October 26, 2016
Special blog on winter retrospective can be found here - http://www.aer.com/winter2016
Dr. Judah Cohen from Atmospheric and Environmental Research (AER) recently embarked on an experimental process of regular research, review, and analysis of the Arctic Oscillation (AO). 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.
With the critical month of October starting this weekend, I plan on resuming a weekly update to the weather discussion. However publication may be somewhat erratic over the coming weeks. Subscribe to our email list or follow me on Twitter (@judah47) for notification of updates.
The Arctic Oscillation (AO) is currently negative and is predicted to remain negative over the next two weeks. The AO is predicted to first trend positive towards neutral and then reverse towards negative to possibly strongly negative the first half of November, with uncertainty in the AO becoming large the second week of November.
The current negative AO is reflective of positive pressure/geopotential height anomalies in the Arctic especially on the Eurasian side of the Arctic initially in the Barents-Kara seas and then breaking into two pieces with one piece sliding eastward into the East Siberian Sea. The North Atlantic Oscillation (NAO) is currently positive because of negative height anomalies near Greenland. However the NAO is predicted to trend negative the next two weeks as the second piece of the positive geopotential height anomalies in the Barents-Kara seas retrogrades towards Greenland.
With the AO firmly negative for the next two weeks, this favors below normal temperatures across large portions of northern Eurasia especially Siberia. In contrast, because North America will be dominated by mostly positive pressure/geopotential height anomalies, temperatures will average above normal much of next week in eastern North America. The following week with heights predicted to build once more near Greenland forcing the NAO into negative territory, lower geopotential heights and colder temperatures are predicted first for Europe and then for eastern North America for week two.
Eurasian snow cover is above normal and is continues to advance at a steady pace mostly across eastern Eurasia. However when the NAO flips negative, I expect a more rapid advance of snow cover westward towards Europe. High Eurasian snow cover and low Arctic sea ice in the Barents-Kara seas favor first, a strengthening Siberian high and then a weakened polar vortex (PV) in winter.
The stratospheric PV is predicted to significantly weaken into early November. All weather models now predict an unprecedented and significant early split of the stratospheric PV. I expect the circulation anomalies associated with the PV split to descend into the mid and lower troposphere later in November. When this occurs expect the cold and snow that has been mostly confined to Siberia so far, to expand into the mid-latitudes resulting in an early start to winter weather for widespread portions of northern Eurasia, including Europe and East Asia, and possibly the eastern United States (US).
The AER preliminary US winter forecast has been posted to the NSF website: https://www.nsf.gov/news/special_reports/autumnwinter/predicts.jsp. I share my current thinking below.
I do believe that the large-scale atmospheric stratospheric circulation is nearly unprecedented for this time of year and a record weak PV s predicted by all the models during the first week of November. Since September, I have been focusing on the unusually weak stratospheric polar vortex so far this fall season and in my opinion the strength of the polar vortex in the coming weeks, both in the stratosphere and the troposphere, is key to this winter’s weather. With strong high pressure centered in the Barents Kara seas, the large-scale tropospheric pattern has been very favorable for poleward heat transport or vertical Wave Activity Flux (WAFz). Record strong poleward heat flux is predicted this week. This will result in a highly anomalous stratospheric PV split in the next seven to ten days. This early stratospheric PV split is possibly unprecedented in the observational record. I have so far not found a PV split so early in the season and if anyone can find one please let me know. If we were observing a record weak PV in early January, when it is more common, I would be writing about the strong potential for widespread and extended severe winter weather across the Northern Hemisphere. Instead I find myself expressing higher uncertainty not unlike the AO blogs early last spring trying to predict the consequences of an unprecedented late record weakening of the PV. However there is one important difference and that is the sun’s strength. Back in late March and early April, the sun was quite strong already and only getting stronger across the Northern Hemisphere (NH) mitigating the dynamical cooling forced by the PV split. Now we have the exact reverse situation with solar radiation already low across the NH and it is only getting weaker. I believe that the potential exists for snow cover and cold temperatures to become unusually expansive in the month of November and the dynamically forced cooling by the PV split will be amplified by the weakening solar radiation. An established snow cover and widespread cold temperatures will become increasingly difficult to dislodge and reverse especially across Eurasia as we approach the core winter months.
Still given the lack of precedence for such an early PV weakening the longer terms impacts are highly uncertain. I still feel strongly that the persistence of the weak PV is key to this winter’s forecast. I am now highly certain that in early November the polar vortex in both the stratosphere and troposphere will be relatively weak. As long as one and/or both are weak in the coming weeks and months this favors a relatively cold winter for large portions of the NH mid-latitudes consistent with our winter forecast. Alternatively if this current weakening stratospheric event is a one-off event this cold season and both the stratospheric and tropospheric polar vortices become relatively strong simultaneously, then this favors a much milder solution and the winter forecast will need to be re-evaluated.
