October 7, 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.

Summary

• The Arctic Oscillation (AO) is currently strongly negative and is predicted to remain negative to strongly negative over the next two weeks.  The AO is predicted to bottom out early next week and then slowly rise but still remain negative.
• The current negative AO is reflective of positive pressure/geopotential height anomalies in the Arctic especially on the North Pacific and European sides of the Arctic and negative pressure/geopotential height anomalies across the mid-latitudes of the Northern Hemisphere (NH).  And because the anomalies are strongly positive on the North Atlantic side of the Arctic near Iceland, the North Atlantic Oscillation (NAO) is currently also negative and is predicted to remain negative for the next two weeks.
• With pressure/geopotential height anomalies predicted to remain positive across much of the Arctic and pressure/geopotential height anomalies predicted to remain negative across the NH mid-latitudes, the AO/NAO are likely to remain in negative territory into the foreseeable future.
• With the AO/NAO firmly negative for the remainder of the next two weeks, favors below normal temperatures across large portions of northern Eurasia including Europe, East Asia and eastern North America. However because North America will be an exception to the remainder of the NH mid-latitudes, with mostly positive pressure/geopotential height anomalies, temperatures will average above normal much of next week.  The following week the low heights in the North Atlantic may retrograde westward into eastern North America and finally cause temperatures to cool.
• Both low sea ice and the negative AO are favorable for the rapid advance of Siberian snow cover.  Eurasian snow cover is above normal and this trend should continue through at least mid-month. 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 in winter.
• The polar vortex both in the stratosphere and the troposphere are unusually weak so far this fall, if this trend continues severe winter weather is likely for widespread portions of northern Eurasia, including Europe and East Asia, and the eastern United States (US).

Impacts

In previous blogs I noted the unusually negative AO in the stratosphere and that the strong positive polar cap height anomalies look to be descending down into the troposphere.  The weak stratospheric polar vortex has now been followed by an unusually weak tropospheric polar vortex and a deeply negative tropospheric AO.  This is not the time of year when tropospheric-stratospheric coupling is considered active but the most recent polar cap height anomaly (PCH) plot is certainly suggestive of a downward propagation of warm/positive PCH from the stratosphere to the troposphere.  Regardless, the tropospheric AO is strongly negative, which favors widespread cold temperatures across the NH mid-latitudes, especially northern Eurasia and a rapid advance in snow cover especially Siberia and to a lesser extent Canada.  October Eurasian snow cover extent is a key predictor in our winter forecast and if our expectations are correct that Eurasian snow cover will be above normal for the month of October, this will bias our model colder for the winter forecast.

 As I have discussed in previous blogs, the Arctic sea ice is well below normal and possibly most importantly for longer term weather sea ice is below normal in the Barents-Kara seas.  This will also bias the winter forecast colder.  Also with the likelihood that the AO will be strongly negative right through the middle of October, October 2016 will probably rival other recent Octobers for the most negative October AO observed, which include October 2002 and 2009.  And though there is not a strong relationship between the October AO and the winter AO, the winter AO in 2002/03 and 2009/10 were both negative and cold temperatures were widespread both across northern Eurasia and the Eastern US.  However the AO was much more negative in the latter winter and the evolution of the polar vortex was quite different.  It is still very early and the outcome of this winter still very much in doubt but in my opinion, the longer that the tropospheric and/or stratospheric polar vortex remain weak and the AO negative, favors a colder winter solution.

In the near term the NH atmospheric circulation looks stable over the next two weeks.  With the AO/NAO solidly negative, positive geopotential height anomalies will dominate the Arctic while negative geopotential height anomalies will be widespread across the mid-latitudes including both the North Atlantic and the North Pacific.  A negative AO most strongly favors below normal temperatures across northern Eurasia especially Siberia.  And because the NAO is also predicted to be strongly negative, below normal temperatures are also favored across Europe.  However because the Aleutian low will be anomalously deep especially to the southeast of its climatological position this will force above normal geopotential heights across much of North America and the overall warm temperature pattern will likely continue for much of North America at least through next week. However the longer that the AO/NAO persists negative does eventually favor lowering heights in the Eastern US with a concomitant drop in temperatures, which is predicted for week two.

Recent and Very Near Term Conditions

The AO is currently strongly negative (Figure 1), reflective of mostly positive geopotential height anomalies both across the Arctic and negative geopotential height anomalies across the mid-latitudes (Figure 2).  Geopotential height anomalies are mostly positive on the European side of the Arctic including Iceland (Figure 2), also producing a strongly negative NAO.

Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 7 October 2016 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 7 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 stretch across the European side of the Arctic including Northern Europe (Figure 2) resulting in mild temperatures for Scandinavia (Figure 3).  Underneath the Scandinavian ridging, a closed area of low pressure spins over Central Europe (Figure 2).  Negative geopotential height anomalies combined with northerly flow are bringing below normal temperatures to much of Central and Southern Europe (Figure 3).  Further east ridging/positive geopotential height anomalies stretch across Western Asia (Figure 2) resulting in above normal temperatures (Figure 3). The rest of Asia is divided into three with ridging/positive geopotential height anomalies draped across Northern Siberia, troughing/negative geopotential height anomalies pulled across Southern Siberia and Northern China and finally ridging/positive geopotential height anomalies stretched across Southern Asia (Figure 2).  Likewise Asia is divided into three temperature zones with above normal temperatures across the north slope of Siberia and Southern Asia and in between below normal temperatures stretch from the southern Urals across Northern China and Southern Siberia and extending to Northern Japan (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 7 October 2016 at 00Z. Note the high heights over the North Pacific and European sides of the Arctic, Northern Siberia and eastern North America with low heights over Europe, Southern Siberia and western North America.

Two areas of ridging/positive geopotential height anomalies dominate North America, with one over eastern North America and the other over Alaska (Figure 2).  Underneath the ridging, temperatures are above normal in eastern North America and Alaska (Figure 3).  Between the two ridges negative geopotential height anomalies stretch across Western Canada and the Western US (Figure 2) resulting in below normal temperatures for much of western North America (Figure 3) except along the coast (Figure 2) where weak ridging is resulting in above normal temperatures (Figure 3).

Figure 3. Analyzed surface temperature anomalies (°C; shading) 7 October 2016 at 00Z. Note the warm temperatures across Northern Europe, Northern Siberia and eastern North America with cool temperatures in Europe, Southern Siberia and western North America.

Hurricane Matthew will bring copious amounts of precipitation to the coast along the Southeastern US (Figure 4). Strong ridging across Northern Europe will bring dry weather to much of Northern and Western Europe, however precipitation will be above normal east of the closed low in Southeastern Europe (Figure 4).  Wet weather is also predicted for the Asia monsoon regions and Eastern Siberia, which will be under a moist southeasterly flow of air (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/day; shading) from 8 – 12 October 2016. Note the wet conditions across Southeastern Europe, Southeastern China, Japan, the Eastern US and Southern Canada. The forecasts are from the 00Z 7 October 2016 GFS ensemble.

With the negative AO snow cover will likely advance across the Northern Hemisphere at an accelerated pace.  Snow cover advance is predicted across much of Siberia, the Tibetan Plateau and Northern Canada (Figure 5).

Figure 5. Forecasted snow depth anomalies (mm/day; shading) from 8 – 12 October 2016. Note the snowfall over parts of Siberia, the Tibetan Plateau and Northwest Canada. The forecasts are from the 00Z 7 October 2016 GFS ensemble.

Near-Term

1-2 week

The AO is predicted to remain strongly negative over the weekend and remain negative all of next week (Figure 1).  The negative AO is a result of positive geopotential height anomalies stretched across the entire Arctic (Figure 6a).  Since the region of positive geopotential height anomalies include Greenland with negative geopotential height anomalies further south across the North Atlantic, the NAO will also be strongly negative.

Figure 6. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 13 – 17 October 2016. (b) Same as (a) except averaged from 18 – 22 October 2016. The forecasts are from the 7 October 2016 00z GFS ensemble.

Ridging/positive geopotential height anomalies are predicted to persist across Northern Europe (Figure 6a).  Widespread positive heights will persist above normal temperatures for Scandinavia (Figure 7).  However the rest of Europe will be under the influence of negative geopotential height anomalies and northerly flow (Figure 6a) that will transport below normal temperatures across Europe (Figure 7).  Meanwhile Asia will remain divided into three zones with ridging/positive geopotential height anomalies draped across Northern Siberia, troughing/negative geopotential height anomalies stretching from the Urals to Northern Japan and finally ridging/positive geopotential height anomalies stretched across Southern Asia (Figure 6a).  Likewise Asia will remain divided into three temperature zones with below normal temperatures stretching from the southern Urals across Northern China and Southern Siberia and extending to Northern Japan sandwiched in between above normal temperatures across the north slope of Siberia and Southern China (Figure 7).

Figure 7. Forecasted surface temperature anomalies (°C; shading) from 13 – 17 October 2016. Note the warm temperatures across much North America, Northern Europe and Southern China with cold temperatures in Europe, Siberia and the Eastern US. The forecasts are from the 00Z 7 October 2016 GFS ensemble.

The dominant circulation feature this period will be an anomalously deep and southeastward displaced Aleutian Low (Figure 6a).  This will help build positive geopotential height anomalies downstream across the US and Southern Canada (Figure 6a).  The resultant southwesterly flow across the continent will result in above normal temperatures for much of the US and Canada (Figure 7). Low pressure that was previously near the North Pole will be shunted south over Hudson Bay this period (Figure 6a). Northerly flow from the closed low could transport enough Canadian air across the border to keep temperatures closer to seasonable or even slightly bellow normal for parts of the Eastern US (Figure 7).

