Yesterday at AER Dr. Judah Cohen provided his Arctic Oscillation and Polar Vortex Analysis and Forecasts biweekly report and outlook regarding the arctic oscillation and the coming winter in Northern Hemisphere. Excerpts with my bolds.
- As is often the case, the current positive AO is associated with a relatively mild weather pattern across the NH continents including Europe and much of North America.
- However over the next two weeks with the predicted overall negative trend in the AO a concomitant cooling trend is predicted across the NH continents including the British Isles and Western Europe but especially the Eastern United States (US).
- Across East Asia troughing will allow a series of fronts to swing through the region keeping temperatures variable but overall close to seasonable.
- Looking ahead to this upcoming winter, in my opinion both below normal Arctic sea ice and above normal Siberian snow cover so far this month favor more severe winter weather especially mid and late winter across the NH mid-latitudes. Though it is still early and there is much uncertainty in predictions of winter weather.
The flow across the NH is currently mostly zonal especially across North America and this is resulting in an overall mild weather pattern including Europe and the US. The exception to the zonal flow is a block over the Laptev Sea resulting in troughing/negative geopotential height anomalies over both Western and Eastern Asia and colder temperatures.
Siberian snow cover has advanced at a relatively rapid pace so far this fall, which has been the recent trend. However snow cover extent this October is so far lagging the pace of last October. My, along with my colleagues and others, research have shown that extensive Siberian snow cover in the fall favors a trough across East Asia with a ridge to the west near the Urals. This atmospheric circulation pattern favors more active poleward heat flux, a weaker PV and cold temperatures across the NH.
Strong negative departures in the Barents-Kara Seas favors cold temperatures in Asia while strong negative departures near Greenland and/or the Beaufort Sea favor cold temperatures in eastern North America. However sea ice is currently more extensive in the Barents-Kara-Laptev Seas than last year at this time and even more than two years ago. I believe that low sea ice in the Barents Kara sea the past two winters helped anchor blocking in the region that favored cold temperatures in Eurasia relative to North America. That same forcing may not be as strong for the upcoming winter.
I would conclude that the three factors that I consider favorable for severe winter weather increased atmospheric blocking in the fall, more extensive Siberian snow cover and low Arctic sea ice have become the norm more than the exception over the past decade. I do believe that the lack of variability in these three factors, likely reduces their utility in winter predictions.
From Post Natural Climate Factors: Snow
Previously I posted an explanation by Dr. Judah Cohen regarding a correlation between autumn Siberian snow cover and the following winter conditions, not only in the Arctic but extending across the Northern Hemisphere. More recently, in looking into Climate Model Upgraded: INMCM5, I noticed some of the scientists were also involved in confirming the importance of snow cover for climate forecasting. Since the poles function as the primary vents for global cooling, what happens in the Arctic in no way stays in the Arctic. This post explores data suggesting changes in snow cover drive some climate changes.
The Snow Cover Climate Factor
The diagram represents how Dr. judah Cohen pictures the Northern Hemisphere wintertime climate system. He leads research regarding Arctic and NH weather patterns for AER.
Conceptual model for how fall snow cover modifies winter circulation in both the stratosphere and the troposphere–The case for low snow cover on left; the case for extensive snow cover on right.
1. Snow cover increases rapidly in the fall across Siberia, when snow cover is above normal diabatic cooling helps to;
2. Strengthen the Siberian high and leads to below normal temperatures.
3. Snow forced diabatic cooling in proximity to high topography of Asia increases upward flux of energy in the troposphere, which is absorbed in the stratosphere.
4. Strong convergence of WAF (Wave Activity Flux) indicates higher geopotential heights.
5. A weakened polar vortex and warmer down from the stratosphere into the troposphere all the way to the surface.
6. Dynamic pathway culminates with strong negative phase of the Arctic Oscillation at the surface.
From Eurasian Snow Cover Variability and Links with Stratosphere-Troposphere
Coupling and Their Potential Use in Seasonal to Decadal Climate Predictions by Judah Cohen.
Observations of the Snow Climate Factor
The animation above shows from remote sensing that Eurasian snow cover fluctuates significantly from year to year, taking the end of October as a key indicator. Snowfall in 2016 was especially early and extensive, 2017 similar but slightly less at this point.
For several decades the IMS snow cover images have been digitized to produce a numerical database for NH snow cover, including area extents for Eurasia. The NOAA climate data record of Northern Hemisphere snow cover extent, Version 1, is archived and distributed by NCDC’s satellite Climate Data Record Program. The CDR is forward processed operationally every month, along with figures and tables made available at Rutgers University Global Snow Lab.
This first graph shows the snow extents of interest in Dr. Cohen’s paradigm. The Autumn snow area in Siberia is represented by the annual Eurasian averages of the months of October and November (ON). The following NH Winter is shown as the average snow area for December, January and February (DJF). Thus the year designates the December of that year plus the first two months of the next year.
Notes: NH snow cover minimum was 1981, trending upward since. Siberian autumn snow cover was lowest in 1989, increasing since then. Autumn Eurasian snow cover is about 1/3 of Winter NH snow area. Note also that fluctuations are sizable and correlated.
The second graph presents annual anomalies for the two series, each calculated as the deviation from the mean of its entire time series. Strikingly, the Eurasian Autumn flux is on the same scale as total NH flux, and closely aligned. While NH snow cover declined a few years prior to 2016, Eurasian snow is trending upward strongly. If Dr. Cohen is correct, NH snowfall will follow. The linear trend is slightly positive, suggesting that fears of children never seeing snowfall have been exaggerated. The Eurasian trend line (not shown) is almost the same.
What About Winter 2017-2018?
These data confirm that Dr. Cohen and colleagues are onto something. Here are excerpts from his October 2 outlook for the upcoming season AER. (my bolds)
The main block/high pressure feature influencing Eurasian weather is currently centered over the Barents-Kara Seas and is predicted to first weaken and then strengthen over the next two weeks.
Blocking in the Barents-Kara Seas favors troughing/negative geopotential height anomalies and cool temperatures downstream over Eurasia but especially Central and East Asia. The forecast for the next two weeks across Central Asia is for continuation of overall below normal temperatures and new snowfall.
Currently the largest negative anomalies in sea ice extent are in the Chukchi and Beaufort Seas but that will change over the next month or so during the critical months of November-February. In my opinion low Arctic sea ice favors a more severe winter but not necessarily hemisphere-wide and depends on the regions of the strongest anomalies. Strong negative departures in the Barents-Kara Seas favors cold temperatures in Asia while strong negative departures near Greenland and/or the Beaufort Sea favor cold temperatures in eastern North America.
Siberian snow cover is advancing quickly relative to climatology and is on a pace similar to last year at this time. My, along with my colleagues and others, research has shown that extensive Siberian snow cover in the fall favors a trough across East Asia with a ridge to the west near the Urals. The atmospheric circulation pattern favors more active poleward heat flux, a weaker PV and cold temperatures across the NH. It is very early in the snow season but recent falls have been snowy across Siberia and therefore I do expect another upcoming snowy fall across Siberia.
In summary the three main predictors that I follow in the fall months most closely, the presence or absence of high latitude blocking, Arctic sea ice extent and Siberian snow cover extent all point towards a more severe winter across the continents of the NH.
Uh oh. Now where did I put away my long johns?