July 8, 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 remain mostly positive the next two weeks as pressure/geopotential height anomalies across the Arctic are currently mostly negative and are predicted to remain mostly negative for the foreseeable future. The North Atlantic Oscillation (NAO) is currently positive with negative pressure/geopotential height anomalies across Greenland and the NAO is predicted to persist mostly negative the next two weeks as pressure/geopotential height anomalies are predicted to remain negative across Greenland also for the foreseeable future.
- Over the next two weeks general troughing/negative geopotential height anomalies across Greenland will support ridging/positive geopotential height anomalies across much of Europe. However, ridging/positive geopotential height anomalies centered near Iceland will support troughing/negative geopotential height anomalies across Northwestern Europe. This pattern will support the next two weeks normal to above normal temperatures across Europe with the exception of normal to below normal temperatures across Northwestern Europe including the including the United Kingdom (UK).
- The general pattern predicted for Asia this week is an omega block pattern with ridging/positive geopotential height anomalies centered across Central Siberia bookended by troughing/negative geopotential height anomalies across Western Siberia and Northeast Asia. Then next week, ridging/positive geopotential height anomalies wil dominate much of Asia with the exception of troughing/negative geopotential height anomalies across Northwestern Asia. This pattern favors the next two weeks normal to above normal temperatures across much of Asia with the exceptions of Western Siberia and Northeastern Asia this week and Western Russia and extending into Siberia next week.
- The next two weeks predicted ridging/positive geopotential height anomalies across Southern Canada and the United States (US) with troughing/negative geopotential height anomalies across Alaska and Northern Canada will create a mostly zonal flow pattern across the region with the exception of troughing/negative geopotential height anomalies across the Central US this week. This pattern favors normal to above normal temperatures across much of Canada and the US the next two weeks with the exceptions of normal to below normal temperatures across the Central US this week and Alaska and far northern Canada the next two weeks.
- In the Impacts section I discuss the summer pattern across the Northern Hemisphere (NH) and at least ask what is going on in the stratosphere.
Plain Language Summary
Record heat in the Western US with heat in the Eastern US and cool in the middle should continue this week (see Figure 3). The European dipole of cool in the west and hot in the east looks to continue (e.g., Figure 6). Overall looks like a warm pattern for the Western and Eastern US while I am less certain about temperatures in the Plains. Overall looking like an overall good summer forecast from the AER (see Figure ii) model but still plenty of summer to go!
Impacts
I do think that the dominant atmospheric circulation pattern for the Northern Hemisphere (NH) this summer is becoming clearer. There was some high latitude blocking in early June that resulted in some cooler weather for Europe and the Eastern US. However, the positive polar cap geopotential height anomalies (PCHs) associated with the high latitude blocking retreated higher into the troposphere creating a vacuum for cold/negative PCHs to dominate the lower troposphere. The dominance of cold/negative PCHs to dominate the lower troposphere and positive AO regime has allowed for ridging to spread across the Eastern US and most of Europe except for far Western Europe.
So despite the initial high latitude blocking, the Central Arctic has been dominated by low pressure centered near the North Pole following more closely the multi-annual trends of low pressure ovver the Arctic surrounded by what I refer to as the ring of fire across the mid to high latitude continents with above normal temperatures in the Western US, Eastern North America, Eastern Europe, Western Asia, East Asia and focused in Siberia (but not Eastern Siberia, with more seasonable temperatures to below normal temperatures in Western Canada (and likely Alaska, Western Europe, around the the Urals region and Eastern Siberia (see Figure i).
Figure i. Observed surface temperature anoamlies 1 June – 5 July 2024 (°C; shading). Computed from NCEP/NCAR reanalysis using plotting tools from https://psl.noaa.gov/data/composites/day/
For what it’s worth this pattern is quite similar to what the AER model predicted as shown in the 28 May 2024 blog (unfortunately it was not archived so I include it here in Figure ii). The one region where AER forecast may or may not accurately predict is the Central US. July has been cool so far but June was much warmer relative to normal.
Figure ii. Summer temperature anomalie forecasts for June, July and August 2024 (°C; shading).
I do want to once again conclude with the PCHs this summer. I really don’t know what to make of the PCH plot as shown in Figure 11 once again this week, which looks very strange to me. To highlight how bizarre it has been I include in Figure iii the same plot from one year ago. This plot of episodic warming or what I like to desribe as “pulsing red”of tropospheric PCHs with cold stratospehric PCHs that occasionally descend to the surface makes much more physical sense to me. Larger amounts of greeenhouse gases in the polar stratosphere should result in cooling of the polar stratosphere punctuated by warming when upwelling wave energy is absorbed and thus warms the polar stratosphere but limited to the winter months but in particular January and February.
Figure iii. 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 5 July 2024 2023 GFS ensemble.
