Winterverwachting 2021-2022 door Judah Cohen
Elk jaar zorgt de gerenommeerde, Amerikaanse wetenschapper Judah Cohen voor een exclusieve winter gastblog op onze website. In de blog schat hij aan de hand van verschillende (soms eigen ontwikkelde) parameters de kans op deftig winterweer in Europa komende winter in. De winterupdate is dus gebaseerd op allerlei parameters (ENSO, QBO …) die belangrijk zijn om de winterkansen op het Noordelijk Halfrond in te schatten. Dit artikel is uitzonderlijk in het Engels, maar is zeker waardevol voor menig weerliefhebber die zich wil verdiepen in winterkansen. Veel leesgenot!
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We are currently in a double dip La Niña (La Niña two years in a row) and many of the same forcings are common to last winter and the dynamical model forecasts are nearly identical to last winter. The most commonly used predictor in seasonal prediction is the El Niño/Southern Oscillation or (ENSO). Though the government forecast centres rely mostly on dynamical models for seasonal forecasts, they are very sensitive to the phase of ENSO.
In Figure 1, I include the multi-model ensemble from the North American models (NNME; includes coupled models from a number of US and Canadian modeling centers) and from the European models (C3S) including the European Centre for Medium-range Weather Forecasting (ECMWF) model, the UK Met Office Hadley Center Unified Model and the and the MeteoFrance model. This winter, a weak to moderate La Niña is predicted and a canonical La Niña pattern can be seen across North America with below normal temperatures in the northwestern part of North America and above normal temperatures elsewhere across the continent including Northeastern Canada and much of the US. Across Eurasia where ENSO influence is weaker the models default to a “global warming” forecast with above normal temperatures throughout the continent.
In contrast, I have focused my research on the contribution of Arctic forcings on mid-latitude weather especially on winter and at seasonal timescales. Much of my research has been on the relationship between October Siberian snow cover extent on winter surface temperature anomalies across the Northern Hemisphere (NH). When October Siberian snow cover is extensive, winter temperatures are relatively cold across Northern Eurasia including Northern Europe and the Eastern US with relatively mild temperatures across the Mediterranean, North Africa and Northeast Canada. This past October Siberian snow cover was slightly above normal but less than one standard deviation and similar to last October. This by itself favours a relatively cold winter across the NH including Northern Europe, but admittedly the signal is once again not strong this year.
Recently I have also been researching the relationship between Arctic sea ice extent and winter weather. In general, reduced Arctic sea ice extent is related to cold temperatures across the NH but regionality is important. Low sea ice in the Barents-Kara Seas favours cold in Asia, low sea ice in the Chukchi and Bering Seas favour cold in eastern North America and low sea ice around Greenland favour’s cold in Northern Europe. This fall the region that has experienced the greatest negative anomalies in sea ice extent has been in and around Greenland especially Baffin Bay and Hudson Bay. I interpret the sea ice anomalies as favouring cold temperatures for Europe. And so far in December, Europe has experienced some cold temperatures, more so than the US for example. However, the region of sea ice considered most influential for influencing the weather across the NH is the Barents-Kara Seas and there the sea ice cover is very close to normal (see Figure 2) and may not be a big factor this winter.
I and others have argued that Siberian snow cover and Arctic sea ice can influence winter weather across the mid-latitudes through interactions with the polar vortex (PV). The PV is a fast-flowing ribbon or river of air that circumnavigates the Arctic high above the earth’s surface in the stratosphere. When the PV is strong, cold air remains confined over the Arctic while the mid-latitudes are relatively mild but when the PV is perturbed or weak the Jet Stream becomes wavier or more meandering allowing cold air from the Arctic to be transported to the mid-latitudes including Europe and the Eastern US. Both extensive October Siberian snow cover and diminished Arctic sea ice extent in the Barents-Kara Seas are related to a weaker or more perturbed winter PV and colder winter temperatures. I don’t think that Arctic sea ice will be a big factor in influencing the PV but instead I do believe that snow depth across Siberia could be a bigger factor. And currently snow depths across most, if not all, of Siberia are above normal (see Figure 3).
