Can we expect an Elfstedentocht soon?

Update September 2021 - original IJsbode prediction September 2009

Can we expect an Elfstedentocht soon?

At the beginning of winter one of the most important questions a ice-skater asks himself is: “Should we prepare for a season with frozen canals and lakes in the Netherlands?”

It has been a while since we had ice-skating tours in our country. Between 2010 and 2013 we had winters with a period with frozen lakes and various skating trips or tours. After that we had some mild winters with only two winters when we could skate for a few days. After the week with severe temperatures and lots of skating last winter, everyone now wants to know whether we can expect another skating tour this year, or even better, an Elfstedentocht.

Predicting a Frisian Eleven Cities Tour, which represent the ultimate skating winters for the last 100 years in the Netherlands, is important for skaters. Really harsh winters occur quite irregularly in time. 24 years have now passed (1997 - present) since the last 'tour of the ice-tours', but also in the past, there were comparable periods (22 years: 1963-1985) without the Elfstedentocht. Here are the eleven cities winters with the different ice thicknesses (cm).

Year Date Ice thickness

1909   Sat   2 Jan           17,9
1912   Wd 7 Feb          20,6
1917   Sat 27 Jan           15,0
1929   Mo 12 Feb          28,1
1933   Sat 16 Dec          20,8
1940   Tue 30 Jan           41,3
1941   Thu   6 Feb          30,5
1942   Thu 22 Jan          34,2
1947   Sat 8 Feb          35,1
1954   Wd 3 Feb          23,2
1956   Tue 14 Feb          23,3
1963   Fri 18 Jan           24,9
1985   Thu 21 Feb          17,4
1986   Wd 26 Feb          24,8
1997   Sat   4 Jan           26,0

There were of course more very cold winters before 1909, but clear information about them is not always available on the internet. Weather data on winters after 1900 has been made available by the KNMI for all winters to date. The cold winters, defined with the so-called Hellmann Number, are shown with various colors on the figure.

Predicting Elfsteden winters turns out to be a difficult task. In the past I sometimes made a prediction based on frequency statistics and I noticed that there are no really regular periods with more than average severe winter in the Netherlands. Predictions based on old wisdom like many migrating geese or beach nuts did also not work out. In 2015 there was a bumper crop of beach nuts, a year with a crappy skating winter. However, last year we had an autumn with many acorns that also fell after St Michiel, but also a winter with severe frost: Do the acorns fall before St –Michiel, then the wind cuts through body and soul.).

What does have some predictable value are sunspots, volcanic eruptions and the global circulation pattern over the Atlantic Ocean (NOA) and possibly even the strength of the trade winds in the tropics (El Niño - La Niña).

Good skating winters and sunspots are plotted on a figure It is clear that periods of absence of sunspots have many severe winters compared to winters with many spots, but there are several exceptions. In particular the winter of 1976 was very disappointing, because there was no severe winter. After that there were elfstedentochts in 1985, 1986 and 1997 at sunspot minima.

Sunspots

Since the figure suggests that sunspots are important for the prediction of ice-skating winters, a more detailed explanation of what exactly is meant by the effect of sunspots on our winters follows.

It is clear to everyone that our climate system on Earth is powered by energy from the sun. However, it is sometimes forgotten that the activity of the sun itself is not a constant factor. It is of course not the intention here to go deeply into interactions of the Sun with the Earth's climate system, but there seems to be a relationship of the Sun's magnetic field with Earth's climate that is not yet well understood. Sunspots are relatively dark spots on the sun's surface that are related to the strength of the magnetic field there. The absence of those spots is associated with a reduced magnetic field from the sun, which in turn weakens the Earth's magnetic field.

On Earth, clear climatic variations can be observed over periods of tens to a hundred years that correspond to activity of the sun's magnetic field. A well-known periodic fluctuation is the sunspot cycle that occurs approximately every 11 (22) years. However, according to the KNMI, there is not yet a good explanation of how this magnetic field can affect our climate. Hypotheses that there is more cloud formation during periods with little or no sunspots are not generally accepted. There are speculations about changes in the trade winds or even the polar and arctic circulation due to a weaker magnetic field. This will be discussed most likely, because we are experiencing an exceptionally quiet period with few or no sunspots, hopefully also with some severe winters. The Earth's magnetic field is lagging slightly behind observable sunspots and has not yet risen sharply.

