Wunderground.com - 4/25/13, Dr. Jeff Masters
Figure 2. The Arctic Oscillation (AO) is a pattern of varying pressure and winds over the Northern Hemisphere that can strongly influence mid-latitude weather patterns. When the AO is in its positive phase, jet stream winds are strong and the jet stream tends to blow mostly west to east, with low-amplitude waves (troughs and ridges.) Since the jet stream marks the boundary between cold Arctic air to the north and warm subtropical air to the south, cold air stays bottled up in the Arctic. When the AO is in its negative phase, the winds of the jet stream slow down, allowing the jet to take on more wavy pattern with high-amplitude troughs and ridges. High amplitude troughs typically set up over the Eastern U.S. and Western Europe during negative AO episodes, allowing cold air to spill southwards in those regions and create unusally cold weather.
During March 2013, residents of Europe and the Southeast U.S. must have wondered what happened to global warming. Repeated bitter blasts of bitter cold air invaded from the Arctic, bringing one of the coldest and snowiest Marches on record for much of northern Europe. In the U.K., only one March since 1910 was colder (1962), and parts of Eastern Europe had their coldest March since 1952. A series of exceptional snowstorms struck many European locations, including the remarkable blizzard of March 11 - 12, which dumped up to 25 cm (10”) of snow on the Channel Islands of Guernsey and Jersey in the U.K., and in the northern French provinces of Manche and Calvados. The entire Southeast U.S. experienced a top-ten coldest March on record, with several states experiencing a colder month than in January 2013. Despite all these remarkable cold weather events, global temperatures during March 2013 were the 9th warmest since 1880, said NASA. How, then, did such cold extremes occur in a month that was in the top 8% of warmest Marches in Earth's recorded history? The answer lies in the behavior of the jet stream. This band of strong high-altitude winds marks the boundary between cold, polar air and warm, subtropical air. The jet stream, on average, blows west to east. But there are always large ripples in the jet, called planetary waves (or Rossby waves.) In the Northern Hemisphere, cold air from the polar regions spills southward into the U-shaped troughs of these ripples, and warm air is drawn northwards into the upside-down U-shaped ridges. If these ripples attain unusually high amplitude, a large amount of cold polar air will spill southwards into the mid-latitudes, causing unusual cold extremes. This was the case in Europe and the Eastern U.S. in March 2013. These cold extremes were offset by unusually warm conditions where the jet stream bulged northwards--over the Atlantic, the Western U.S., and in China during March 2013. In fact, the amplitude of the ripples in the jet stream reached their most extreme value ever recorded in any March during 2013, as measured by an index called the Arctic Oscillation (AO).
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