The warm PCHs in the stratosphere I assume have the potential to descend to the surface and increase high latitude blocking resutling in cooler temperatures in the Eastern US and Northern Europe, yet for the most part this has not happened and it is looking less likely to ocuur for a significant aount of time this summer.
This pattern or low pressure over the Central Arctic surrounded by high pressure across the mid-latitudes, which results in a quasi zonal flow is favorable for preserving Arctic sea ice. We can see in Figure iv that sea ice extent is comparable to the best two summers and I would expect the trends to continue. Arctic sea ice is tracking below normal and should continue right through the September minimum. However, the possibility of a new record low sea ice minimum is looking less and less likely.
Figure iv. Arctic sea ice extent as of 2 July 2024, along with daily ice extent data for four previous years and the record low year of 2012. 2024 is shown in blue, 2023 in green, 2022 in orange, 2021 in brown, 2020 in magenta, and 2012, the record low year, in dashed brown. The 1981 to 2010 median is in dark gray. The gray areas around the median line show the interquartile and interdecile ranges of the data. Plot taken from https://nsidc.org/arcticseaicenews/
Near-Term
This week
The AO is predicted to be positive this week with the possible excetion of Wednesday (Figure 1) with mostly negative geopotential height anomalies across the Arctic and with mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). With predicted negative geopotential height anomalies across Greenland (Figure 2), the NAO is predicted to be positive this week.
Figure 1. The predicted daily-mean AO at 1000 hPa from the 00Z 8 July 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, troughing/negative geopotential height anomalies across Greenland will support ridging/positive geopotential height anomalies across much of Europe this week with the exception of troughing/negative geopotential height anomalies across Northwestern Europe forced by ridging/positive geopotential height anomalies centered near Iceland (Figures 2). This pattern favors normal to above normal temperatures across much of Europe with the exception of normal to below normal temperatures across Northern and Western Europe including the UK (Figure 3). This weekthe predicted pattern across Asia is ridging/positive geopotential height anomalies in Central Siberia sandwiched by troughing/negative geopotential height anomalies across Western Siberia and Northeastern Asia with more ridging/positive geopotential height anomalies across Southern Asia (Figure 2). This pattern favors normal to above normal temperatures across much of Asia with normal to below normal temperatures across Western Siberia and Western Kazakhstan and parts of Northeastern Asia including Eastern Siberia (Figure 3).
Figure 2. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 9 – 13 July 2024. The forecasts are from the 00z 8 July 2024 2024 GFS ensemble.
This week predicted troughing/negative geopotential height across Alaska and Northern Canada with ridging/positive geopotential height anomalies across Canada and extending south into the Western and Eastern US with more troughing/negative geopotential height anomalies across the Central US will create a mostly zonal flow pattern across North America (Figure 2). This pattern will favor normal to below normal temperatures across Alaska, Northern Canada, and the Central US with normal to above normal temperatures across Southern Canada and the Western and Eastern US (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 9 – 13 July 2024. The forecast is from the 00Z 8 July 2024 2024 GFS ensemble.
Troughing will support above normal rainfall across Western Europe, Western Siberia, parts of Northeast Asia, the Tibetan Plateau and Southeast Asia with normal to below normal rainfall elsewhere across Eurasia this week (Figure 4). Troughing will support above normal rainfall across the Western Alaska, Central Canada, the Central US (enhanced by the remnants of Beryl) and the US East Coast 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 9 – 13 July 2024. The forecast is from the 00Z 8 July 2024 2024 GFS ensemble.
Near-Mid Term
Next week
With geopotential height anomalies remaining mostly negative across the Arctic and with mixed geopotential height anomalies across the mid-latitudes this period (Figure 5), the AO will likely continue to be positive this period (Figure 1). With predicted negative pressure/geopotential height anomalies across Greenland (Figure 5), the NAO will likely be positive this period as well. 
Figure 5. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 14 – 18 July 2024. The forecasts are from the 00z 8 July 2024 2024 GFS ensemble.
Persistent troughing/negative geopotential height anomalies across Greenland will continue to support ridging/positive geopotential height anomalies across Southern and Eastern Europe while ridging/positive geopotential height anomalies centered across Iceland will support troughing/negative geopotential height anomalies across Northern and Western Europe this period (Figure 5). This pattern will favor normal to below normal temperatures across Northern and Western Europe including the UK with normal to above temperatures across Southern and Eastern Europe (Figures 6). The predicted omega block pattern of ridging/positive centered on Central Siberia bookended by troughing/negative geopotential height anomalies centered on the Urals and across Northeastern Asia with more ridging/positive geopotential height anomalies across Southern Asia is predicted to continue this period (Figure 5). This pattern favors normal to above normal temperatures widespread across Asia with normal to below normal temperatures mostly limited to the Urals region, Eastern Kazakhstan and parts of Northeastern Asia but mostly in Eastern Siberia this period (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 14 – 18 July 2024. The forecasts are from the 00z 8 July 2024 2024 GFS ensemble.
