpoint of a troposphere-stratosphere-troposphere coupling event. They start with a tropospheric precursor which is seen by the enhanced red shading in the tropopshere the third week of September (see Figure 11) and the Ural block in Figure ii. The intermediary step is the disrupted PV, seen by the enhanced red shading in the stratosphere predicted at the very end of September (see Figure 11) and in the PV animation (see Figure iv). Given that the warming is focused in the Central Arctic and not the North Pacific, it resembles more of an SSW (or possibly a Canadian Warming) than a stretched PV.
Figure iv. (a) Observed and predicted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere from 18 March 2024 . (b)
Same as (a) except forecasted averaged from 23 Spetember – 8 March 2024. The forecasts
are from the 00Z 23 Spetember 2024 GFS model.
Typically the last step is the downward propagation of the circulation anomalies from the stratosphere to the troposphere that are consistent with a negative AO (including high latitude blocking and colder temperatures in Northern Europe and/or Northern Asia and/or eastern North America eventually). That is not shown in the models. Figure 11 shows some bizarre fork in the warm/postive PCHs with one branch going up and another going down. This seems like an unlikely scenario to me.
Instead downward propagation could happen and this is something to monitor for mid to late October. If there is downward propagation it could lead to cooler temperaters likely persisting across Siberia and possibly in Europe and returning to the Eastern US after a very long hiatus. It would also likely help to accelerate snow cover advance across Siberia, a key predictor in our winter temperature forecast. Snowfall is currently above normal across Siberia but I think October and November snow cover are much more important for winter temperartures.
This would then be followed by a likely strengthening PV, hard to imagine that the PV would persist in a perpetual weakened state. But I think best to proceed cautiously with predictions for now since the atmospheric behavior is unusual and in a highly anomalous state which are even harder to predict.
Near-Term
This week
The AO is predicted to be negative this week (Figure 1) with mostly positive geopotential height anomalies across the Arctic and with mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). With predicted strongly positive geopotential height anomalies across Greenland (Figure 2), the NAO is predicted to be strongly negative this week.
Figure 1. The predicted daily-mean AO at 1000 hPa from the 00Z 23
September 2024 GFS ensemble. Gray lines indicate the AO index from each individual
ensemble member, with the ensemble-mean AO index given by the red line with
squares.
This week, rdging/positive geopotential height anomalies across Greenland will support troughing/negative eopotential height anomalies centered on Western Europe this week (Figures 2). This pattern will favor normal to below normal temperatures across Northern and Western Europe including the UK while induced southwesterly flow favors normal to above normal temperatures across Southern and Eastern Europe (Figure 3). This week the predicted pattern across Asia is ridging/positive geopotential height anomalies centered over Wester Asia forcing troughing/negative geopotential height anomalies across Central Asia with more ridging across East Asia (Figure 2). This pattern favors normal to above normal temperatures across much of Asia with normal to below normal temperatures limited to Eastern Kazakhstan and Western Siberia (Figure 3).
Figure 2. Forecasted average 500 mb geopotential heights (dam; contours) and
geopotential height anomalies (m; shading) across the Northern Hemisphere from 24 – 28 Spetember 2024. The forecasts are from the 00z 23 September 2024 GFS ensemble.
This week troughing/negative geopotential height anomalies across Alaska, Western Canada and the US West Coast will force ridging/positive geopotential heights across Eastern Canada and much of the US with the exception of troughing in the US Southern Plains (Figure 2). This pattern will favor normal to below normal temperatures across western Alaska, the West Coast of Canada, the US Pacific Northwest and the US Southern Plains with widespread normal to above normal temperatures across much of Canada and the US (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 Spetember 2024. The forecast is from the 00Z 23 September 2024 GFS ensemble.
Troughing will support above normal rainfall across the Northern and Western Europe, Central Siberia, the Tibetan Plateau and parts of India and East Asia with normal to below normal rainfall elsewhere across Eurasia this week (Figure 4). Troughing will support above normal rainfall across the Northern Canada, West Coast of Canada, the Pacific Northwest and the Southeastern US with normal to below normal rainfall across the remainder of North America this week (Figure 4).
Figure 4. Forecasted precipitation rate (mm/day; shading) from 24 – 28 September 2023.
The forecast is from the 00Z 23 September 2023 GFS ensemble.
