| 英文摘要: | Arctic warming has reduced cold-season temperature variability in the northern mid- to high-latitudes. Thus, the coldest autumn and winter days have warmed more than the warmest days, contrary to recent speculations. Pictures of partly frozen Niagara Falls and ice-covered orange trees in Florida dominated the news for weeks last winter. The jet stream — a ribbon of strong winds at high altitude — followed an unusual route and brought bitterly cold weather to the eastern US and unusually warm temperatures to Alaska. Likewise, Eurasia experienced some very cold winters in recent years. A number of studies proposed that strong Arctic warming and declining sea ice extent caused the jet stream to meander more1, thereby making temperatures more volatile and causing more intense cold spells in mid-latitudes of the northern hemisphere2, 3. As he reports in Nature Climate Change, James Screen4 challenges this hypothesis and provides observational evidence for the opposite effect. Again, anomalous Arctic warming is put forward as the primary driver, but for making northern hemispheric autumn and winter temperatures less, rather than more, variable. Screen observed a decline in daily temperature variance during the cold season at northern mid- to high-latitudes. The coldest autumn and winter days have warmed more than the warmest days. This behaviour is explained with a remarkably simple mechanism4 — cold days predominantly occur when winds are blowing from the north and warm days occur when they are blowing from the south. Arctic amplification, meaning that warming is greater in the Arctic than at low latitudes, causes northerly winds to warm to a greater degree than southerly winds. Consequently, the coldest days warm more rapidly than the warmest days at the latitudes between. A comparable mechanism has been used to explain the simulated variance reduction in central European winters5, where the east–west land–sea warming contrast — that is, the land warms more than the surrounding ocean — is considered instead of the north–south contrast.
TODD BANNOR/ALAMY
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Affiliations
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Erich M. Fischer and Reto Knutti are at the Institute for Atmospheric and Climate Science, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland
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