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article imageLink between Pacific Ocean temperatures and tornado locations

By Robert Myles     Oct 20, 2013 in Science
Columbia - A researcher at the University of Missouri (MU) has found a link between Pacific Ocean temperatures and the locations where tornadoes occur in the United States.
The new discovery could help meteorologists attempting to predict tornado activity and, in so doing, help protect lives and property.
Often meteorologists use data about warm and cold fronts to ascertain if a tornado will occur in a particular location. For their research, Laurel McCoy, an atmospheric science graduate student at the MU School of Natural Resources, and Tony Lupo, professor and chair of atmospheric science in the College of Agriculture, Food and Natural Resources, cast their net back in time examining 56,457 tornado-like events spanning over 60 years from 1950 to 2011.
In analyzing six decades of data, they found that when Pacific Ocean temperatures were warmer than average, the US experienced 20.3 percent more tornadoes that were rated EF-2 to EF-5 on the Enhanced Fuijta (EF) Scale, which measures the strength or violence of tornadoes on the basis of the likely damage they will cause.
Developed in 1971 by severe storms researcher Dr.Tetsuya Theodore "Ted" Fujita, sometimes known as Mr. Tornado, the original Fujita Scale categorized the total violence of a tornado based on its intensity, wind speed and area covered. The Fujita Scale ranges from F0, classed as a gale, upwards to the most destructive tornadoes ranked F4, described ominously as “Devastating,” and the apocalyptic-sounding “Incredible” of an F5.
In 2007 the Enhanced Fujita Scale was put in place based on the original thesis of Mr. Tornado with a finer gauge of the types of buildings that might be expected to suffer structural damage during a tornado.
McCoy and Lupo discovered that when Pacific Ocean surface temperatures were above average, when tornadoes occurred, they tended to be located to the west and north of Tornado Alley.
Tornado Alley, taking in a large swathe of the Midwest, experiences more tornadoes than any other part of the US. There’s no official meteorological or geographical description of Tornado Alley, but its extent is usually defined as sweep of the Midwest covering northern Texas, western Oklahoma, most of Kansas and Nebraska, and a large slice of South Dakota.
McCoy also found that when Pacific Ocean temperatures were cooler, more tornadoes tracked from southern states, like Alabama, into Tennessee, Illinois and Indiana.
McCoy said, “Differences in sea temperatures influence the route of the jet stream as it passes over the Pacific and, eventually, to the United States. Tornado-producing storms usually are triggered by, and will follow, the jet stream. This helps explain why we found a rise in the number of tornadoes and a change in their location when sea temperatures fluctuated.”
In their research, McCoy and Lupo investigated the relationship between tornadoes and a climatic episode called the Pacific Decadal Oscillation (PDO). As its name implies, the PDO is a long-term climate phenomenon linked to warm or cool surface waters of the Pacific Ocean north of 20 degrees north. The phases of the PDO take place over an inter-decadal timescale, usually varying between 20 and 30 years. Such are the effects of the PDO on the climate of North America that scientists have been able to reconstruct its inter-decadal phases as far back as 1661 based on tree-ring measurements in Baja California.
According to NASA scientists, the current PDO phase has just entered into a cool state. McCoy described these cool phases thus: “PDO cool phases are characterized by a cool wedge of lower than normal sea-surface ocean temperatures in the eastern Pacific and a warm horseshoe pattern of higher than normal sea-surface temperatures extending into the north, west and southern Pacific.”
Contrasting the cool phase with the PDO’s warm phase, McCoy said. “In the warm phase, which lasted from 1977 to 1999, the west Pacific Ocean became cool and the wedge in the east was warm.”
The MU research, which will be presented at the National Weather Association Conference this fall, will be a useful additional mechanism for predicting tornadoes.
As McCoy said, “Now that we know the effects of PDO cool and warm phases, weather forecasters have another tool to predict dangerous storms and inform the public of impending weather conditions.”
Apart from the tragic loss of life caused by tornadoes, the damage they wreak on the American economy is significant.
2011 was one of the worst years on record for tornadoes in the US. In that one year there were 1,704 confirmed tornadoes, together causing an estimated $10 billion of damage—and leaving 553 dead in their wake. 2011 was also a significant year for the frequency of the most violent EF5 tornadoes. Six of these "Incredible" class monster storms ripped through Mississippi, Alabama, Missouri and Oklahoma.
Armed with the new data, the hope is that it will help tornado-prediction services prevent future loss of life.
More about tornado forecasting, predicting tornadoes, Extreme weather, tornado alley, Fujita scale
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