Continued troughing/negative geopotential height anomalies across Alaska and Northern Canada with ridging/positive geopotential height anomalies across Southern Canada and across much of the US will persist a zonal flow pattern this period (Figure 5). This pattern favors normal to above normal temperatures across much of Canada and the US with normal to below normal temperatures mostly limited to Alaska (Figure 6).
Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 14 – 18 July 2024. The forecast is from the 00Z 8 July 2024 2024 GFS ensemble.
Troughing will support above normal rainfall across Central Europe, Southern Siberia, 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 across Alaska, Northwest Canada, the US Great Lakes, and Quebec with normal to below normal rainfall across the rest of North America this period (Figure 7).
Mid Term
Week Two
With predicted mostly negative geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes this period (Figure 8), the AO will likely persist positive this period (Figure 1). With predicted negative pressure/geopotential height anomalies across Greenland (Figure 8), the NAO will likely be positive as well this period.
Figure 8. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 19 – 23 July 2024. The forecasts are from the 00z 8 July 2024 2024 GFS ensemble.
Persistent troughing/negative geopotential height anomalies across Greenland will continue to support ridging/positive geopotential height anomalies across much of Europe this period (Figure 8). This pattern should favor widespread normal to above normal temperatures across much of Europe with closer to seasonable temperatures across Northwestern Europe including the UK this period (Figures 9). Ridging/positive geopotential height anomalies are predicted to dominate much of Asia with troughing/negative geopotential height anomalies centered on the Urals and Western and Northern Siberia this period (Figure 8). The predicted pattern favors widespread normal to above normal temperatures across much of Asia with normal to below normal temperatures mostly confined to the Urals region, Western and Northern Siberia this period (Figure 9).
Figure 9. Forecasted surface temperature anomalies (°C; shading) from 19 – 23 July 2024. The forecasts are from the 00z 8 July 2024 2024 GFS ensemble.
Troughing/negative geopotential height anomalies are predicted to remain entrenched across Alaska and the Gulf of Alaska this period supporting ridging/positive geopotential height anomalies across much of Canada and much of the US with downstream troughing/negative geopotential height anomalies across Northeastern Canada (Figure 8). This pattern favors widespread normal to above normal temperatures across most of Canada and the US with normal to below normal temperatures limited to Alaska and Northwestern Canada this period (Figure 9).
Figure 10. Forecasted precipitation anomalies (mm/day; shading) from 19 – 23 July 2024. The forecast is from the 00Z 8 July 2024 2024 GFS ensemble.
Troughing will support above normal rainfall across the Alps, the Tibetan Plateau and widespread across East Asia with normal to below normal rainfall elsewhere across Eurasia this period (Figure 10). Troughing will support above normal rainfall for Alaska, Northwestern Canada, the US Upper Midwest and the Southeastern US 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 and lower troposphere (Figure 11). Then for the next two weeks warm/positive PCHs will persist throughout most of the stratosphere and troposphere with the exception of cold/negative PCHs in the upper stratosphere and the lower troposphere (Figure 11). This PCH sandwich certainly looks bizarre to me as I discussed in the Impacts section.
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 8 July 2024 2024 GFS ensemble.
The predicted cold/negative PCHs near the surface this week (Figure 11) are consistent with the predicted positive surface AO the next two weeks (Figure 1). However, there is always the possibility that the warm/positive PCHs in the mid to upper troposphere will descend to the surface forcing a period of a negative AO. Something to watch for the remainder of the summer.
Figure 12. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for August 2024. The forecasts are from the 00Z 8 July 2024 2024 CFS.
I include in this week’s blog the monthly 500 hPa geopotential heights (Figure 12) and surface temperatures for August (Figure 13) from the Climate Forecast System (CFS; the plots represent yesterday’s four ensemble members). The forecast for the troposphere is ridging centered near Iceland, Western Asia and Eastern Siberia, Western Canada and the Western US with troughing in Western Europe, East Asia, Western Siberia, the Aleutians, Eastern Canada and the Northeastern US (Figure 12). This pattern favors seasonable to relatively warm temperatures across Northern and Eastern Europe, Western and Central Asia, Alaska, Western Canada and the Western US with seasonable to relatively cool temperatures for Western Europe, Kazakhstan, Central Siberia, East Asia, Eastern Canada and the Central US (Figure 13). 
Figure 13. Forecasted average surface temperature anomalies (°C; shading) across the Northern Hemisphere for August 2024. The forecasts are from the 00Z 8 July 2024 CFS.
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) anomalies are below normal, especially along the Equator near South America, indicating emerging La Niña conditions (Figure 14) and neutral to La Niña 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 daily-mean global SST anomalies (ending 4 July 2024). Plot taken from https://psl.noaa.gov/map/clim/sst.shtml
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 briefly pop into phase 5 but otherwise remain weak for the next two weeks. Phase 5 only forces weak anomalies (mostly troughing) and therefore the MJO seems to be having little 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 8 July 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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