Quasi Biennial Oscillation
Another factor that could be important this winter is the phase of the quasi biennial oscillation (QBO). The phase of the QBO is determined by the sign of the winds in the equatorial stratosphere. When the winds are easterly or negative so is the QBO and when the winds are westerly or positive so is the QBO. Often the winds are from different directions at different levels but the winds in the lower stratosphere are considered most important for the behaviour of the PV. The winds in the lower equatorial stratosphere are currently easterly or negative. There is both observational and modelling support that an easterly QBO favours a weak PV and relatively cold temperatures across the NH mid-latitudes including Europe. So even though Arctic forcing for a disrupted PV is weak, the QBO could be amplify this forcing.
Pacific Decadal Oscillation
The last factor that I considered for the winter forecast is the Pacific Decadal Oscillation or PDO. The PDO was anomalously negative this past fall, more so than in many decades. A negative PDO favours a similar temperature pattern as La Niña across North America with below normal temperatures in the northwestern part of North America and above normal temperatures in the Southern and Eastern US. And like La Niña it seems that the impacts from the PDO are weaker across Eurasia than North America. In summary it does seem to me that PDO can amplify the influence of La Niña.
In Figure 1, I also include the winter surface temperature anomalies from the AER statistical model. The major predictors used in the model are predicted winter ENSO (Niño 3.4 index), estimated October Arctic sea ice concentration anomaly in the Barents-Kara Seas, October Eurasian snow cover extent anomaly, the October sea level pressure (SLP) anomaly in northwestern Asia and the observed October value of the PDO. The AER statistical model predicts relatively warm temperatures across Southeastern Europe, Southern Asia, Northeastern Canada and the Southern and Eastern US with seasonable to cold temperatures across most of Europe but especially Northern Europe, Northern and East Asia, Western and Central Canada and the Northwestern United States.
Last winter there was strong high latitude blocking (stagnant high pressure) centered near the Urals and the Barents-Kara Seas in December. High latitude blocking is critical for cold weather in the mid-latitudes. This resulted in a large and extended PV disruption beginning in late December and continuing into early February. This winter the occurrence of high latitude blocking in particular near the Urals has been absent. There are signs of the return of Ural blocking but it’s ability to disrupt the PV will be later than last winter or even possibly completely absent.
However, I think there will be at least a minor disruption of the PV in early January in the form of a stretched PV (see Figure 4). During these events the shape of the PV is not circular, which is the typical state, but instead the PV becomes stretched resulting in cross polar flow from Siberia to North America acting as a conduit for cold air from Siberia to Canada. A stretched PV favors cold and possibly extreme cold in Central to East Asia and especially North America mostly east of the Rockies. For Europe there doesn’t seem to be much of a relationship between a stretched PV and temperatures. Also, the cold anomalies tend to last for one to two weeks and not longer. I do think that if Ural blocking can persist for long enough in the coming weeks, then a larger and longer lasting PV disruption either can occur either later in January or in February.
Northern Europe is currently cold in large part due to strong Greenland blocking. However, the forecasts are for the Greenland blocking to wane and for temperatures to turn milder across Europe heading into the New Year. As I have discussed previously, the best chance for Northern and Western Europe to have a cold winter is for a PV split to occur. There are two types of PV disruptions: displacements and splits. In a PV displacement the PV remains intact but is displaced off the North Pole and weakens. In a PV split the original one PV splits into two sister vortices that then tend to meander closer to the mid-latitudes. A nice example of a PV split occurred in February 2018. Following the PV split it turned much colder across Europe including Western Europe and at eventually in the Eastern US. There are no credible signs of a PV split but one could still occur in the coming weeks and months.
In conclusion, based on above normal October snow cover extent and La Niña/negative PDO our model is predicting the best chances of a cold winter are in Central and East Asia, Central Canada, the northwestern US and Western and Central Europe. The best chance for a cold winter in Northern Europe would be the occurrence of a PV split in my opinion. There are no signs that a PV split will occur in January based on the dynamical models. However, I do think there is a possibility of one in late January and into February. We do run operationally an experimental PV forecast model and it suggests a good chance of a significant weakening of the PV in late January. If a PV split does occur that would greatly increase the chance of a cold winter across Northern Europe but not guarantee it. If a PV split does not occur this winter, then I do believe that an overall mild winter is more likely across Europe.