Hellmann development Dutch winters shows cold declining

In climatology development of the average may differ substantially from the extremes – both as a characteristic of the normal distribution – and the possibility of skewness increases.

On average the Dutch climate shows a clear warming trend – but compared to spring, summer and autumn, the winter shows somewhat better resilience – with a smaller increase of average temperatures. In scientific literature it has been suggested a few negative feedbacks could be at play, whereby Arctic warming may actually increase the chance of experiencing cold European winters.

However, based on Hellmann data by the Royal Netherlands Meteorological Institute KNMI, such theories are not supported by multi-decadal evidence. Not only are average winter temperatures rising, also the cumulative frost is declining – albeit that here the trend indeed seems a little less steep:

Dutch winters Hellmann number graph trend
Declining Hellmann trend for Dutch winters between 1900 and 2012 suggest cold winters are becoming increasingly rare. The green winters have a substantial chance of snow and ice – and an ice skating period in one of the winter months, but can still be mild overall. The orange winters are mild and rainy, with a very dominant influence of Atlantic depressions. The blue dots represent a very different category, actual cold winters, some of which can last for months on end. It shows the erratic nature of the Dutch climate – meteorology is a far greater influence on temperature and winter ice conditions than temperature trends. Cold winters have lasting high pressure blockades, mostly over Scandinavia. The high Hellmann score of the cold winters has the potential to disturb the trend, but a weak decline is visible – although not statistically relevant on the presented timescale*. The last cold winter was the winter of 1997. Despite very low temperatures in February 2012 the frost score of the current winter is ‘normal’. Graph by Dorien Bartels – data from Royal Netherlands Meteorological Institute KNMI.

[*) The trend would be steeper on a shorter timescale, as the coldest winters occured in the mid-20th century – and climate warming has been most pronounced since the 90s. The trend could even be steeper on a longer timescale, as in the Netherlands (much) colder winters occured before 1850.]

Elfstedentocht: is cultural heritage at stake?

After a substantial two-week cold snap in Eastern Europe, Germany and the Low Countries, with temperatures in the Netherlands sinking to -23 degrees Celsius lows, the Dutch had a good chance to enjoy their national sport and cultural tradition: ice skating.

The legendary Elfstedentocht* – an ice-skating event across 200 kilometres of frozen canals, lakes and slow-running rivers in the northern province of Friesland (a race, followed by 16,000 well-trained amateurs, a predicted two million-people crowd of supporters along the ice) was cancelled because of critical ice conditions.

[*) Actual name in Frisian language: Âlvestêdetocht – translating as a tour along eleven (Frisian) cities.]

Although the entire IJsselmeer lake and much of the salty Wadden Sea are frozen, with shipping lanes requiring ice breakers, and despite massive snow-clearing efforts [snow hinders ice growth underneath] the minimally required ice thickness of 15 centimetres was not reached along the entire Elfstedentocht route.

Many other long-distance skating races and events on lakes and canals with between one and two million participants across the country did take place last week and several are still scheduled for Monday – when the thaw is forecast to set in, as cold air from Arctic Russia is pushed aside by wet and mild air from Iceland.

Although it was the fourth winter that the Dutch could enjoy the ice, many are aware that winters are becoming milder, as December and January were very mild and these cold snaps are nothing compared to ‘the true Elfstedentocht winters’ in remembrance, like the November-to-March frost of 1963.

As the Dutch ice-skating culture actually dates back to the Little Ice Age that had cold winters as the norm, some are concerned a defining tradition could be at stake under global climate change and continued warming in the 21st century.

‘The last Elfstedentocht’

The last Elfstedentocht was held in 1997. Based on the Hellmann score that was the last winter the KNMI defines as ‘cold’. Recent snow and ice-skating winters classified as ‘normal’ based on Hellmann numbers – and mostly as mild based on combined average temperature. The last winter to define as ‘very cold’ was the Dutch winter of 1985.

Both cold winters and (near) ‘Elfstedentochten’ seem to come in groups in the Dutch climate. The strongest statistical relation to explain such groups is through the solar cycle, as the North Atlantic Oscillation is somewhat more easily inclined towards a negative state during a solar minimum.

The cold Februari of 2012 cannot be explained along this theory – and although general weather stabilisation in that month can sometimes be attributed to a delayed La Niña effect, it simply shows the erratic nature of the local climate.

© Rolf Schuttenhelm | www.bitsofscience.org

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