Samstag, April 17, 2010

Quiet sun puts Europe on ice

BRACE yourself for more winters like the last one, northern Europe. Freezing conditions could become more likely: winter temperatures may even plummet to depths last seen at the end of the 17th century, a time known as the Little Ice Age. That's the message from a new study that identifies a compelling link between solar activity and winter temperatures in northern Europe.

The research finds that low solar activity promotes the formation of giant kinks in the jet stream. These kinks can block warm westerly winds from reaching Europe, while allowing in winds from Arctic Siberia. When this happens in winter, northern Europe freezes, even though other, comparable regions of the globe may be experiencing unusually mild conditions.
Northern Europe freezes, even though comparable regions experience unusually mild conditions

Mike Lockwood at the University of Reading in the UK began his investigation because these past two relatively cold British winters coincided with a lapse in the sun's activity more profound than anything seen for a century. For most of 2008-9, sunspots virtually disappeared from the sun's surface and the buffeting of Earth by the solar magnetic field dropped to record lows since measurements began, about 150 years ago.

Lockwood and his colleagues took average winter temperatures from the Central England Temperature dataset, which extends back to 1659, and compared it with records of highs and lows in solar activity. They found that during years of low solar activity, winters in the UK were far more likely to be colder than average. "There is less than a 1 per cent probability that the result was obtained by chance," says Lockwood, in a paper to appear in Environmental Research Letters (DOI: 10.1088/1748-9326/5/2/024001).

Judith Lean, a solar-terrestrial physicist at the US Naval Research Laboratory in Washington DC, says the analysis is statistically robust, and reckons it forms a piece in the larger puzzle of how solar activity influences weather. Often cited by climate-change sceptics as a cause of global warming (see "What are you up to, sunshine?"), the effects of solar cycles have largely evaded the grasp of climate modellers. Lockwood found that when he removed 20th-century warming due to industrial emissions from his models, the statistical link between solar lows and extreme winters was stronger, suggesting the phenomenon is unrelated to global warming. But the sun undeniably has a big influence on weather systems: it is, after all, the energy source that powers them.

"All the little pieces are adding up into something much bigger," says Lean. "People are beginning to realise that European weather is particularly susceptible to solar activity." A study she published in 2008 found that warmer-than-average temperatures were more likely in northern Europe when solar activity is high (Geophysical Research Letters, DOI: 10.1029/2008GL034864).

Lean says research like hers and Lockwood's is helping to overcome a long-standing reticence among climate scientists to tackle the influence of solar cycles on the climate and weather. A big clue to the nature of this influence may lie in work published in 2008 by David Barriopedro at the University of Lisbon, Portugal, and colleagues. They investigated so-called "blocking events" in the mid-latitude jet stream during the winters of 1955-99.

The jet stream brings winds from the west, over the Atlantic, and into northern Europe. Blocking occurs when the meanders in the jet stream become so large that they double back on themselves, halting the prevailing westerly winds and allowing cold north-easterlies to take control (see diagram). Barriopedro found that when solar activity is low, the blocking events move eastwards across the Atlantic towards Europe, effectively opening an atmospheric corridor to the frigid Siberian Arctic.

But how can solar variability influence the jet stream? One finger of suspicion is pointing at the stratosphere, the layer of the atmosphere that lies 20 to 50 kilometres above our heads. There, patterns of winds and temperature are known to be influenced by solar activity, says Lockwood. This is because peaks in ultraviolet radiation emitted by the sun boost ozone formation in the stratosphere, which in turn absorbs more ultraviolet and heats up. The heating is greatest in the region of the stratosphere nearest to the sun and so a temperature gradient appears across the stratosphere and winds are born.

How this affects the weather below is still debated. Very little is known about the physics that governs the stratosphere, but one pattern that is emerging is that stratospheric "weather" is linked to the troposphere below it - where our everyday weather and currents like the jet stream reside. Edwin Gerber of New York University studies these interactions. He and colleagues showed in 2009 that upward movements of air in the troposphere can change the patterns of stratospheric winds. These changes, in turn, can be reflected back down to the troposphere and influence weather at the surface of the Earth (Geophysical Research Letters, DOI: 10.1029/2009GL040913). "Changes in the stratospheric winds influence the path of winter storms across Europe," Gerber says.

If researchers can prove that the sun can similarly induce changes in the troposphere via the stratosphere, which Gerber thinks they will, this could solve one of the biggest puzzles of the Little Ice Age - namely, that it appeared to have been a peculiarly European phenomenon, with other parts of the globe largely spared. The effects of the sun on the stratosphere are not global, says Lockwood. "They change the way the atmospheric energy is distributed around the world rather than change the total amount of energy being put into it."

Future studies may show that the effects of changes in solar activity can be felt further afield, but for now it seems that Europe is particularly susceptible because it happens to sit under the northern jet stream at a longitude where its meanders can grow into kinks.

Although sunspot activity is gradually returning, astronomers are not expecting it to reach its previously high levels. So if Lockwood is correct, while the general temperature trend in the northern hemisphere will increase in line with global warming over the coming decades, northern Europe can look forward to some more harsh winters. It may be time to buy some decent gloves.

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