September 19, 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.
The Arctic Oscillation (AO) is currently positive but is predicted to trend negative the remainder of the week. The AO is predicted to remain near neutral all of next week, a likely sign of large model uncertainty.
The current positive AO is reflective of mostly negative pressure/geopotential height anomalies in the Arctic especially on the North American side including Greenland and mostly positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic sector.
The forecasts of a near-term negative AO trend are being driven by strengthening positive pressure/geopotential height anomalies both in Western Siberia and Northwestern Canada that eventually push north and converging in the Arctic basin forcing height rises across much of the central Arctic.
With the AO currently positive, temperatures are averaging above normal across much of Asia and North America. One exception is Europe where negative pressure/geopotential height anomalies are resulting in below normal temperatures.
Towards the end of this week and into next week, with the AO trending negative, colder air will move southward across eastern North America and especially Asia. Some cooler air will filter into Europe as well. However with the AO predicted to remain near neutral, temperatures should start to rebound by middle and end of next week.
With the Arctic sea ice melt season now officially coming to an end, the 2016 sea ice minimum will settle tied for second lowest extent observed. Below normal Arctic sea ice in the fall favors a strengthened Siberian high and/or a weaker winter polar vortex (PV).
With both the AO and North Atlantic Oscillation (NAO) positive for the first half of this week, temperatures will average above normal across much of Asia and North America. The positive AO is reflective of negative geopotential height anomalies in the Arctic and positive geopotential height anomalies across much of the mid-latitudes of the Northern Hemisphere (NH) especially in the North Atlantic sector.
One notable exception to the positive geopotential height anomalies across the NH mid-latitudes is a cutoff area of low pressure/geopotential heights centered over Northern Italy, which is resulting in below normal temperatures for much of Central and Southern Europe. A second exception is a trough/area of negative geopotential height anomalies over Western Canada and the northwestern United States (US), which is also resulting in seasonably cool temperatures for the northwestern US and southwestern Canada. A third trough/below normal geopotential heights in East Asia is also currently resulting in seasonably cool temperatures across East Asia, especially along the coast.
However by the end of the week and into next week, those three troughs will be replaced by two new troughs/areas of below normal geopotential heights one in eastern North America and the other in Central Siberia that extends into Central Asia with the Asian trough being the dominant trough across the NH. These two troughs are in response to positive geopotential heights expanding northward from both Northwestern Canada and Western Siberia respectively. As positive geopotential heights converge over the central Arctic, cold air currently over the Arctic (this cold air ended the Arctic sea ice melt season) will be displaced southward over eastern North America and especially over northern Asia.
The trough in eastern North America is predicted to be migratory and temperatures will quickly rebound by early next week. The Asian trough looks more persistent but as geopotential heights are predicted to begin falling again across the Arctic next week, the Asian trough should start to fill and temperatures moderate as well. Even though temperatures across Europe are currently cool, a mostly westerly and southwesterly flow will prevent Arctic air spilling into Europe as well. However, ridging/above normal geopotential heights just west of Europe in the eastern North Atlantic could draw some modified Arctic air into Europe and keep temperatures seasonably cool for next week.
With the start of meteorological fall, I have begun to explore signs for the upcoming winter season. Despite some publicity of forecasts of a severe upcoming winter, I believe it is still too early to make such a forecast with confidence. However one potentially important boundary forcing is already at an extreme level - Arctic sea ice. Arctic sea ice has likely reached its minima for the year, which is tied with 2007 for the second lowest minima ever observed. Our research and that of other groups have shown that below normal sea ice in the fall and winter favor more extensive Siberian snow cover in the fall, a strengthened Siberian high and a weak winter polar vortex/negative winter AO. Also of note, sea surface temperatures (SSTs) in the northern North Atlantic are considerably warmer than this time last year. I believe this favors a weaker polar Jet Stream allowing for the greater possibility of blocking downstream over Europe relative to recent winters. SSTs are even warmer relative to normal in the northern North Pacific and this may favor high latitude blocking in the North Pacific sector as well.
Recent and Very Near Term Conditions
The AO is currently positive (Figure 1), reflective of negative geopotential height anomalies across the Arctic with negative anomalies near the North Pole, Eastern Siberia and the North American side of the Arctic including Greenland (Figure 2). Geopotential height anomalies are mostly positive across the mid-latitudes of the North Atlantic sector (Figure 2).
Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 19 September 2016 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 19 September 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.
Though ridging/positive geopotential height anomalies in the North Atlantic sector extend into westernmost Europe, the dominant circulation feature across Europe is a closed low/area of negative geopotential height anomalies spinning over Northern Italy and the Adriatic Sea (Figure 2). The resultant northerly flow will bring below normal temperatures to much of Central and Southern Europe this week (Figure 3). Further north, ridging/positive geopotential height anomalies across Northern Europe (Figure 2) are resulting in above normal temperatures for Northern Europe (Figure 3). The area of positive geopotential height anomalies across Northern Europe extends eastward into northwest Asia, Western and Central Siberia and then southward into Central Asia (Figure 2) resulting in above normal temperatures for northwestern and Central Asia and much of Siberia (Figure 3). However, a second cutoff area of low pressure in southwest Asia is undercutting the ridging in northwest Asia (Figure 2) resulting in below normal temperatures for southwestern Asia (Figure 3). Meanwhile Eastern Siberia and East Asia are dominated by negative geopotential height anomalies (Figure 2) resulting in seasonable temperature for Eastern Siberia and along the coast of East Asia (Figure 3).
Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 19 September 2016 at 00Z. Note the low heights over the Central Arctic, Western Canada, Southern Europe and Southwestern Asia with high heights over Northern Europe, Western Siberia and Eastern North America.
The week begins with a western trough and eastern ridge across Canada (Figure 2) resulting in above normal temperatures for eastern Canada and more seasonable temperatures for Western Canada (Figure 3). The US is dominated by positive geopotential height anomalies with the exceptions of the northwestern US and a weak trough in the Tennessee Valley (Figure 2). For the week this will result in mostly above normal temperatures for much of the US with the exception of more seasonable temperatures in the northwestern US (Figure 3).
Figure 3. Analyzed surface temperature anomalies (°C; shading) from 19 September 2016 at 00Z. Note the warm temperatures across much of Northern Europe, Northern Asia and North America with cool temperatures in Central and Eastern Europe and Southwestern Asia.
Across North America the heaviest precipitation is predicted in the vicinity of the two troughs, one in western North America including Alaska and the other in the mid-Atlantic ahead of the Tennessee valley trough (Figure 4). Strong ridging across northern Europe will bring dry weather to much of Europe with the exception of the Balkans and Turkey in the vicinity of the closed low pressure system (Figure 4). Wet weather is also predicted for the Asia monsoon regions (Figure 4).
Figure 4. Forecasted precipitation anomalies (mm/s; shading) from 20 – 24 September 2016. Note the wet conditions across southeastern Europe, Japan, the monsoon region of North America, the Rockies, Southwestern Canada and Alaska. The forecasts are from the 00Z 19 September 2016 GFS ensemble.
After ending the week in negative territory, the AO is predicted to oscillate close to neutral (Figure 1). The near neutral AO is a result of mixed positive geopotential height anomalies both in the high latitudes and the mid-latitudes, especially in the North Atlantic sector (Figure 5a).
Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 25 – 29 September 2016. (b) Same as (a) except averaged from 30 September – 4 October 2016. The forecasts are from the 619 September 2016 00z GFS ensemble.
The ridging/positive geopotential height anomalies that were in the eastern North Atlantic the previous week, expand further eastward deeper into Europe (Figure 5a). Widespread positive heights will usher warmer temperatures across Europe but still close to seasonable levels (Figure 6). One exception could be southeastern Europe where persistent negative geopotential height anomalies (Figure 5a) will maintain below normal temperatures in Southeastern Europe and Southwestern Asia (Figure 6). Meanwhile further downstream, the positive geopotential height anomalies from the previous period over northwestern Asia will expand northward (Figure 5a), not only persisting above normal temperatures across northwestern Asia (Figure 6) but will force negative geopotential height anomalies, previously in the central Arctic to spiral into Central Siberia. This will carve out a large area of negative geopotential height anomalies over Central and Eastern Siberia and eventually into Central Asia (Figure 5a). The large area of negative geopotential height anomalies and northerly flow will allow below normal temperatures to become widespread across much of Siberia and then later into Central Asia (Figure 6). Downstream of the Central Siberian trough, positive geopotential height anomalies will strengthen across Eastern Siberia and far Eastern Asia (Figure 5a) allowing for temperatures to warm across Eastern Siberia and East Asia (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 25 – 29 September 2016. Note the warm temperatures across much North America, Northern Europe and East Asia with cool temperatures in Eastern Europe, Southwestern Asia, Siberia and the Northeastern US. The forecasts are from the 00Z 19 September 2016 GFS ensemble.
