The discussion of climate change continues to be disputed internationally. The 2013-14 winter season has been quite whacky across North America and Europe. Some regions have been warm and dry while other areas have been inundated by a parade of storms. From an outsider’s view, there is no disputing that predictable and consistent winters may very well be a thing of the past.
“Mother Nature in the last several years has dished up an incredible smorgasbord of extreme weather,” stated US climatologist Jennifer Francies. Her view is shared by others who also study weather including BBC science correspondent Pallab Ghosh who reported in February at the American Association for the Advancement of Science that “we may have to get used to winters where spells of weather go on for weeks or months.”
The reason according to Francies, caused by the complex interaction of land, ice and sea, is pretty simple. For much of this winter the jet stream, a meandering ribbon of air that sweeps storms across the globe, has been stuck. The consequences have been impressive especially in the small country of Austria where they are experiencing dramatic highs and lows.
Jet streams form at the boundary of cold, dry arctic air intruding southward, and warm, wet sub-tropic air pushing north. Along this boundary narrow bands, like rivers, strong winds of 200-300 km/hr form at 10km (30,000 feet) and steer storms across the northern hemisphere from west to east.
By the end of January the roadblock was so strong that some storms had forced their way into the Mediterranean where they were unable to advance. The storms were cut off from the jet stream and parked into an upper-level low pressure system that pumped huge amounts of moisture into central Europe. Switzerland, Austria, Italy, Slovenia, Bulgaria and Croatia experienced their heaviest snows in decades, but only on the south side of the mountains. The north side stayed dry and for a country the size of Austria – only 300km north to south – the differences were stark.
The Alps run east and west and are perfectly situated to wring out moist air from the Mediterranean on the windward (south) side of the mountains. Rain in the valleys measured in centimeters and snowfall in mountains measured in meters. On the north side of the range, Kitzbühel got practically no snow while just 100 km (60 miles) south in the Dolomites deep snows blanketed the ground. To the east in Slovenia snows reached depths not seen in 50 years. At the Zelenica Hut the weather recording instruments were buried by more than a meter of snow.
Historically the jet stream shifts locations every few days, but as the climate changes there seems to be reasons, according to Francies, for the jet stream to become stuck for weeks on end. The spells of weather going on for weeks or months, as mentioned by Ghosh, means that some traditionally snowy locations will receive snow and winter weather while others will not. Those locations will depend upon the location of the jet stream, which can only be accurately forecasted days to a week or two in advance. This unfortunate ambiguity means that next winter conditions will likely be different.