Blame the volcano trouble on sun and global warming
Ash-laden Arctic air is blowing over Europe because the usual westerly winds are being “blocked” by a high-pressure weather system, and such blockages may become more common. “We predict that the frequency and length of blocking events will increase in a warmer climate,” says Christophe Cassou of the European Centre for Research and Advanced Training in Scientific Computation in Toulouse, France.
Using IPCC simulations of air flow, Cassou and colleague Éric Guilyardi show that global warming will increase summer blocking events over Europe.
Blocking occurs when the jet stream, which carries winds from the west, is forced to slow down suddenly. “It catches up on itself and starts to meander,” says Mike Lockwood from the University of Reading, UK. Sometimes the meanders double back on themselves, allowing north-easterly winds to fill the gap.
When solar activity is low this jet stream “pile-up” shifts eastwards across the Atlantic Ocean, bringing blocking events to Europe. The reasons seems to be that solar activity influences high-level stratospheric winds, and these eventually feed through to the troposphere, where the jet stream lies.