Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:57 PM GMT on February 07, 2012
Europe's winter onslaught continues unabated this week, with very cold temperatures and heavy snows over much of the continent. Yesterday, a rare snow storm hit North Africa, bringing 2 - 3 inches of snow to Tripoli, Libya. It was the first snow in Tripoli since at least 2005, and may be the heaviest snow the Libyan capital has seen since February 6, 1956. Across Europe, at least 250 deaths have been blamed on the winter weather since the cold spell began on January 26. Hardest hit has been Ukraine, with 135 deaths--mostly of homeless people. According to weather records researcher Maximiliano Herrera, the current cold snap is the most severe for Europe since February 1991.
Figure 1. The scene in Tripoli, Libya, on February 6, 2012, after a rare snowstorm. Image credit: libyall.com.
Unusual jet stream kink causing Europe's harsh winter weather
The reason for the exceptionally cold and snowy winter weather in Europe lies in the behavior of the jet stream. The jet stream--the band of strong west-to-east blowing upper-level winds that circles the globe at mid-latitudes--acts as the dividing line between cold, polar air to the north, and warmer subtropical air to the south. On average, the jet blows straight west to east. But this winter, the jet has had a highly convoluted shape, with unusually large excursions to the north and south. When the jet bulges southwards, it allows cold air to spill in behind it, and that is what has happened to Europe over the past two weeks. The jet often gets "stuck" in one of these highly convoluted shapes, allowing a persistent period of extreme weather to occur. The latest predictions from the GFS and ECMWF models show the unusual jet stream pattern over Europe persisting for at least another week.
Figure 2. The jet stream pattern over Europe shows that the jet is taking a major dive southwards across France and into North Africa, keeping almost all of Europe on the cold (north) side of the jet.
The AO and NAO
A good measure of the tendency of the jet to form major bulges in winter is the Arctic Oscillation (AO) Index, which has ranged between -1 and -3 over the past two weeks. A strongly negative AO Index like this means the winds of the jet are relatively weak, allowing it to sag southwards over Europe and allow cold air to plunge southwards behind it. Usually, a negative AO also means cold winter weather over North America, but not this winter. In North America, we're better off paying attention to the North Atlantic Oscillation (NAO) Index, which we can think of as the Northern Atlantic portion of the AO. Ordinarily, the AO and NAO are in phase during winter (about 80 - 90% of the time), meaning that Europe and North America experience similar winter weather. However, over the past two weeks, the NAO has been positive while the AO has been negative. The positive NAO means that jet stream winds have been strong over North America and the North Atlantic, keeping cold air bottled up to the north over Canada and the Arctic. This pattern is predicted to persist for at least another week. We've seen many record extremes of both the positive and negative phase of the AO since 2006; see that latest post by Andrew Freedman of climatecentral.org to learn more.
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