A key predictor for our winter forecast is October Eurasian snow cover. We are almost four weeks into the month of October and Eurasian snow cover is above normal. Over the past few years I have especially focused on snow cover south of 60°N and so far the snow cover advance has been particularly impressive south of 60°N. My interpretation of the early indications of the snow cover advance across Eurasia is a bias to a weaker winter PV and a relatively cold winter. Furthermore with the prediction of a negative AO/NAO to end the month of October, I expect that Eurasian snow cover to continue its advance at a relatively accelerated rate, especially westwards towards Europe. Furthermore I continue to be impressed with the persistence of the cold temperatures across Siberia for the month of October. The longer the cold temperatures persist across Siberia, the harder it becomes to dislodge and the more likely those cold Siberian temperatures will be a player in the hemispheric weather outside of Siberia, especially if the polar vortex weakens simultaneously in both the stratosphere and troposphere. I do expect that the circulation anomalies from the early November stratospheric PV to propagate to down into the middle and lower troposphere during the month of November. If and when this occurs I do expect the cold temperatures and snow, mostly confined to Siberia so far, to spread to the NH mid-latitudes including East Asia and Europe. I expect a similar transition in the eastern United States (US) but it will take longer since the circulation in the North Pacific is a more important player and the predicted strong negative geopotential height anomalies in the Gulf of Alaska favor an overall warm pattern across North America in the near term.
Arctic sea ice remains well below normal and possibly most importantly for longer-term weather, below normal sea ice anomalies have only become greater in extent in the Barents-Kara seas over the past few weeks. This also biases the winter forecast colder. Therefore based on above normal October snow cover extent across Eurasia, persistent below normal sea ice in the Barents Kara seas through the fall and early winter and the early tendency for high latitude blocking all favor a colder winter forecast for Northern Eurasia including Europe and East Asia and the eastern US, consistent with the winter forecast posted on the NSF website.
In the near term the PV split will already be influencing the atmospheric circulation across Eurasia. The persistent positive geopotential height anomaly center across northwestern Eurasia will split into two, with one piece spreading westward towards Greenland and the other eastward towards the East Siberian Sea. This will allow geopotential heights to fall significantly downstream of both centers of positive geopotential height anomalies across Europe and East Asia. The circulation across North America will be dominated by ridging/positive geopotential height anomalies centered over the interior of the continent forced by deepening heights in the Gulf of Alaska. However as geopotential heights rise across Greenland starting next week this will favor lowering geopotential heights in the Eastern US with a concomitant drop in temperatures and an expanding snow cover.
Recent and Very Near Term Conditions
The AO is currently weakly negative (Figure 1), reflective of mostly positive geopotential height anomalies both across the Arctic and negative geopotential height anomalies scattered across the mid-latitudes especially Asia (Figure 2). Geopotential height anomalies are currently negative between Iceland and Greenland (Figure 2), and therefore the NAO is positive.
Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 26 October 2016 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 13 October 2016 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.
Strong ridging/positive geopotential height anomalies are centered south of Iceland with positive heights spreading eastward across much of Europe with the lone exception of a trough/negative geopotential height anomalies in the Baltic Sea (Figure 2) resulting in mild temperatures for much of Europe (Figure 3). However the strongest positive geopotential height anomalies of the NH are centered further downstream near the Urals and the Barents Kara seas (Figure 2) resulting in mild temperatures for northwest Asia (Figure 3). Underneath the northwest Asia ridging, a closed low spins near the Caspian Sea (Figure 2) resulting in below normal temperatures for southwest Asia (Figure 3). But the main impact of the northwestern Asia ridging/positive geopotential height anomalies is to force troughing/negative geopotential height anomalies downstream stretching across all of Siberia and south into Northern China and even Northern Japan (Figure 2) resulting in extensive below normal temperatures across Northern Asia including much of Western and Central Siberia, Northern China and northeast Asia (Figure 3). Weak ridging/positive geopotential height anomalies across southeastern China (Figure 2) are resulting in above normal temperatures for much of Southern China (Figure 3). Finally southerly flow out ahead of the Siberian trough (Figure 2) is ushering in mild air to Eastern Siberia (Figure 3).
Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 26 October 2016 at 00Z. Note the high heights over the Barents-Kara seas, the Urals, much of North America with low heights over Siberia and eastern North America.
