AIRCURRENTS A WINDY WINTER SEASON EDITORS NOTE: Dr. Gerhard Zuba, AIR Principal Scientist, reviews the 2011–2012 windstorm season in Europe and discusses the climatological influences that lead to storm clustering. By Dr. Gerhard Zuba Edited by Nan Ma 02.2012 SEASON REVIEW akin to a Category 2 hurricane on the Saffir-Simpson Although quiet and bitterly cold in later weeks, the winter scale) were reported, with gusts exceeding 260 km/h (160 of 2011–2012 has seen several damaging windstorms pass mph). Friedhelm left more than 100,000 homes without in quick succession across the European continent. These power and caused massive closures and travel disruptions. low pressure extratropical systems (also called winter storms) Fortunately, Scotland’s wind design provisions are stringent form when warm moist air from the subtropical region and well enforced, and damage to the predominantly converges with cold air from the subarctic region. They track masonry building stock was not very significant. across the Atlantic and arrive in Western Europe, bringing high winds, heavy precipitation, and coastal flooding. One week later, another powerful storm struck Europe, this Because windstorms are relatively fast-moving and do not time affecting France, Switzerland, and Germany. Off the lose their energy when they exit the ocean, they can cover coast of Brittany, the winds from Joachim were so severe vast expanses of land and affect several countries over the that a cargo ship ran aground, spilling more than 200 course of a few days. tons of fuel. The storm caused widespread power outages in northern and western France, with some 300,000 The first significant storm of the season was Xaver, which households losing electricity. Flooding was reported in formed in the late November and brought strong winds several coastal departments. While wind speeds from to Scotland. High waves caused by the storm forced Joachim were not as high as those from other major storms some oil platforms off the coast of Norway to shut down. to affect France in recent years (like Klaus in 2009 and Storm activity picked up in early December with the Xynthia in 2010, both of which caused insured losses in arrival of Friedhelm in Scotland after a process of rapid excess of 1 billion Euros), strong winds were recorded over strengthening called explosive cyclogenesis. Considered much of the country and into Germany and Switzerland. Scotland’s worst storm in more than a decade, Friedhelm This led to widespread—but mostly minor—damage, mainly brought violent winds to central and southern portions to nonstructural building elements. PERILS issued an initial of the country, prompting a rare Red Alert from the U.K. insured loss estimate of 300 million Euros. Met Office. Sustained winds of over 170 km/h (105 mph, AIRCURRENTS 02.12|A WINDY WINTER SEASON BY DR. GERHARD ZUBA EDITED BY NAN MA A Note on Naming Fast on the heels of Ulli, yet another storm—Andrea— In Europe, the Institute for Meteorology at Freie University formed southwest of Iceland on January 3 and passed to of Berlin is a well-accepted authority on identifying and the north of the U.K., bringing strong winds to central tracking winter storms and has been naming all high and England. As it moved southeast, Andrea intensified and low pressure systems to affect Central Europe since the struck Germany, bringing wind, rain, sleet, and snow. As is 1950s. In 2002, the Institute introduced the “Adopt a typical of storms approaching Germany from the North Sea, Vortex” program, allowing public patrons to submit storm the storm pushed water against the coast, causing flooding names (on average, 50–60 highs are named each year and in low-lying areas in the north, including Hamburg. While 150 lows). power outages, traffic accidents, and travel disruption were reported, structural damage from winds was limited. To These Freie University names are widely used by the media date, Andrea was the last damaging storm of the season. when covering potentially damaging events and by many weather agencies throughout Europe. Some countries, however, will use a different name to refer to a storm that enters their domain. The Norwegian Meteorological Institute, for example, assigns its own name to significant storms that affect Scandinavia. Thus, storms can have two names, including this season’s Patrick (named Dagmar by the Norwegian Meteorological Institute) and Ulli (also called Xaver, November 22 Patrick, December 24 Emil). The names used in this article are those assigned by the Freie University. A week and a half later on Christmas Day, Patrick swept through northern Scandinavia, causing tree damage, landslides, travel disruptions, and massive power outages that disrupted gas processing operations. Described as Norway’s third-worst storm in the past 50 years by the Friedhelm, December 7 Ulli, December 31 Norway Meteorological Institute, wind gusts reached hurricane