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Blame the jet stream for the coldest winter since '78/79

Paul Hudson | 16:06 UK time, Monday, 1 March 2010

Across the UK, using the UK national temperature dataset which started in 1914, it's been the coldest winter since 1978/79 and was the 7th coldest winter since these records began. Even more impressive were the figures for Scotland, which show it was the coldest winter there since 1962/63.

I've been asked many times what has caused this winter to be the coldest for over 30 years, coming so soon after last winter which was the coldest since 1996/1997. Furthermore is there a connection between these two cold winters, and the succession of poor summers we have experienced?

There is a common factor in both this winter's cold, and the wet summers we experienced in 2007, 2008 and 2009. It's down to the position of the jet stream. In summer when it's to the north of us, our weather is usually warm and settled. When it's over or to the south of us, summers are usually wet and cooler. In winter, a more southerly jet stream leaves us open to cold conditions to the north and east. Normally the jet stream in winter ensures generally mild, and at times wet and windy weather across our shores.

So what has forced the jet stream to assume, on average, a more southerly track over the last 2 or 3 years?

The answer, it seems, could, at least in part, be down to solar influences. Over the last 3 years solar activity has been very low, a subject I've written about before.

In 1972, Professor Lamb of the University of East Anglia conducted research into the effect of solar activity on weather patterns across the Atlantic. He found in his paper that there was a tendency for higher pressure on average in Iceland during solar cycle minima, based on data from 1750-1958. When pressure is higher over Iceland, the jet stream takes on a more southerly track.

More recently in 2005, Bochnicek and Hejda showed that during Jan to March from 1963-2001 high geomagnetic activity was nearly always associated with a positive North Atlantic oscillation (NAO); with low geomagnetic activity associated with negative NAO.

The NAO has been strongly negative this year, and often leads to more blocking (colder) weather in winter. The earth's geomagnetic field is strongly influenced by solar activity.

According to Professor Joanna Haigh at Imperial College London there is now evidence that, on average, the jet stream is slightly further north when the Sun is more active, and further south during periods of lower solar activity. This signal is small, and difficult to detect in innately very variable meteorological data, but the Imperial team is now beginning to explain how it may come about. When the Sun is active it emits much more ultraviolet radiation and this UV is absorbed by the ozone in the stratosphere, causing that region of the atmosphere (above about 15 km altitude) to warm up. This results in a change in temperature structure which influences the winds at this level, the jet streams and the circulation of air below.

More interestingly, the cold winters of 1986/87, and 1995/96 all occurred during solar minimum.

The relationship is not perfect, indeed the cold winters of 1978/79 and 1981/82 actually occurred close to a solar cycle maximum, an indication that there are other factors involved. But it's interesting that the only 4 winters of any note from a cold point of view in the last 25 years, in which most winters have been mild or very mild, actually occurred during these solar cycle minima. And the last 3 poor summers have also being during this solar minima, too.

Winter 2009-2010 statistics

Across the UK, using the UK national temperature dataset which started in 1914 it's been the coldest winter since 1978/79, and was the 7th coldest winter on record. The mean temperature (average of daytime and night-time temperatures) for 2009/2010 was 1.59C; colder than 1981/82 (2.12C) but not as cold as 1978/79 (1.17C).

For Scotland, the mean temperature for winter came in at 0.27C, making it the coldest since 1962/63 (0.16C).

The local statistics for winter in Yorkshire indicate it has been the coldest for over 30 years at Linton-on-Ouse in North Yorkshire, too. The mean figure from 1st December 2009 to 28th February 2010 was 1.56C, colder than 1981/82 (Mean 1.64C), but not as cold as 1978/79 (mean 0.8C).

Because of technical issues Leeming's figures are not yet available.

For Waddington, in South Lincolnshire, (this winter 2.3C), it's been the coldest since 1981/82 (2.1C).

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