Ridging/positive geopotential height anomalies dominate much of the interior of North America centered over Central Canada (Figure 2). Under the ridging, temperatures are above normal across much of North America (Figure 3). However a strong trough/negative geopotential height anomalies swinging through the Northeastern US and Canadian Maritimes (Figure 2) is resulting in below normal temperatures for much Eastern Canada and the US East Coast. However, at the end of the week, the trough will swing out into the North Atlantic allowing the milder air to the west to reach the east coast of North America as well, with the possible exception of New England where cold temperatures may linger longer (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 27 – 31 October 2016. Note the warm temperatures across much of North America, Northern Europe and Southern China with cold temperatures in Siberia, the Canadian Maritimes and the Northeastern US. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
With the negative AO and eventually NAO, snow cover will likely advance more rapidly across Eurasia compared to this past week. Snowfall is predicted across much of Siberia, with an expansion westward across western Russia and Scandinavia (Figure 4). Snowmelt is predicted in Southern Kazakhstan and Western and Eastern Canada (Figure 4). Also after a long delay, snow cover is predicted to advance across Alaska (Figure 4) as heights begin to lower.
Figure 4. Forecasted snow depth anomalies (mm/day; shading) from 27 – 31 October 2016. Note the snowfall over parts of Siberia, Western Asia and Scandinavia with snowmelt in Northern Canada. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
The AO is predicted to trend negative to strongly negative all of next week (Figure 1). The negative AO is a result of positive geopotential height anomalies stretched across the entire Arctic except in the Chukchi and Beaufort seas (Figure 5a). And since the region of positive geopotential height anomalies now includes Iceland and Greenland, the NAO will turn negative as well.
Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 1 – 5 November 2016. (b) Same as (a) except averaged from 6 – 10 November 2016. The forecasts are from the 26 October 2016 00z GFS ensemble.
But to understand fully what is happening across western Eurasia including Europe, it is helpful to look at the stratospheric PV (below Figure 12). The PV is predicted to split into two pieces with one piece over northwestern Eurasia and the other over Northern Canada. As I discussed in last year’s blog, it is my observation that there is an immediate response to a sudden stratospheric warming (SSW)/PV weakening event and a delayed response to a SSW. The immediate response will be a crash in geopotential heights over northwestern Eurasia under the stratospheric PV center in the eastern hemisphere (Figure 5a). This will force the ridging/positive geopotential height anomalies so persistent across the Barents Kara seas and Northern Europe, to split into two pieces with one piece sliding east towards the East Siberian Sea and the other piece retrograding westward towards Greenland (Figure 5a). In its place strong negative height anomalies are predicted to stretch across Western Asia and Eastern Europe (Figure 5a). Northerly flow between the northwestern Asia low and the building Greenland high will funnel colder air from the Arctic across Europe and Western Asia (Figure 6). One exception could be northern Scandinavia as lingering above normal geopotential height anomalies (Figure 5a) persist above normal temperatures (Figure 6). Meanwhile downstream of the northwestern Eurasia troughing/negative geopotential height anomalies, ridging/positive geopotential height anomalies will converge across Central Asia and Eastern and Central Siberia (Figure 5a). Above normal heights will help to maintain relatively mild temperatures across much of Northern and Eastern Siberia and Central Asia (Figure 6). Meanwhile the large area of troughing/negative geopotential height anomalies previously stretched across Siberia will be displaced eastward into East Asia and the northern North Pacific (Figure 5a). Negative geopotential height anomalies and northerly flow will likely result in an early season Arctic outbreak to northeast Asia including Northern China, Korea and Japan (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 1 – 5 November 2016. Note the warm temperatures across much North America, Northern Europe and Central Asia with cold temperatures in Siberia, Europe and the Northeastern US. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
The response of the PV split in the western Hemisphere will be a deep closed low over Northern Canada, reflecting the other PV center over Northern Canada, however it does not show up clearly in the five day averages and is not predicted to be as important on the sensible weather as the piece over northwestern Eurasia. Instead the trough from the previous period in the eastern North Pacific, is predicted to further deepen as energy coming off of East Asia is incorporated (Figure 5a). The resultant positive heights downstream and southwesterly flow of maritime North Pacific air across the continent will result in above normal temperatures for much of the US and Canada (Figure 6). One exception could be the US Northeast as northerly flow in the wake of the exiting trough will pull down colder air across the region (Figure 6).
Figure 7. Forecasted snow depth anomalies (mm/day; shading) from 1 – 5 November 2016. Note the snowfall over Siberia, Western Asia, Scandinavia, Alaska and Northern Canada. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
With lowering geopotential heights and the NAO turning negative, snow cover should expand westward across northwestern Asia and even extend into Europe (Figure 7). Lowering heights and a strong Jet Stream across western Canada should produce ample snowfall in the mountains of Western Canada and the US Pacific Northwest (Figure 7). And in spite of rising geopotential heights and warming temperatures, the GFS is predicting mostly new snowfall across Eastern Canada (Figure 7).