strength at every recording station in the country (a maximum of 233 km/h, or 145 mph, was recorded). In Finland, Patrick affected the entire south portion of the country and was the worst storm since Janika in 2001. A few days later, windstorm Ulli formed off the coast of Newfoundland, Canada. Ulli steadily strengthened as it sped Joachim, December 14 Andrea, January 3 across the Atlantic Ocean, reaching Scotland with a central pressure of just 952 mb. The storm generated a sting jet—a Figure 1. Tracks and maximum recorded wind gusts of major storms from the 2011–2012 winter storm season (Source: AIR) localized strong downdraft that brings cold, dry air from the mid-troposphere down to the surface—that caused intense STORM TRACKS AND THE NAO localized wind gusts in Scotland. Strong winds toppled trees This season’s storm frequency and tracks are unlike those and power lines, overturned tall trucks, and caused some of the previous two European winter storm seasons, which damage to buildings. Ulli also brought heavy precipitation, saw only one major windstorm, Xynthia, impact central and coastal and river flooding. The region affected by Ulli Europe in the winter of 2009/10. Compared to recent was similar to that of Windstorm Friedhelm, although Ulli’s years, this season saw a significant northward shift in storm wind speeds were generally lower. Disruption to travel was tracks, resulting in an unusually high number of damaging widespread as roads, bridges and rail lines were forced to events in northern Europe (extratropical systems typically close. exhibit the highest wind speeds on the right-hand side 2 AIRCURRENTS 02.12|A WINDY WINTER SEASON BY DR. GERHARD ZUBA EDITED BY NAN MA of the track, so regions south of the storm track tend to Under NAO+ conditions like the 2011–2012 season, experience the most damage). This shift can be attributed extratropical cyclones tend to track toward Europe in a to a change in the North Atlantic Oscillation (NAO) from a northeastward direction, bringing wind and precipitation negative to positive phase in late 2011. to northern Europe. This has resulted in a disproportionate number of storms affecting the U.K. and Scandinavia. On the other hand, during NAO- conditions, storms tend to track due east, which takes them into the Mediterranean region. For example in 2009, which had a strongly negative NAO during the winter months, Klaus made landfall on France’s Aquitaine coast, causing billions of Euros of damage in northern Spain and southern France. Figure 2. North Atlantic Oscillation Index, three-month moving average 2001–2011 (Source: Climate Prediction Center http://www.cpc.ncep.noaa.gov/) The NAO is a climate signal that influences atmospheric flow in the North Atlantic Ocean and drives much of the weather variability over parts of Europe and North America. (It is also responsible for the mild winter season this year Figure 4. Monthly NAO during winter, 2009–2012 (Source: Climate Prediction Center http://www.cpc.ncep.noaa.gov/) in the U.S.) The NAO is determined by the difference in pressure between a subtropical high pressure system This year, as shown in Figure 4, the NAO remained positive located near Bermuda, the Azores High, and a subarctic low through January, and daily indices are still showing this pressure system located near Iceland, the Icelandic Low (see trend. However, no significant storms have formed after Figure 3). The clockwise rotation of the Azores High and the Andrea in early January, indicating that weather factors counterclockwise rotation of the Icelandic Low directs air other than the NAO are influencing storm activity. Indeed, eastward to Europe. The strength of this flow depends on the low pressure system around Iceland has shifted the pressure difference between the two. A well-developed westward, and continental high pressure over Russia has Azores High (strong high pressure) and a well-developed extended through central Europe and into parts of western Icelandic Low (very low pressure) corresponds to the NAO Europe. This weather phenomenon is called Scandinavian positive (NAO+) phase. The NAO negative (NAO-) phase blocking, whereby the high pressure that has moved over occurs when both systems are weak. Europe is blocking storms from tracking eastward into Europe. This blocking is also causing extremely cold air from the Arctic and Russia to flow into central and western Europe. This resulted in the severe cold snap that gripped the continent—the worst in several decades—that claimed hundreds of lives since late January. NATURE IS NOT AN OPTION: MODELING STORM CLUSTERS In addition to influencing storm tracks, NAO+ phases are also often associated with above average counts of storms, particularly over northwestern Europe, as well as the tendency for storms to follow closely on each other’s heels, Figure 3. The pressure difference between the Azores High and the Icelandic Low or cluster.