Near-Mid Term
Next week
With geopotential height anomalies turning more mixed across the Arctic and with mixed geopotential height anomalies across the mid-latitudes this period (Figure 5), the AO will likely turn neutral this period (Figure 1). With predicted persistent positive pressure/geopotential height anomalies across Greenland (Figure 5), the NAO will likely be persist negative this period.
Figure 5. Forecasted average 500 mb geopotential heights (dam; contours) and
geopotential height anomalies (m; shading) across the Northern Hemisphere from 29
September – 3 October 2024. The forecasts are from the 00z 23 September 2024 GFS
ensemble.
Persistent ridging/positive geopotential height anomalies across Greenland will continue to support troughing/negative geopotential height anomalies centered on Western Europe this period (Figure 5). This pattern will favor normal to below normal temperatures across Northern and Western Europe including the UK while induced southwesterly flow favors normal to above normal temperatures across Southern and Eastern Europe (Figures 6). The predicted pattern across Asia is ridging/positive geopotential height anomalies centered over Western Asia forcing troughing/negative geopotential height anomalies across Central Asia now extending across Siberia with more ridging/positive geopotential height anomalies across Far East Asia (Figure 5). This pattern favors normal to above normal temperatures across all of Western Asia and far East Asia but especially across Western Russia with normal to below normal temperatures across much of Siberia and East Asia (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 29 September
– 3 October 2024. The forecasts are from the 00z 23 September 2024 GFS ensemble
Predicted troughing/negative geopotential height anomalies across Alaska and the Gulf of Alaska will force ridging/positive geopotential height anomalies across most of Canada and the US with the exception of troughing in the Southeastern US this period (Figure 5). This pattern will favor normal to below normal temperatures across southern Alaska, the West Coast of Canada, the US Pacific Northwest and parts of the Southeastern US with widespread normal to above normal temperatures across much of northern Alaska, Canada and the US (Figure 6).
Figure 7. Forecasted precipitation rate (mm/day; shading) from 29 September – 3
October 2024. The forecasts are from the 00z 23 September 2024 GFS ensemble.
Troughing will support above normal rainfall across Northern Europe, Siberia, the Tibetan Plateau, northern India and parts of East Asia with normal to below normal rainfall elsewhere across Eurasia this period (Figure 7). Troughing will support above normal rainfall across southern Alaska, and parts of the Southeastern US and Ohio Valley with normal to below normal rainfall across the rest of North America this period (Figure 7).
Mid Term
Week Two
With predicted mixed geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes this period (Figure 8), the AO will likely persist near neutral this period (Figure 1). With predicted persistent positive pressure/geopotential height anomalies across Greenland (Figure 8), the NAO will likely be negative this period.
Figure 8. Forecasted average 500 mb geopotential heights (dam; contours) and
geopotential height anomalies (m; shading) across the Northern Hemisphere from 4 – 8
October 2024. The forecasts are from the 00z 23 September 2023 GFS ensemble.
Persistent ridging/positive geopotential height anomalies across Greenland will once again continue to support troughing/negative geopotential height anomalies Northern Europe with ridging/positive geopotential height anomalies across Southern this period (Figure 8). This pattern should favor normal to below normal temperatures across Northern Europe including the UK with normal to above normal temperatures across Southern Europe this period (Figures 9). The predicted pattern across Asia this period is ridging/positive geopotential height anomalies centered on Western and Southern Asia forcing troughing/negative geopotential height anomalies across Northern and Eastern Asia this period (Figure 8). The predicted pattern favors widespread normal to above normal temperatures across Western and Southern Asia with normal to below normal temperatures widespread across Siberia this period (Figure 9).
Figure 9. Forecasted surface temperature anomalies (°C; shading) from 4 – 8 October
2024. The forecast is from the 00Z 23 September 2023 GFS ensemble.
Predicted persistent troughing/negative geopotential height anomalies across Alaska and the Gulf of Alaska will force ridging/positive geopotential height anomalies across most of Canada and the US (Figure 8). This pattern will favor normal to below normal temperatures across southern Alaska, the West Coast of Canada, the US Pacific Northwest and parts of the Southeastern US with widespread normal to above normal temperatures across much of northern Alaska, Canada and the US (Figure 9).
Figure 10. Forecasted precipitation rate (mm/day; shading) from 4 – 8 October 2024.
