Nog een uitgebreidere verklaring door Jeff Masters
Bericht van: Saskia (Diepenveen) - moderator - , 22-03-2012 11:38Why the record early-season warmth?
The unusual warmth is due to a loop in the jet stream that has created a large upper-level ridge of high pressure that is stuck in place over the Eastern U.S.--a phenomenon known as a "blocking pattern." Since the jet stream acts as the boundary between cold air to the north and warm air to the south, and the large loop in the jet places its axis far to the north of the eastern U.S., summer-like warmth has developed over the eastern half of the U.S. Conversely, colder than average temperatures have developed over the western third of the U.S. behind the southwards-dipping loop of the jet stream. There are at least three large-scale patterns working together right now to create an unusually strong ridge over the eastern half of the U.S.:
1) La Niña. The on-going La Niña event in the Eastern Pacific has weakened considerably over the past month, but ocean temperatures there are still cool enough to affect the jet stream pattern, favoring high pressure and warm temperatures over the Eastern U.S., and low pressure and cold temperatures over the Western U.S.
2) The Madden-Julian Oscillation( MJO). The MJO is a 2-month cycle of thunderstorm activity that travels west to east along the Equator. The MJO is currently in phase with La Niña, and is helping create warmer temperatures over the Eastern U.S.
3) The North Atlantic Oscillation (NAO.) The NAO is in its positive phase, which means the difference in pressure between the Icelandic Low and the Azores High is stronger than usual. This tends to increase the jet stream winds and keeps the jet from sagging southwards over the Eastern U.S.
While the blocking pattern responsible for the heat wave is natural, it is very unlikely that the intensity of the heat would have been so great unless we were in a warming climate. Climate scientist Dr. James Hansen has posted an interesting (as yet unpublished) paper discussing how the odds of such extreme heat events have shifted in recent years.
Extreme jet stream patterns like this often lead to tornado outbreaks in the Midwest, at the boundary of where warm, moist air flowing north from the Gulf of Mexico collides with cold, dry air flowing south. Cold dry air aloft, combined with warm, moist air at the surface, makes the atmosphere unstable, since air rising in thunderstorm updrafts will be less dense than the surroundings, allowing the air to accelerate upwards and increase the intensity of the thunderstorm. This will be the case today, when NOAA's Storm Prediction Center has highlighted a Moderate Risk area for severe weather over much of Texas today, including the Dallas-Fort Worth area.
Bron: Dr. Jeff Masters' WunderBlog: Tornadoes, extreme flooding, unprecedented heat expected in U.S. today
