May 15, 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.
Summary
- The Arctic Oscillation (AO) is currently positive and is predicted to trend negative towards neutral later this week as pressure/geopotential height anomalies across the Arctic are currently mostly negative but then pressure/geopotential height anomalies across the Arctic turn more mixed. The North Atlantic Oscillation (NAO) is currently positive with mostly negative pressure/geopotential height anomalies across Greenland and the NAO is predicted to trend negative the next two weeks as pressure/geopotential height anomalies turn more mixed to positive across Greenland.
- Weak troughing/negative geopotential height anomalies across Greenland will generally support ridging/positive geopotential height anomalies across Northern Europe with weak troughing/negative geopotential height anomalies across Southern Europe the next two weeks. This pattern will support normal to above normal temperatures across Northern Europe including the United Kingdom (UK) with normal to below normal temperatures across Southern Europe the next two weeks.
- The next two weeks general ridging/positive geopotential height anomalies predicted across Northern Europe and then Northwestern Asia will support troughing/negative geopotential height anomalies across Northern and Eastern Asia with more ridging/positive geopotential height anomalies across Southern Asia. This pattern favors normal to above normal temperatures across Western and Southern Asia with normal to below normal temperatures across Northern and Eastern Asia the next two weeks.
- The predicted general pattern across North America the next two weeks is ridging/positive geopotential height anomalies across Alaska, Northern and Eastern Canada and the Eastern United States (US) forciing troughing/negative geopotential height anomalies across Southwestern Canada and the Western US. This pattern favors normal to above normal temperatures across Alaska, Northern and Eastern Canada and the Eastern US with normal to below normal temperatures across Southwestern Canada and the Western US the next two weeks.
- In the Impacts section I discuss the upcoming pattern and how it may influence weather across the Northern Hemisphere (NH).
Plain Language Summary
Episodic cooler and/or wetter temperatures is predicted to continue in parts of the US, Europe and Asia the second half of May thanks in part to high-latitude high pressure ridging in late May (e.g., Figures 3 and 4). This does seem unusual to me, and our summer weather will be impacted by how long the high-latitude high pressure ridging can persist in the summer months.
Impacts
In previous years, I would discuss how spring seems to be advancing aggressively across the Northern Hemisphere (NH) with rapidly declining snow cover and the rapid advance northward of warm temperatures. It does seem to me that this spring the advancement of spring might be a bit slower and snow cover is hanging in there. The one region where this seems to be most true, is Western Russia. It has been not just seasonably cold there but well below normal and that is the region where snow cover remains above normal for the date, with the other exception being the Tibetan Plateau (see Figure i). And some of that cold air has spilled across Scandinavia as well. This despite the ongoing streak of record warm months globally.
Figure i. Observed snow cover anomalies for 14 May 2024. Plot taken from http://climate.rutgers.edu/snowcover/index.php
I do ask myself what is the reason for the exceptional cold and even snow in Northwestern Eurasia and could it have longer term implications for the summer. One plausible reason that I can come up with is that the remnants of the polar vortex (PV) remain over western Eurasia (see Figure ii); something that I discussed two weeks ago. It should disappear soon so maybe the impressive cold that has been around all winter and now spring will finally abate. But this is the region where you could attribute a warming hole to during the summer, the other being the interior of the North America (though not last summer). So not sure if it will have implications for regions away or remote from Western Russia, but seems to me favors another summer where temperatures are closer to seasonable if not below normal in Western Russia in contrast to almost everywhere else.
Figure ii. Predicted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere from 9 – 13 May 2024. The forecasts are from the 00z 8 May 2024 2024 GFS ensemble.
But the region of below normal temperatures is not limited to Western Russia but are widespread across Siberia, Europe and even parts of Canada and the US (see Figure 3). Seems like more widespread below normal temperatures than typical of recent springs but then again, my recollection could be faulty. And for the summer I expect the region of below normal temperatures to shrink. I will post the AER summer temperature forecast in the next blog.
