by Doug
The winter of 2009/10 was a very good one...Cairngorm enjoyed a longer season than Courcheval – with an amazing 147 days of operation. Nevis Range managed to run the mythical Braveheart chairlift for a record number of days. Glencoe opened before new year for the first time in 14 years – and enjoyed a fantastic long season under new ownership. While over in the east Glenshee and the Lecht attracted impressive skiers numbers last seen a decade ago in the late 1990s.
So what caused the legendary and memorable winter of 2010? In the current climate of media fueled ‘global warming' no one predicted the early snow and persistent sub-zero temperatures that delighted Scottish snow-sport enthusiasts. The season was a long snowy one with the ski centres reporting “all runs complete” from Christmas until Easter.. As many commented, it was like a return to the snowy winters of the 1980s!
Astute weather watchers will remember the cold easterly weather systems that were a defining feature of the winter 2009/10. This was almost un-heard of.. For 3 months the freezing level hardly rose above 1000ft and Loch Morlich remained frozen for exactly 100 days. The left chart below shows a ‘typical’ winter weather chart for the UK. In this chart there is an obvious low pressure centered just south of Iceland and another high pressure near the Azores - the famous "Azores High". This typical scenario results in the UK experiencing predominately westerly weather systems from the Atlantic, and the resulting mild maritime climate that can often be the curse of skiers. The think black line on the chart marks the approximate direction of the Jet Stream. The Jet Stream is a constant flow of air in the upper atmosphere – it plays a big role in determining the prevalent trajectory of storms in the lower atmosphere.
Now consider the chart on the right. A low pressure is now straddled over the Azores replacing the usual “Azores High”. The Jet Stream takes a much more southerly path, leaving Spain and the Mediterranean to be blasted by the windy, and mild, Atlantic systems. Meanwhile the UK is sitting under higher than normal pressure – allowing cold Siberain air to be sucked from the east across the country. This unusual weather pattern was prevalent from mid December 2009 until the end of March 2010.
A simple way to quantify the state of the charts above is to measure the atmospheric pressure difference between Iceland and the Azores. This is called the NAO Index, and it is related to a climatic cycle called the North Atlantic Oscillation (NAO) [1], [2]. A weather chart like the one on the left will produce a large positive NAO index (i.e. the pressure over Iceland is much lower than that over the Azores) and a chart like the one on the right will produce a zero or negative NAO index (i.e. the pressure over the Azores is equal to or lower than that over Iceland) .
A defining feature of the legendary winter of 2009/10 was the remarkable negative North Atlantic Oscillation index. The southerly Jet Stream (i.e negative NAO index) can be attributed to several notable weather events.
- An exceptionally good ski season in Scotland
- Record snowfalls in southern alpine regions, and floods in the Mediterranean
- Below average snow fall in central alpine regions
- A very poor ski season in Norway and Iceland (too cold and dry)
In many ways the uber-snowy winter of 2010 was a freak weather event! Much like the snowless winter of 2003 was also a freak event. In 2010 the persistent easterly winds, and cold weather, resulted in some very notable conditions. The Cairngorms and Grampians experienced record breaking snowfall, cold temperatures and lots of misty days – meanwhile the Western Highlands tended to sit in a weather shadow on the lee side of the country, enjoying long periods of drier and generally sunny weather. Basically a role reversal of normal conditions!.
So what controls the N.A.O ? The simple answer is that meteorologists don’t yet know for sure… But there are lots of theories. Some scientists believe that solar cycles (measured by the strength of sun spots) control the flow of air in the Earths upper atmosphere – and there certainly seem to be a greater number of colder UK winters when solar activity is low [3]. Others studies claim the atmosphere and ocean are in a constant feedback loop, passing information from one to the other. It is speculated that warmer sea temperatures, and melting ice caps, could directly affect the NAO [4].
Are we likely to see similar -ve NAO scenarios in future years ? Is the gulf stream shutting down ? Could global warming actually result in colder Scottish winters !? Are sun spots to blame ? Or is it just natural variation ? Your guess is as good as mine... But interestingly there are signs of similar weather patterns developing once more during November 2010!
Either way it will be several decades before anyone really knows whats really happening to our weather patterns and climate. One cold winter certainly does not mean that the threat of climate change has suddenly disappeared over night.
[1] North-Atlantic-Winter-Exposed - http://magicseaweed.com/North-Atlantic-Winter-Exposed-Content/2343/
[2] North Atlantic Oscillation - http://en.wikipedia.org/wiki/North_Atlantic_oscillation
[3] Link Between Solar Activity and the UK's Cold Winters
- http://www.sciencedaily.com/releases/2010/04/100415080848.htm
[4]Global warming could cool northern hemisphere
- http://www.pik-potsdam.de/news/press-releases/global-warming-could-cool-down-temperatures-in-winter