Positive pressure/geopotential height anomalies are predicted to persist across the Arctic this period with the strongest positive anomalies stretching from the southern tip of Greenland to Northern Canada and with another positive center in the East Siberian Sea (Figure 5b). Widespread positive heights across the Arctic and negative heights across the mid-latitudes ocean basins will likely maintain both the AO and the NAO in negative territory heading in to the second week of November (Figure 1).
The deep center of negative geopotential height anomalies originating over northwestern Asia will continue to drift further west this period towards Europe (Figure 5b). Coupled with the dominant center of above normal geopotential heights in the northern North Atlantic will create persistent northerly flow and widespread below normal temperatures across much of Europe (Figure 8). Further east, ridging/positive geopotential height anomalies are predicted to stretch from north to south across much of Western Asia and into the Middle East (Figure 5b). Meanwhile the persistent area of negative geopotential height anomalies is predicted to continue to drift eastwards into Eastern Siberia and the northern North Pacific (Figure 5b). Therefore persistent northerly flow will help to maintain the pattern of widespread below normal temperatures across much of Northern Asia including Siberia (Figure 8). The return of ridging/positive geopotential height anomalies across southeastern China (Figure 5b) is predicted to result in above normal temperatures for much of Southern China (Figure 8).
Figure 8. Forecasted surface temperature anomalies (°C; shading) from 6 – 10 November 2016. Note the warm temperatures much of North America and Northwestern Asia with cold temperatures in Europe, Siberia, northeast Asia and the Northeastern US. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
Deep negative geopotential height anomalies in the Gulf Alaska will continue to force both ridging/positive geopotential height anomalies and southwesterly flow across much of North America (Figure 5b) that will result in above normal temperatures for much of the continent (Figure 8). However building ridging/positive geopotential height anomalies across Greenland and Northeastern Canada will contribute to increased troughing/negative geopotential height anomalies in the Northeastern US and Southeastern Canada (Figure 5b). Increasingly northerly flow should favor temperatures averaging normal to below normal for much of the Northeastern US and Southeastern Canada (Figure 8).
Figure 9. Forecasted snow depth anomalies (mm/day; shading) from 6 – 10 November 2016. Note the snowfall over much of Siberia and Scandinavia and Canada. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
With the AO is predicted to persist in negative territory this period, snow cover advance will likely continue across the Northern Hemisphere including northern Eurasia, especially northwestern Asia and Europe, and across and Southern Canada and possibly into parts of the Northern US (Figure 9).
The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows well above normal PCHs in the troposphere and especially in the lower to mid stratosphere (Figure 10) consistent with the PV split and SSW. The reason for the warming PCHs in the stratosphere is record strong upward pulse of Wave Activity Flux (WAFz) or poleward heat transport predicted through the end of the month (Figure 11). In my opinion the tropospheric pattern, the high snow cover and low sea ice are textbook for driving increased poleward heat flux and is the reason for the record WAFz. The record WAFz is predicted by all the models to result in what is possibly an unprecedented PV split in early November (Figure 12a) forcing the stratospheric AO into near record low territory in late October and early November (Figure 1).
Figure 10. Observed and predicted daily polar cap height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
Given that this is an unprecedented event, complicates the forecast, but my expectations are for the PV split to dominate the large scale hemispheric circulation over the next thirty days. As I discussed above, there is a near immediate impact and a delayed impact from a SSW/PV weakening. The near immediate impact I discussed above, the most important of which is the likely start of winter-like weather for parts of Europe. However I do expect there to be significant delayed impacts as well. Typically it takes about two weeks from the circulation anomalies associated with a SSW/PV weakening to translate from the stratosphere to the troposphere. The PCHs show the largest warm/positive anomalies getting trapped near the tropopause (Figure 10). This is a known model bias where positive PCHs fail to penetrate the tropopause into the troposphere. Instead I expect that the positive PCHs to propagate more fully into the troposphere. A good rule of thumb is to translate what happens in the stratosphere eventually into the troposphere. Longer term the models are predicting the PV to once again consolidate into one center but remain off the Pole over the northern North Atlantic (Figure 12b). This would promote cross-polar flow and drive cold air from Siberia into western and then eventually eastern North America. This is my expectation for the latter half of November. But the evolution of the Gulf of Alaska trough needs to be monitored closely as to when and whether this scenario plays out.