But what really has me scratching my head this morning is the ongoing forecast for the polar cap geopotential height anomalies (PCHs) (see Figure 11). I can certainly understand warm/positive PCHs in the troposphere but why the entire stratosphere as well? Why are the maximum postiive values predicted in the low to mid-stratosphere? Please keep in mind that increasing greenhouse gases are fully expected to lead to cooling of the polar stratospehre in all seasons. It is also my understanding that only upwelling wave energy from the troposphere can result in episodic heating of the stratosphere. Upwelling wave erngy should be blocked or repelled from entering the mid straotpshere by easterly winds (currently around 50hPa). But maybe my physical understanding of the situation is lacking. It could be as simple as even small anomalies look much larger when converted to standardized anomalies. And as I discussed in the most recent blog, it could be that wave refelction explains some of the predcited warming in the stratosphere, especially in the lower stratosphere.
Regardless, this full atmospheric warming seems to be related to high latitude blocking that has contributed and will likely contribute to some cooler and/or wetter weather in parts of the the US, Europe and Asia. And at least the PCH plot in Figure 11 suggested this could continue into June. The 11-15 day forecat exhibits some impressive ridging/ blocking in the Cenrtal Arctic (see Figure 8). This would be in contrast to recent summer circulations. The summer mid-tropospheric circulation in recent years has been characterized by low-pressure troughing centered over the North Pole/Central Arctic with high-pressure ridging along centered in the mid- to high-latitude continents especially over western North America, the east coast of North America, Eastern Europe and East Asia coupled with widespread above normal temperatures across the continents with only regional below normal temperatures.
Hard for me to imagine that the high latitude blocking will dominate the summer circulation, but it is certainly plausible and it will be interesting to monitor when and if the transition takes place from the presence of high latitude blocking to its absence, at least in the Central Arctic.
The summer mid-tropospheric circulation in recent years has been characterized by low-pressure troughing centered over the North Pole/Central Arctic with high-pressure ridging along centered in the mid- to high-latitude continents especially over western North America, the east coast of North America, Eastern Europe and East Asia coupled with widespread above normal temperatures across the continents with only regional below normal temperatures. From what I can tell the weather models are trying to transition into different variations of this pattern. However, as I discussed just above, I wonder if the Final Warming might support some wave reflection that would result in more high latitude high-pressure blocking than currently predicted by the models and therefore more widespread below normal temperatures at least in some regions of the NH continents like the Eastern US and/or Europe.
Near-Term
This week
The AO is predicted to be positive to neutral this week (Figure 1) with initially negative but then mixed geopotential height anomalies across the Arctic and with mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). With predicted mostly negative geopotential height anomalies across Greenland (Figure 2), the NAO is predicted to be positive to neutral this week.
Figure 1. The predicted daily-mean AO at 1000 hPa from the 00Z 15 May 2024 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, weak troughing/negative geopotential height anomalies across Greenland will support ridging/positive geopotential height anomalies across Northern Europe with troughing/negative geopotential height anomalies across Southern Europe (Figures 2). This pattern favors normal to above normal temperatures across Northern Europe including the UK with normal to below normal temperatures across Southern Europe (Figure 3). This week ridging/positive geopotential height anomalies predicted in Northern Europe will support troughing/negative geopotential height anomalies across Northern and Western Asia with more ridging/positive geopotential height anomalies across Southern and Eastern Asia (Figure 2). This pattern favors normal to below normal temperatures across Northern and Western Asia with normal to above normal temperatures across Southern and Eastern Asia (Figure 3).
Figure 2. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 16 – 20 May 2024. The forecasts are from the 00z 15 May 2024 2024 GFS ensemble.
