A cyclical drop in the sun's radiation can trigger unusually cold winters in parts of North America and Europe, scientists say, a finding that could improve long-range forecasts and help countries prepare for blizzards。
Scientists have known for a long time that the sun has an 11-year cycle during which radiation measured by sunspots on the surface reaches a peak then falls. But pinning down a clear link to weather has proved harder。
"Our research confirms the observed link between solar variability and regional winter climate," lead author Sarah Ineson of the UK Met Office told the reporters in an email. The study was published in the journal Nature Geoscience on Monday。
Her team focused on data from the recent solar minimum during 2008-10, a period of unusual calm for the sun and intense winters in the United States and parts of Europe that shut down air travel and disrupted businesses。
The researchers found that a reduction in ultraviolet (UV) radiation from the sun can affect high-altitude wind patterns in the Northern Hemisphere, triggering cold winters。
"While UV levels won't tell us what the day-to-day weather will do, they provide the exciting prospect of improved forecasts for winter conditions for months and even years ahead. These forecasts play an important role in long-term contingency planning," Ineson, a climate scientist, said。
Ineson and colleagues from the Imperial College London and the University of Oxford used satellite data that more accurately measures UV radiation from the sun and found a much greater variability than previously thought。
They found that in years of low activity, unusually cold air forms high in the atmosphere over the tropics. This causes a redistribution of heat in the atmosphere, triggering easterly winds that bring freezing weather and snow storms to northern Europe and the United States and milder weather to Canada and the Mediterranean。
When solar UV radiation is stronger, the opposite occurs。
Ineson's team used the data in a complex computer model that simulates long-term weather patterns. The model successfully reproduced what scientists have observed happening in the upper atmosphere during changes in solar radiation。
More study was needed, though. A key uncertainty in the experiment lay in the satellite data used, because it spans only a few years. "So questions remain concerning both accuracy and also applicability to other solar cycles," she said。
科学家表示，太阳辐射的周期性减弱可能导致北美洲和欧洲部分地区出现异常“冷冬”。这项科学发现可以改进长期天气预报，帮助各国准备好应对风雪。
早前研究发现，太阳的活动周期为11年，在此期间，以太阳表面黑子为衡量标准的太阳辐射达到峰年和谷年。但证实太阳活动周期与天气之间的明确联系一直存在困难。
研究负责人、英国气象局的莎拉•伊尼森在电邮中告诉记者：“对已观测到的太阳活动变化与地区冬季气候之间的联系，研究予以了证实。” 研究结果本周一发表在英国《自然—地球科学》杂志上。
研究小组对最近的一次太阳活动谷年，也就是2008年至2010年的数据进行了分析。这段时间太阳活动出奇平静，而美国出现极端寒冬，欧洲部分地区还曾关闭机场，这也打乱了经济活动。
研究人员发现，太阳紫外线辐射水平的降低可以影响北半球高纬度的环流形态，导致冷冬。
气候学家伊尼森说：“紫外线水平无法预测每日的天气状况，但可以使我们对未来数月甚至数年的冬季气候预测更加准确。这种预测可以使我们在较长时期内对极端天气做好准备。”
伊尼森和来自帝国理工学院以及牛津大学的同事借助卫星数据，更为准确地测得太阳紫外线辐射，发现数据波动超过先前预计。
他们发现，在太阳活动谷年，异常冷空气在热带高层大气形成，造成大气热量重新分配，引发东风，令欧洲北部和美国遭遇异常低温和暴风雪，加拿大和地中海地区气候则变得更为温和。
太阳紫外线进入活动峰年时，情况则相反。
伊尼森的研究小组利用一个复杂计算机模型得出数据，这种模型可以模拟长期气候类型。模型成功重现了在太阳辐射变化时，科学家已经在高层大气观测到的状况。
但这一结论仍需要更多研究来印证。卫星数据中存在一个关键的不确定因素，因为数据只覆盖了最近几年。她说：“因此研究结果的准确性和对太阳其他周期的适用性仍有待验证。”