We have just witnessed the three largest tornado outbreaks in history. The April 25 - 28 outbreak of 327 tornadoes and the April 14 - 16 outbreak of 162 tornadoes smashed the previous record set by the April 3 - 4, 1974 Super Outbreak of 148 tornadoes.
In my quest to find out why so many tornadoes happened this year I came across a pre-publication report on line which may indicate that the United States is entering a period of increased numbers of tornadoes, a period of drought in the southwestern states and period of possibly more Atlantic hurricanes. A combination of climate change and changing natural cycles may be responsible for the extreme weather.
Sky before tornado devastated Joplin, Mo; Source: Wunderground user thestig1
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The preliminary tornado count for the 5-day outbreak that began Saturday is 243.
Preliminary tornado reports are an overestimate, since some storms get counted multiple times. These over-counts were 35% - 40% in the case of the April 14 - 16 tornado outbreak and April 25 - 28 Super outbreak, so we can expect that the May 21 - 25, 2011 outbreak will end up with close to 150 tornadoes. This would rank as the third largest tornado outbreak in history, giving 2011 the three largest tornado outbreaks of all-time. Prior to 2011, NOAA rated the April 3 - 4, 1974 Super Outbreak as the largest tornado outbreak of all-time, with 148 tornadoes. According to a list of tornado outbreaks maintained by Wikipedia, only two other tornado outbreaks have had as many as 150 twisters prior to 2011--the May 2004 outbreak (385), and the May 2003 outbreak (401). However, these outbreaks occurred over an eight-day and eleven-day period, respectively, and were not due to a single storm system.
The historical record of tornado outbreaks is so variable that scientists have been unable, to date, to establish a statistical relationship between tornadoes and climate change. However, the chance of the three greatest tornado outbreaks in history occurring randomly this April and May would seem to be very small. When asked in April if the extreme weather observed in 2010 and 2011 is caused by climate change or natural variability, NOAA Climate Data Center climatologist, Dr Kevin Trenberth answered,
For extreme events, the question isn’t, "Is it global warming or natural variability?" It is always both. The question is just how much each is contributing.
Climate Data Center records show that the climate of the U.S. east of the Rocky mountains has become wetter by as much as an inch and a half of precipitation per decade. As the waters of the Gulf of Mexico and western north Atlantic have warmed in response to increasing levels of greenhouse gases, increased evaporation rates have added more precipitable water to storms. The wetter climate caused by global warming has increased the numbers of floods west of the Rocky Mountains.
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Because humid low-level air is a key ingredient of tornadic storms, global warming has likely contributed to this year's record tornado numbers. Near record warmth this April and May in the Gulf of Mexico and western Atlantic ocean is one of the factors that has led to the record flooding and record numbers of tornadoes.
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This year's Pacific ocean sea surface temperature anomalies reveal another factor involved in this year's extreme weather. Very warm water is in the northern central Pacific while it's cool in the central equatorial Pacific. This pattern, which happens in La Nina years, is associated with weather patterns where the jet stream that moves up towards Alaska then dives down towards the Rocky Mountain states.
The devastating tornado outbreak this week was triggered when a large bubble of exceptionally warm air pushed up over Alaska into the Arctic ocean. While Alaska and the Canadian Yukon had very warm weather, the cool air from Alaska pushed south into the Rocky Mountains as the jet stream dove south. When the strong jet stream wave swung off the Rocky mountains into the plains it encountered a low level southerly stream of humid air off of the Gulf of Mexico. The combination of strong shear and very humid low level air triggered the destructive tornado outbreak.
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The pattern of the jet stream pushing far north over the central Pacific then diving down towards the Pacific northwest has persisted for the past 3 months. The record breaking tornado outbreaks in April were associated with this pattern. Intense winds at jet stream levels blowing off the Rockies spun up humid low level air blowing off the Gulf in a massive outbreak of destructive atmospheric convection.
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Hidden in the sea surface data is a pattern shift that may indicate we are entering a period of increasingly destructive weather. La Nina is ending in the tropical Pacific, but the warm anomaly continues in the north central Pacific and the water along America's Pacific coast is cold.
This may indicate a shift in a poorly understood natural oscillation called the Pacific Decadal Oscillation or PDO. Bromirski, Miller, Flick, Auad, Dynamical Suppression of Coastal Sea Level Rise, JGRC, sub. Oct. 2010, rev. 1 Jan. 2011, rev. 2 Mar. 2011 1
Dynamical Suppression of Sea Level Rise Along the Pacific Coast of North America: Indications for Imminent Acceleration
Long-term changes in global mean sea level (MSL) rise have important practical implications for shoreline and beach erosion, coastal wetlands inundation, storm-surge flooding, and coastal development. Altimetry since 1993 indicates that global MSL rise has increased about 50% above the 20th century rise rate, from 2 to 3 mm/yr. At the same time, both tide gauge measurements and altimetry indicate virtually no increase along the Pacific coast of North America during the satellite epoch. Here we show that the dynamical steric response of North Pacific eastern boundary ocean circulation to a dramatic change in wind stress curl, τxy, that occurred after the mid-1970’s regime shift, can account for the suppression of regional sea level rise along this coast since 1980.Alarmingly, mean τxy over the North Pacific recently reached levels not observed since before the mid-1970’s regime shift. This change in wind stress patterns may be foreshadowing a PDO regime shift, causing an associated persistent change in τxy that will result in a concomitant resumption of sea level rise along the U.S. West Coast to global or even higher rates.
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The cold phase of the PDO affects sea surface temperatures and jet stream patterns in a manner similar to La Nina, but the PDO may stay cold for 30 years or more. The warm phase of the PDO may have kept wind shear over the plains relatively low while the Gulf of Mexico warmed due to global warming. However, if we are now entering the cold phase of the PDO, the number of severe storms and tornadoes may increase rapidly. This year's extreme weather may be the beginning of a new pattern of more extreme weather.
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Moreover, we are now in the warm phase of the Atlantic Multidecadal Oscillation. The combination of a warm western Atlantic and a cold eastern Pacific is associated with long term droughts in the southwestern U.S. Cool tropical eastern Pacific temperatures reduce east Pacific hurricane numbers. However, low numbers of east Pacific hurricanes mean that wind shear over the tropical Atlantic is less, so more hurricanes tend to form over the tropical Atlantic.
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The combination of global warming and the warm phase of the Atlantic combined with the cold phase of the Pacific may trigger a period of extremely destructive weather and drought.