The forecast is from the 00Z 23 September 2024 GFS ensemble
Troughing will support above normal rainfall across the Baltics region, Tibetan Plateau and across parts of India and East Asia with normal to below normal rainfall elsewhere across Eurasia this period (Figure 10). Troughing will support above normal rainfall for the West Coast of Canada and the US Pacific Northwest with normal to below normal rainfall across the remainder of North America this period (Figure 10).
Longer Term
30–day
The latest plot of the polar cap geopotential height anomalies (PCHs) currently shows warm/positive PCHs through much of the stratosphere and troposphere with the exception of cold/negative PCHs in the upper stratosphere (Figure 11). Then next warm/positive PCHs will strengthen and extend throughout the stratosphere but weaken somewhat reverse in the lower troposphere and surface (Figure 11).
Figure 11. 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 23 September 2024 GFS ensemble.
The predicted warm/positive PCHs near the surface this week (Figure 11) are consistent with the predicted negative surface AO this week (Figure 1). However, next week when the PCHs in the lower troposphere are predicted to trend cold/negative, this will force a period of a more neutral AO next week.
Figure 12. Forecasted average 500 mb geopotential heights (dam; contours) and
geopotential height anomalies (m; shading) across the Northern Hemisphere for
October 2024. The forecasts are from the 00Z 23 September 2024 CFS.
I include in this week’s blog the monthly 500 hPa geopotential heights (Figure 12) and surface temperatures for October (Figure 13) from the Climate Forecast System (CFS; the plots represent yesterday’s four ensemble members). The forecast for the troposphere is ridging centered between Greenland and Iceland (i.e., a negative NAO pattern), Southern Europe, Southern and Eastern Asia across the North Pacific to the Aleutians and much of Canada and the US with troughing across Western Asia, Siberia, Alaska, the Gulf of Alaska and the Southeastern US (Figure 12). This pattern favors seasonable to relatively warm temperatures across Southern Europe, Southern and Eastern Asia, northern Alaska much of Canada and the US with seasonable to relatively cool temperatures for Northern Europe, parts of Siberia and East Asia, southern Alaska. The West Coast of Canada and parts of the Southeastern US (Figure 13).
Figure 13. Forecasted average surface temperature anomalies (°C; shading) across
the Northern Hemisphere for October 2024. The forecasts are from the 00Z 23 September 2024 CFS.
Boundary Forcings
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) anomalies are below normal, between the Dateline and the South America coast, indicating that an La Niña is emerging (Figure 14) and weak La Niña conditions are expected through the winter. Observed SSTs across the NH remain well above normal especially in the central North Pacific centered on teh Dateline and the western North Pacific, much of the North Atlantic and offshore of eastern North America though below normal SSTs exist regionally especially in the South Pacific.
Figure 14. The latest daily-mean global SST anomalies (ending 23 September 2024).
Data from NOAA OI High-Resolution dataset. Source: https://psl.noaa.gov/map/clim/sst.shtml
Madden Julian Oscillation
Currently the Madden Julian Oscillation (MJO) is in phase seven (Figure 1). The forecasts are for the MJO to weaken where no phase is favored. Therefore it seems that the MJO is having limited influence on North American weather next week. But admittedly this is outside of my expertise.
Figure 15. Past and forecast values of the MJO index. Forecast values from the 00Z 23 September 2024 ECMWF model. Yellow lines indicate individual ensemble-member
forecasts, with the green line showing the ensemble-mean. A measure of the model
“spread” is denoted by the gray shading. Sector numbers indicate the phase of the
MJO, with geographical labels indicating where anomalous convection occurs during
that phase. Image source: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/ecmf.shtml
Get Detailed Seasonal Weather Intelligence with sCast
We appreciate your taking the time to read the public Arctic Oscillation blog from Dr. Judah Cohen and the AER Seasonal Forecasting team.
Dr. Cohen’s detailed monthly seasonal forecast, sCast, is also available for purchase. sCast provides a monthly 30-60-90-180-day outlook into temperature and precipitation, solar flux and wind anomalies across the globe, and regional population weighted cooling and heating degree forecasts for the US.
Our sCast principal engineer, Karl Pfeiffer, can help you use sCast and other AER seasonal forecast products to deliver important, long-lead time weather intelligence to your business. Please reach out to Karl today!