Figure 11. Observed and predicted daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N. The forecasts are from the 00Z 26 October 2016 GFS ensemble.
A mature AO circulation pattern is not favorable for increased WAFz or poleward heat flux, which is consistent with the relatively quiet WAFz predicted for week 2 (Figure 11). In the short term, the relatively quiet WAFz should allow the downward propagation in the stratospheric circulation anomalies, in contrast to last winter when the constant active WAFz interrupted downward propagation. However longer term, the ongoing evolution in the PV will be critical (in my opinion) as to how much the PV strengthens. However, (also in my opinion) the extensive Eurasian snow cover and low Arctic sea ice favor a tropospheric pattern that produces more active poleward heat transport and renewed weakening of the PV resulting once more in cold temperatures from Siberia expanding into the mid-latitudes.
Figure 12. a) Forecasted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) for 1 – 5 November 2016 b) same as a) except forecasted for 6 – 10 November 2016 across the Northern Hemisphere. The forecast is from the 00Z 26 October 2016 GFS operational model.
Surface Boundary Conditions
Arctic Sea Ice
Arctic sea ice reached its annual minimum on September 10th. Despite an initial spurt of ice growth, overall Arctic sea ice has been expanding at an anemic rate. One area to note on the North Atlantic side is in the Barents-Kara Seas, where negative sea ice anomalies have been growing (Figure 13). Recent research has shown that regional anomalies are important and the sea ice region most highly correlated with the winter AO is the Barents-Kara seas region where low Arctic sea ice favors a negative winter AO. Given that sea ice is running well below normal, this currently favors more extensive Eurasian snow cover in the coming weeks, followed by a strengthened Siberian high and a weakened polar vortex/negative AO this upcoming winter.
Figure 13. Observed Arctic sea ice extent on 25 October 2016 (white). Orange line shows climatological extent of sea ice based on the years 1981-2010. Image courtesy of National Snow and Ice Data Center (NSIDC). Snow and Ice Data Center (NSIDC).
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) continue to be weakly cooler than average (Figure 14). The cool anomalies (i.e., La Niña-like) are not terribly impressive and most ENSO forecasts are for either neutral or more likely weak La Niña conditions. La Niña conditions favor a negative Pacific/North American (PNA) pattern that produces cold anomalies in the northwestern US and warm anomalies in the Southeastern US. In addition, the North Pacific SST pattern resembles more of a positive Pacific Decadal Oscillation (PDO) pattern, which favors a positive PNA opposite of La Niña. Therefore forcing from the Pacific SSTs seems to be contradictory or weak at best so far this fall season.
Figure 14. The latest weekly-mean global SST anomalies (ending 24 October 2016). Data from NOAA OI High-Resolution dataset. The tropical Pacific shows La Niña SST structure with cool waters near the equator in the eastern and central tropical Pacific. Warmer than normal waters also extend into the subtropical North Pacific and along the western coast of North America and along the East Asian coast. Well above normal waters extend across the subpolar North Atlantic near Greenland and Iceland.
I also note that northern North Atlantic SSTs are warmer and shifted south compared to the same time last year. Northern North Atlantic SSTs for the past few winters have been exceptionally cold, which I believe favors an enhanced longitudinal temperature gradient, a strengthened Jet Stream, a positive NAO and a warm Europe. Potentially significantly warmer SSTs this winter could favor opposite conditions this upcoming winter with a weakened longitudinal temperature gradient, a weakened Jet Stream, a negative NAO and a colder Europe. And so far this fall, the NAO has had a negative tendency. But it is still very early and there are many other complicating factors still unknown. Even warmer SSTs are present in the northern North Pacific, which could also favor high latitude blocking this upcoming winter. But it is still very early and there are many other complicating factors still unknown. Also air and ocean temperatures remain near record levels across the NH and without dynamically forced cold (i.e., a weak polar vortex), the streak of warm weather will persist.
I will also begin soon presenting the phase of the Madden Julian Oscillation (MJO), however for yet another week I will simply mention that the ECMWF model shows no favored phase of the MJO in the foreseeable future and the MJO is unlikely to have a material impact on extra-tropical weather.
Northern Hemisphere Snow Cover
Snow cover advance across Eurasia continues at a rapid pace the first three weeks of October. Also because much of the advance has occurred at latitudes south of 60°N the snow advance index is also above normal. Predicted atmospheric conditions continue to favor the rapid advance of Eurasian snow cover through the end of the month. Therefore confidence is high that October Eurasian snow cover will be well above normal once again this year and in the range of the past four Octobers. Above normal snow cover favors a strengthened Siberian high and a weakened polar vortex/negative AO this upcoming winter with cold temperatures across the continents of the NH.