The predicted pattern this week is ridging/positive geopotential height anomalies across Alaska, Northern and Eastern Canada and the Southern and Eastern US with troughing/negative geopotential height anomalies limited to Southwestern Canada and the Northwestern US (Figure 2). This pattern will favor normal to above normal temperatures across Alaska, Northern and Eastern Canada and the Southern and Eastern US with normal to below normal temperatures across Southwestern Canada and the Northwestern US (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 16 – 20 May 2024. The forecast is from the 00Z 15 May 2024 2024 GFS ensemble.
Troughing will support above normal rainfall across the parts of Central and Southern Europe, the higher elevations of Central Asia and Southern China with normal to below normal rainfall elsewhere across Eurasia this week (Figure 4). Troughing will support above normal rainfall across Southwestern and Central Canada and the Southeastern US with normal to below normal rainfall across the remainder of North America this week (Figure 4).
Figure 4. Forecasted precipitation anomalies (mm/day; shading) from 16 – 20 May 2024. The forecast is from the 00Z 15 May 2024 2024 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 remain close to neutral this period (Figure 1). With more mixed pressure/geopotential height anomalies across Greenland (Figure 5), the NAO will be neutral to slightly negative this period.
Figure 5. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 21 – 25 May 2024. The forecasts are from the 00z 15 May 2024 2024 GFS ensemble.
Weakening troughing/negative geopotential height anomalies across Greenland will continue to support weakening ridging/positive geopotential height anomalies across Northern Europe with weak troughing/negative geopotential height anomalies across Southern Europe this period (Figure 5). This pattern will favor normal to above normal temperatures widespread across Europe including the UK with near seasonable temperatures across Southeastern Europe (Figures 6). Persistent ridging/positive across Northern Europe and now extending into Northwestern Asia will support weak troughing/negative geopotential height anomalies across Southwestern Asia and Northeastern Asia with more ridging/positive geopotential height anomalies across Southern and Eastern Asia this period (Figure 5). This pattern favors normal to above normal temperatures across Northwestern and Southeastern Asia with normal to below normal temperatures across Southwestern and Northeastern Asia this period (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 21 – 25 May 2024. The forecasts are from the 00z 15 May 2024 2024 GFS ensemble.
Persistent ridging/positive geopotential height anomalies across Alaska, Northern and Eastern Canada and the Eastern US will continue to support troughing/negative geopotential height anomalies across Southwestern Canada and the Western US this period (Figure 5). This pattern favors normal to above normal temperatures across Alaska, Northern and Eastern Canada and the Eastern US with normal to below normal temperatures across Southwestern Canada and the Western US (Figure 6).
Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 21 – 25 May 2024. The forecast is from the 00Z 15 May 2024 2024 GFS ensemble.
Troughing will support above normal rainfall across Southeastern Europe, the Tibetan Plateau and parts of East Asia with normal to below normal rainfall elsewhere across Eurasia this period (Figure 7). Troughing will support above normal rainfall along the Canadian and US border, the Great Lakes, the Ohio Valley and the Northeastern US with normal to below normal rainfall across the rest of North America this period (Figure 7).
Mid Term
Week Two
With predicted mostly mixed to positive geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes this period (Figure 8), the AO will likely be close to neutral this period (Figure 1). With weak and mixed pressure/geopotential height anomalies across Greenland (Figure 8), the NAO should be also close to neutral to weakly negative this period.
Figure 8. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 26 – 30 May 2024. The forecasts are from the 00z 15 May 2024 2024 GFS ensemble.
Some weak troughing/negative geopotential height anomalies in the northern North Atlantic will support mostly zonal flow across Europe this period (Figure 8). This pattern should favor widespread normal to above normal temperatures across Europe including the UK though relatively weak anomalies are predicted across Europe this period (Figures 9). Persistent ridging/positive geopotential height anomalies are predicted across Northwestern Asia supporting weak troughing/negative geopotential height anomalies with more ridging/positive geopotential height anomalies across Southern Siberia and Northern China this period (Figure 8). The predicted pattern favors widespread normal to above normal temperatures across much of Asia with normal to below normal limited to Southern Siberia and Northern China this period (Figure 9).