Other explanations for cold winters

Another independent explanation for a cold winter could be that a large eruption of a volcano causes cooling over large areas. Really large volcanic eruptions in tropical areas have effects on the weather that are noticeable all over the world. In large eruptions, where the eruption is more than twenty kilometers high, small ash particles and sulfur oxide are released into the stratosphere. These ash particles block light from the sun, so that the earth can warm up less. More importantly, sulfur oxides create aerosols at high altitudes in the stratosphere. These particles reflect heat from the sun and can sometimes lower the Earth's temperature for several years afterwards. It is confusing that before this cooling starts, the earth can warm up for a few months because the heat radiation from earth to space is also prevented by the aerosols.

The figure shows that these weather changes due to volcanic eruptions may have played a role for some severe winters. Volcanoes can therefore sometimes serve as an explanation for cold winters, but that such eruptions generally do not seem to be the best predictors of the Elfsteden tour. What makes forecasting complex is that the consequences for Dutch winters of an eruption in the southern hemisphere are different from large eruptions in the tropics or in the northern hemisphere. Recent years have been characterized by a series of small eruptions where the effect on our winter weather is unclear. Larger volcanic eruptions can certainly have a cooling effect. The last major eruption was the Puyehue in Chile (southern hemisphere)

If we compare the Dutch cold winters with large eruptions of volcanoes (Krakatau, which showed a large eruption in August 1883, caused at least two bad summers in the Netherlands), the relationship is not really clear. Volcanoes may explain the winters of 1903 and 1963. I have fond memories of skating in the early 1990s, possibly caused by the impressive eruption of Pinatubo in the Philippines.

Ocean circulation

The Dutch winter weather is also highly dependent on the high-altitude circulation pattern over the Atlantic Ocean (jet current), which is usually fixed for extended periods of time due to blocking high pressure areas. We all familiar with vast Scandinavian high pressure areas that allows us to have severe winters for a few weeks. The drought of 2018 and 2019 can be explained by such a different jet stream pattern.

The deviating weather pattern in the 1940s to 1970s (indicated in blue on the figure) may be the cause of a higher frequency of above-average cold winters in that period.

At the moment, the figure indicates that we have had a strongly negative NAO for several months, in contrast to the previous period from 2011 to 2018, where the NAO was often positive. The NAO provides a picture of the circulation in the Atlantic Ocean and has been showing a negative trend for several months this year. This would be favorable for a cold spell in NW Europe if it persists into winter.

Winter weather in the Northern Hemisphere is highly dependent on the high-altitude circulation pattern above the North Pole, the Arctic Vortex. It influences the jet stream over the Atlantic Ocean, which determines the weather in winter. The predictability of the behavior of this vortex is not great.

Globale weather patterns

We can also look at a more global picture. Three years ago there was a very strong El Niño that caused the higher temperatures over large parts of the earth. At the moment, the situation regarding the ENSO index is neutral to slightly negative (La Niña). The hope we can draw from this is that the heat in the tropics will have less effect than in the winter of 2015/2016. A neutral or negative index has been associated with some harsh winters in the past.

This winter, slightly cooler temperatures than normal can be expected from the tropical areas Statistically, this is therefore no obstacle to a severe winter

A Frisian Elfstedentocht or not?

A good prediction for a skating winter in 2021/22 will have to come from the small numbers of sunspots (a weak magnetic field). Other factors that can help for a prediction are also partly favorable. However, a volcanic eruption that has reached the stratosphere has been a while ago and the remaining aerosols still in the stratosphere will only have a positive effect on a cold winter. A really big volcanic eruption has not been experienced for a while and will have little effect in the coming winter.

So based on the small number of sunspots, there is a reasonable chance that we will get a good skating winter for the winter of 2021/22. The chance of a severe winter seems at least comparable to last year. A neutral El Niño or even a La Niña situation and a negative NAO does not seem to stand in the way of such a forecast.

In short, however uncertain, I must recommend that it is still good to prepare for an “it giet oan”.

Heerlen September 2021 - Fokko van Hulten