Once again positive geopotential height anomalies are predicted to dominate North America (Figure 5a). High heights and a mostly zonal flow across the continent will result in above normal temperatures for much of the US and Canada (Figure 6). One exception could be Southeast Canada and the Northeastern US where below normal temperatures are predicted (Figure 6). Strong ridging in Northwest Canada is predicted to force a sharp if brief trough in eastern North America (Figure 5a). Northerly flow will drag cold air out of the Arctic as far south as the Northeastern US. However due to the brevity of the event, temperatures for week are only predicted to be slightly below normal (Figure 6).
Mixed pressure/geopotential height anomalies are predicted to persist across the Arctic this period (Figure 5b). That combined with mixed pressure/geopotential height anomalies across the mid-latitudes are predicted to anchor the AO close to neutral into early October (Figure 1).
Above normal geopotential heights that have been stationary across the eastern North Atlantic are predicted to persist across Western Europe (Figure 5b). Positive heights favor seasonably warm temperatures across Western Europe (Figure 7). However, as the positive heights in the eastern North Atlantic expand northward this period, this will force a lowering of geopotential heights across Scandinavia and into Eastern Europe resulting in seasonable to seasonably cool temperatures across Scandinavia, Central and Eastern Europe (Figure 7). Downstream of the European trough, the persistent positive geopotential height anomalies across Northwestern Asia will maintain seasonable to seasonably warm temperatures for Northwestern Asia (Figure 7). Further east, troughing/negative geopotential height anomalies and northerly flow are predicted to persist for Central and Southern Siberia (Figure 5b). This should result in in a continuation of below normal temperatures for Central and Southern Siberia and even Northern China (Figure 7). However weak ridging/positive geopotential height anomalies in Eastern China and Japan (Figure 5b) will impede the southerly progression of the colder temperatures across the remainder of East Asia (Figure 7).
Figure 7. Forecasted surface temperature anomalies (°C; shading) from 30 September – 4 October 2016. Note the warm temperatures across western North America and Northern Europe, with cool temperatures in Central Europe, Southern Siberia and the Northeastern US. The forecasts are from the 00Z 19 September 2016 GFS ensemble.
Models predict that the pattern of positive geopotential height anomalies dominating most of North America to persist with the exception of negative geopotential height anomalies near the US Pacific Northwest (Figure 5b). This should continue to favor temperatures averaging seasonable to above normal for most of North America (Figure 7). One exception could be the Northeastern US where weak troughing in Eastern Canada (Figure 5b) will help transport cooler air out of Canada (Figure 7).
The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows slightly above normal PCHs currently but turning warmer/more positive for the following week and peaking in late September (Figure 8), consistent with the predicted overall negative AO trend. 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 bias towards warm/positive PCHs.
Figure 8. 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 19 September 2016 GFS ensemble.
In Figure 1 I did include the AO in the middle stratosphere at 10 hPa. This time of year troposphere-stratosphere coupling is inactive and I usually do not include the stratospheric AO in my discussions. However due to increased greenhouse gases the stratosphere tends to be relatively cold and the stratospheric AO is positive outside of active periods of troposphere-stratosphere coupling. Therefore I am intrigued that the stratospheric AO is deeply negative in September. Last winter the strongly positive stratospheric AO during the early part of the winter contributed to the record warmth. It will be interesting to monitor if the stratospheric AO retains a more negative tendency heading into the winter.
Surface Boundary Conditions
Arctic Sea Ice
Arctic sea ice has likely achieved its minimum for this year last week, which tied 2007 for the second lowest minimum. 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 9). 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 months, followed by a strengthened Siberian high and a weakened polar vortex/negative AO this upcoming winter. The lowest sea ice years – 2007 and 2012 were both followed by a strengthened Siberian high the following winter but differences in the location and orientation resulted in different patterns of temperature anomalies across Eurasia.
Figure 9. Observed Arctic sea ice extent on 18 September 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 10). The cool anomalies (i.e., La Niña-like) are not terribly impressive and most ENSO forecasts are for neutral or 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 early in this fall season.
Figure 10. The latest weekly-mean global SST anomalies (ending 17 September 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 noticeably 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. 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.