A look at weather extremes and the big-picture climate connections.

Tornado Outbreak Raises Climate Change Questions

With at least 340 dead from dozens of tornadoes, including at least 249 deaths in Alabama alone, the tornado outbreak that tore across the South on Wednesday was certainly one for the record-books. Storm surveys are still going on, but it appears likely that this outbreak spawned several violent tornadoes, of the EF-4 to EF-5 category on the Enhanced Fujita Scale. Already one tornado has been designated an EF-5. It destroyed much of the town of Smithville, Mississippi, killing at least 14. The tornado that decimated parts of Tuscaloosa and Birmingham, Ala. is likely to be rated at least an EF-4, with maximum winds of at least 165 mph.

Such tornadoes are capable of ripping a home off its foundation and leaving just a concrete slab behind. The 1974 "Super Outbreak", which was the most recent outbreak of a similar magnitude, killed 330 people and featured a whopping 24 F4 and F5 tornadoes, including the infamous Xenia, Ohio F5 tornado, which destroyed most of that Ohio town.

The images of the tornadoes and the destruction they have wrought have sparked plenty of questions: why did this outbreak occur, and why has April been such an active month for tornadoes? And, might global climate change be making tornadoes stronger or more frequent, or perhaps be shifting Tornado Alley out of the Great Plains and into more heavily populated areas of the South?

Aerial footage of damage from the Tuscaloosa, Ala. tornado. Credit: ABC 33/40 TV.

Those of us who write about climate change are often accused of attempting to link every unusual weather event to climate change, as if increasing air and ocean temperatures can explain everything from hurricanes to snowstorms. In this case, with the second-deadliest tornado outbreak in US history, and with the most tornadoes for any April since records began in the early 1950s, it's important to understand that the scientific evidence indicates that climate change probably played a very small role, if any, in stirring up this violent weather. This might disappoint some advocates who are already using this to highlight the risks of climate change-related extreme weather.

Footage of the massive tornado that tore through Tuscaloosa, Ala. on Wednesday.

As I wrote at washingtonpost.com, tornadoes form when a combination of ingredients are present in just the right amounts. These include: abundant moisture, winds that change speed and/or direction with height (also known as "wind shear"), and a trigger to force air to rise into towering thunderstorms — such as a cold front. All of these ingredients were present in abundance on Wednesday, and in fact they have come together again and again this month across the South, Southeast, and Carolinas. In fact, so much wind shear was present on Wednesday that nearly every thunderstorm that formed in Alabama and Mississippi showed signs of rotation, meaning they were capable of producing tornadoes, and aircraft approaching Atlanta's Hartsfield International Airport — the world's busiest — had to break off their landings because of tailwinds that quickly turned into headwinds and back again, altering their airspeed and posing a serious risk of a serious accident.

A major factor in spawning all of the massive tornadoes was unusually powerful jet stream winds, which both helped trigger the storms and gave them a supercharged dose of wind shear to work with. The airflow in the upper atmosphere has steered numerous dips or "troughs" in the jet stream far to the south, helping to bring warm, moist air on a collision course with cold and dry air to the north.

One climate cycle that is being eyed as a contributor to the deadly April weather is La Niña, which is a natural cycle of climate variability that is characterized by cooler than average waters in the Pacific Ocean. Here's how I described La Niña's influence on tornado season over at the Capital Weather Gang blog:

Like its sibling El Niño, La Niña can influence weather patterns far from the Pacific Ocean. A study that examined the relationship between sea surface temperatures in the Pacific and the number of tornadoes in the U.S. found a weak correlation between La Niña and a greater number of tornadoes. Another study found tornadoes during La Niña years had longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes. Brooks points out that both the Palm Sunday tornado outbreak in 1965 and the Super Outbreak in 1974 occurred during La Niña years.

Another broader factor that may be aiding and abetting the destructive weather is a very warm Gulf of Mexico, where sea surface temperatures have been between 1 and 2.5°C above average for this time of year. This is important because it means there is more moisture flowing northward off the Gulf, and a humid environment is necessary for severe thunderstorms to form. As I wrote earlier this week: 

“Tornado Alley” owes its existence in part to humid air transported northward from the Gulf of Mexico, since without this moisture, you would not have the combustible clash of warm, humid air and cool, dry air masses in the Plains. Farther east, into “Dixie Alley”, the Gulf also serves as the fuel for many tornadic storms.

How Might Climate Change Affect Tornadoes?

Climate change is already changing the environment in which severe thunderstorms and their associated tornadoes form, and it's bound to have some sort of influence on tornado frequency or strength. But as of now, no discernible trend has been detected in the observational data, and studies of how tornadoes will fare in a warmer world show somewhat conflicting results.

While overall tornado counts have been increasing in the U.S. in recent years, the number of strong tornadoes has actually been going down, leading researchers to conclude that they are catching more of the weaker tornadoes that are far more common than the mile-wide "wedge" tornadoes that decimated communities like Tuscaloosa, Ala. this week. The trends in tornado counts is likely a result of improvements in observational technology, such as Doppler radar networks and a national network of trained storm spotters. Also, historical tornado data is not considered very reliable, since it's thought that many tornadoes — particularly weaker ones —  were missed in the early decades of record-keeping, and construction methods have also changed with time. This is important since the strength of a tornado is determined by a damage survey, rather than actual instrument measurements of tornadic winds (no wind-recording instrument has ever survived a direct hit by a tornado).

As I reported for the Capital Weather Gang:

As for global warming, this is an active area of scientific research, with some conflicting projections so far about whether a warming atmosphere will make it more or less likely that tornadoes will form. Since more moisture gets added to the atmosphere as the climate warms, additional water vapor may help severe thunderstorms and tornadoes to form. On the other hand, wind shear is expected to decline due to climate change, which would argue against an increase in tornado numbers.

According to some studies, though, by the end of the present century, the added water vapor will be enough to overcome the lower wind shear, and create more opportunities for severe thunderstorms to form.

Tornadoes are a bigger wild card for climate scientists than other types of extreme weather and climate events, such as heat waves and flooding. (Studies have consistently found that both of these hazards will occur more frequently and severely as the world warms.)

Online Resources:

Studies: A 2008 paper in EOS Transactions, a publication of the American Geophysical Union, discussed challenges involved in analyzing tornado trends and climate change.

This 2009 paper in Geophysical Research Letters found that climate change may make many parts of the U.S. more susceptible to severe thunderstorms, and possibly to tornadoes as well.

This 2007 study in Proceedings of the National Academy of Sciences found that severe thunderstorms may become more frequent as the climate warms, particularly in areas close to the Gulf of Mexico and along the Atlantic Seaboard.

Weather Underground chief meteorologist Jeff Masters detailed these studies and others in a 2008 post on this topic.

New York Times blogger Andy Revkin has extended comments from some of the top researchers studying climate change and tornadoes. 

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