Figure 9. Forecasted surface temperature anomalies (°C; shading) from 26 – 30 May 2024. The forecasts are from the 00z 15 May 2024 2024 GFS ensemble.
Ridging/positive geopotential height anomalies are predicted to persist across Alaska Northern and Eastern Canada and the Eastern US with weak troughing/negative geopotential height anomalies across Southwestern Canada and the Western US this period (Figure 8). This pattern favors widespread normal to above normal temperatures across Alaska, much of Canada and the US with normal to below normal temperatures limited to the Southern Canadian Plains and the US Northern Plains this period (Figure 9).
Figure 10. Forecasted precipitation anomalies (mm/day; shading) from 12 – 16 May 2024. The forecast is from the 00Z 1 May 2024 2024 GFS ensemble.
Troughing will support above normal rainfall across Southeastern Europe, the Tibetan Plateau and East Asia with normal to below normal rainfall elsewhere across Eurasia this period (Figure 10). Troughing will support above normal rainfall from the US Plains into the Mid-Atlantic 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 cold/negative PCHs throughout the stratosphere and troposphere (Figure 11). Then for the next two weeks warm/positive PCHs will spread throughout the stratosphere and troposphere but especially in the lower stratosphere and upper troposphere (Figure 11). To be honest not sure what to make of the warm/positive PCHs anomalies predicted to emerge from the upper stratosphere through the troposphere (Figure 11) this week.
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 15 May 2024 2024 GFS ensemble.
The predicted cold/negative PCHs in the lower troposphere this week (Figure 11) are consistent with the predicted positive surface AO this week (Figure 1). However, starting over the weekend, the near neutral surface AO is consistent with switch to weak warm/positive PCHs in the lower troposphere.
Figure 12. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for June 2024. The forecasts are from the 00Z 14 May 2024 2024 CFS.
I include in this week’s blog the monthly 500 hPa geopotential heights (Figure 12) and surface temperatures for June (Figure 13) from the Climate Forecast System (CFS; the plots represent yesterday’s four ensemble members). The forecast for the troposphere is ridging across Northern Europe, Central Asia, much of the North Pacific sector of the Arctic, Eastern Siberia, Alaska, Western Canada and the Central US with troughing in Southwestern Europe, Western Asia, the Gulf of Alaska into the US West Coast, Eastern Canada and the Eastern US (Figure 12). This pattern favors seasonable to relatively warm temperatures across much of Europe, much of Asia, Alaska, much of Canada, the Western US and New England with seasonable to relatively cool temperatures across the Iberian Peninsula, Western Russia, Kazakhstan, the Canadian Southern Plains, the US West Coast, and the Eastern US (Figure 13). 
Figure 13. Forecasted average surface temperature anomalies (°C; shading) across the Northern Hemisphere for June 2024. The forecasts are from the 00Z 14 May 2024 CFS.
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) anomalies are above normal, especially along the Equator, indicating El Niño conditions but it is weakening (Figure 14) and neutral conditions are expected through the summer. Observed SSTs across the NH remain well above normal especially in the central North Pacific (west of recent years), the western North Pacific, the eastern North Atlantic and offshore of eastern North America though below normal SSTs exist regionally especially in the South and North Pacific and the North Atlantic.
Figure 14. The latest weekly-mean global SST anomalies (ending 14 May 2024). Data from NOAA OI High-Resolution dataset.
Madden Julian Oscillation
Currently the Madden Julian Oscillation (MJO) is weak where no phase is favored (Figure 15). The forecasts are for the MJO to remain weak and then emerge into phase four and then five. Phases four and five favor troughing in the interior of Canada with ridggin across Northern Canada and the Southern US and therefore the MJO may be having some influence on the weather across North America 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 14 May 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/clivar_wh.shtml
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