Two days ago I wrote about the first part of an analysis (on Germany winter temperatures) by Kowatsch and Kämpfe appearing here at EIKE. Winter temperatures in Germany have been falling for a quarter of a century now. Much to my satisfaction, that post has been widely shared among social media.
Today I’m writing about the second part: What is the primary driver behind Europe’s variability, i.e. what causes periods of cold winters and periods of milder winters? The main drivers, Kowatsch and Kämpfe conclude, are oceanic cycles.
Figure 1 below shows a plot of German winters since 1881. Shown is the temperature lower curve and the number of days with westerly winds (upper curve) – along with their corresponding smoothed curves.
Figure 1: Germany’s mean winter temperature (lower blue curve) follows the course of the frequency of days with mild westerly winds (W, SW and NW, violet upper curve). Both are accompanied by a smoothed curve).
It’s no surprise that the more days a winter has with winds coming from the west (Atlantic), the milder the winters turn out to be. A correlation here does not surprise us. Here the mechanism that drives Europe’s winters is the North Atlantic Oscillation (NAO), which is the pressure difference between southwest Europe (Portugal to the Azores) and northwestern Europe (Iceland).
When the NAO is very positive it means there is a powerful Azores high and a powerful Icelandic low which serve to pump Atlantic air eastwards into Central Europe (Figure 2, right). If the Azores high and the Iceland low are both weak, then cold air from Eastern Europe or Siberia can make its way over across Europe and the winters tend to be much colder (Figure 2, left).
Figure 2: Prevailing weak NAO pattern shown left leads to cold Europe winters. Strong positive NAO pattern shown right leads to mild winters (Source: UKMO).
Next Figure 3 shows the NAO chart for the past winter, which was most of the time was highly positive, meaning many mild westerly winds swept in from the Atlantic and over Europe.
Figure 3: Winter 2014/15 saw an overwhelmingly positive NAO, thus producing a mild winter for Western and Central Europe.
So what drives the NAO air pressure difference? Kowatsch and Kämpfe have analyzed this and found there is a strong correlation between NAO and the Atlantic Multidecadal Osciallation (AMO). Figure 4 below shows the inverse relationship between the AMO and the winter-time westerly wind frequency over Europe:
Figure 4: The higher the AMO value, the less westerly weather that occurs.
Not only does the AMO drive the NAO, but it is also is a major factor driving Arctic sea ice extent. Arctic sea ice extent does not drive the winters over Europe, as some scientists have been hypothesizing over the recent years. Rather it is the AMO that is driving the Arctic sea ice and the European winters as well.
Although good satellite sea ice data records for the Arctic go back only 35 years, one sees a distinct relationship between the AMO and wintertime Arctic sea ice, see Figure 5 below:
Figure 5: As AMO values rise (green curve), sea ice area (blue) reduces significantly.
And there are clear indications that this relationship applies over the long-term as well. During the 1930s, i.e. during the last AMO positive phase, large melting of the sea ice and strong melting of the Greenland glaciers were observed.”
And when the severe winters of 2009/10 and 2012/13 caused the proponents of the global warming theory to scramble for an explanation, they concocted and put out the tale that “melting Arctic sea ice was disturbing the large scale circulation and thus favored winter cold at the middle latitudes“.
The scientists claimed that especially the low levels of Arctic sea ice in September were suddenly responsible for causing cold winters. Yet, the following chart shows no relationship at all:
Figure 14: The extent of September Arctic sea ice has no impact whatsoever on winter temperatures over Central Europe (Germany). Arctic sea ice cover in blue; Germany winter temperatures in red. The same is true for other times of the year (autumn ice cover or winter ice cover to winter temperatures show no relationship).
EIKE warns that the climate system is much more complicated than meets the eye: “Still the complicated and yet to be researched relationship between ocean currents, AMO, sea ice and large weather patterns have with a high probability an impact on Europe’s climate and weather, and there exists no easy explanations”. Studies have shown that solar activity also play a role in Europe’s winters.
At the end, Kowatsch and Kämpfe look at the (lack of) success that institute’s and experts have had in forecasting the winter of 2014/15. It shows that the science of forecasting is lacking terribly. Of the 7 forecasts examined, 2 were completely faulty, 3 were poor, and 2 were only about half correct and would not earn a grade any higher than a C -.