Are La Nina and global warming behind the extreme tornado activity?

NOAA’s Storm Prediction Center is indicating a 15% chance of a tornado within 25 miles of a point within the marked area, which represents a serious threat. This area includes portions of northeast Texas and the mid-South (NOAA SPC)

In order for tornadoes to form, several factors have to combine in just the right way. These ingredients include: a warm and humid atmosphere, strong jet stream winds, and atmospheric wind shear (winds that vary with speed and/or direction with height), as well as a mechanism to ignite this volatile mixture of ingredients - such as a cold front.

As Jason discussed Friday, an unusually powerful jet stream has steered a cascade of frontal systems across the midsection of the country. “There’s been a ton of systems coming through, that’s I think one of the biggest single things” leading to the recent severe weather outbreaks, says Harold Brooks, a research meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma.

But what is causing all of these systems to be so intense? Is it La Nina, a natural climate cycle featuring cooler than average water temperatures in the tropical Pacific Ocean? Is global warming playing some role?

La Nina’s link

Although Pacific water temperatures have been rising since La Nina peaked last winter, its influence on the atmosphere can persist for a month or two after water temperatures warm up.

Like its sibling El Nino, La Nina 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 Nina 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 Nina years.

Role of the Gulf of Mexico, water vapor, and global warming

Sea surface temperature departure from normal on April 23, 2011 (NOAA)

“Having the Gulf warm doesn’t hurt, because that’s a good moisture source,” Brooks says.

“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.

As the world warms, more moisture is evaporated from the oceans into the atmosphere, providing more moisture for storm systems to work with. The likelihood that a warmer atmosphere will also be a wetter one, meaning that it will contain more water vapor, is a robust conclusion of many climate change studies, that also has a firm foundation in physical science.

Both an increase in water vapor and a rise in temperature will boost a metric that meteorologists use to forecast severe thunderstorms, known as Convective Available Potential Energy, or “CAPE.” A higher CAPE value indicates that there is more potential energy in the atmosphere to fuel thunderstorm development, should some trigger come along and set them off.

Less tornado-producing wind shear in a warming world?

At the same time, though, modeling studies indicate that a warmer world may also have less wind shear, which is necessary in order to transform ordinary thundershowers into organized squall lines and supercells, capable of dropping large hailstones, producing damaging straight line winds, and spawning tornadoes. So which one will win out?

According to some studies, 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. One such paper, published in the journal Geophysical Research Letters, states, “... Our modeling results show that the net severe thunderstorm forcing continues to increase through the late 21st Century even with time-decreasing shear.” This may particularly be the case in the Gulf Coastal states and along the Atlantic Seaboard, closest to the main moisture sources, according to another study. Both of these studies found that by reducing greenhouse gas emissions during the next several decades, the uptick in severe thunderstorm activity could be limited.

Tornadoes are particularly sensitive to the amount of wind shear, which means that less shear could translate into greater increases in non-tornadic severe thunderstorms - which can still be deadly - rather than tornadic storms. Brooks says recent work he has done using newly available datasets shows that, across a portion of Tornado Alley, there has been slight increases in CAPE values and slight decreases in wind shear from 1960 to 2008 in a portion of Tornado Alley, but the variability from year-to-year overwhelms the longer-term signal.

“My gut feeling is that we’ll likely see changes, but that the interannual variability is still going to be so large that you’ll have to be collecting statistics for many years to see an average affect,” he says.

Long term trends

There is no clear indication that severe thunderstorms and tornadoes have become more common due to climate change, in part because of major limitations in relying on the historical record of severe weather reports. While the number of tornadoes recorded in the U.S. has just about doubled during the past 50 years, the number of strong tornadoes (EF2 and above) has actually been decreasing. It may be the case that more tornadoes are being noticed today, given a network of trained storm spotters and a national Doppler radar network, both of which didn’t exist as recently as the early 1980s.

Also, it’s quite possible that tornado strength was overestimated in previous years, before the National Weather Service began sending out storm damage surveyors to rate each tornado’s intensity.

A 2008 paper in EOS Transactions, a publication of the American Geophysical Union, discussed this problem of deciphering recent trends in tornado statistics. As Brooks, a coauthor of the Eos paper, told me, “The changes in reporting practices make it impossible to tell anything about frequency and strength changes” of tornadoes to date.

Still another complicating factor is that in addition to shifts in the environmental conditions that give rise to severe thunderstorms, the mechanisms that kick off the storms - the cold and warm fronts, for example - may also change in unexpected ways. “We don’t know a whole lot about initiation of storms,” Brooks says, noting that the “efficiency” with which Mother Nature converts ordinary thunderstorms into severe thunderstorms may change.

“Given that we have good environments, not all [thunderstorms] actually turn into severe thunderstorms.”

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