Figure 8. Forecasted snow depth anomalies (mm/day; shading) from 13 – 17 October 2016. Note the snowfall over Siberia, Scandinavia and Northern Canada. The forecasts are from the 00Z 7 October 2016 GFS ensemble.

With the AO bottoming out this period, snow cover will likely continue at a rapid advance across the Northern Hemisphere this period.  Snow cover advance is predicted widespread across Siberia, Scandinavia and Northern Canada (Figure 8).

3-4 week

Positive pressure/geopotential height anomalies are predicted to persist across the Arctic this period (Figure 6b). Widespread positive heights across the Arctic and negative heights across the mid-latitudes will likely maintain a negative AO bias into the third week of October (Figure 1).

Little change is predicted in the general circulation pattern across Eurasia this period (Figure 5b). The above normal geopotential heights across Scandinavia previously, will recede northward allowing for almost all of Europe to be dominated by negative geopotential height anomalies (Figure 6b).  Negative heights should result in seasonable to seasonably cold temperatures for much of Europe (Figure 9).  Asia will remain divided into three zones with ridging/positive geopotential height anomalies draped across Northern Siberia, troughing/negative geopotential height anomalies stretching from the Urals to Northern Japan and finally ridging/positive geopotential height anomalies stretched across Southern Asia (Figure 6b).  Some increased ridging will allow temperatures to be more seasonable in Western Asia including Western Siberia while northwesterly flow will transport below normal temperatures from Central Siberia to Northern China (Figure 9).    Above normal temperatures will persist across Southern Asia (Figure 9).

Figure 9. Forecasted surface temperature anomalies (°C; shading) from 18 – 22 October 2016. Note the warm temperatures across North America and Northern Europe, with cool temperatures in Western Asia, Eastern Europe and Southern Siberia.   The forecasts are from the 00Z 7 October 2016 GFS ensemble.

Models predict that a deep Aleutian low will help to persist positive geopotential height anomalies across most of North America with the exception of the Eastern US, as troughing/negative geopotential height anomalies in the North Atlantic retrograde westward (Figure 6b). This should continue to favor temperatures averaging seasonable to above normal for most of North America (Figure 9). One exception could be the US East Coast where weak troughing and northerly flow (Figure 6b) may result in more seasonable temperatures (Figure 9).

Figure 10. Forecasted snow depth anomalies (mm/day; shading) from 18 – 22 October 2016. Note the snowfall over much of Siberia and Scandinavia with snow melt over Northern Canada. The forecasts are from the 00Z 7 October 2016 GFS ensemble.

With the AO still predicted to be negative this period, snow cover advance will likely continue across northern Eurasia (Figure 10).  However rising geopotential heights across Canada could cause snow melt across Northern Canada (Figure 10).

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows well above normal PCHs in the troposphere (Figure 11) consistent with the strongly negative AO. And as discussed earlier, the strongly positive PCHs seem to have descended from the stratosphere.  Low Arctic sea ice will continue to favor warm/positive PCHs.  Therefore I expect little change in the tropospheric PCHs in the coming weeks with a continued bias towards warm/positive PCHs.

Figure 11. 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 7 October 2016 GFS ensemble.

A negative AO both in the stratosphere and the troposphere (Figure 1), a weak polar vortex in the troposphere and possibly the stratosphere and positive PCHs favor below normal temperatures across the NH mid-latitudes, especially across Northern Eurasia.  In the near term negative geopotential height anomalies in the Gulf of Alaska will keep much of North America mild.  However if the low heights retrograde west with time that will allow more troughing in eastern North America and colder temperatures.  However if the low heights remain nearly stationary or even push further east temperatures will continue to run mild.

Surface Boundary Conditions

Arctic Sea Ice

Arctic sea ice reached its annual minimum on September 10th. Despite that Arctic sea ice is now growing, large swaths of the North Pacific side of the Arctic basin are ice-free.  One area to note on the North Atlantic side is in the Barents-Kara Seas, where sea ice anomalies are also negative (Figure 11). 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 below normal, this currently favors more extensive Siberian snow cover in the coming weeks, followed by a strengthened Siberian high and a weakened polar vortex/negative AO this upcoming winter.

Figure 12. Observed Arctic sea ice extent on 6 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 12). The cool anomalies (i.e., La Niña-like) are not terribly impressive and most ENSO forecasts are for neutral or weak La Niña conditions. Though it does seem that SSTs have cooled along the equator favoring a recent tilt towards La Niña.   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 early in this fall season.

Figure 13. The latest weekly-mean global SST anomalies (ending 5 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.

We also note that northern North Atlantic SSTs are warmer than 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 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 colder Europe.  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, the streak of warm weather will persist.

Northern Hemisphere Snow Cover

October snow cover advance across Eurasia has so far been more rapid than normal. The predicted negative AO and below normal heights across Siberia favor snow cover to continue advancing at a normal to above normal rate.  With the AO/NAO predicted to be negative at least up to the third week of October confidence is growing that October Eurasian snow cover will be above normal once again this year.  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.