Climate change in general and a warming climate in particular catches the blame for many types of weather phenomena on the increase. Oftentimes, the blame is at least partially deserved. This year, tornadoes are way up through mid-April over the average of the last 25 years, and especially since 2012.
A good deal of the activity has been over the southern states. One driving force behind the heavy rainfall and occasional severe storm outbreaks is greater amplitude this spring of the jet stream with longer north-to-south waves in its flow. These high altitude winds can energize the air near the surface, creating lift and buoyancy where the highest velocity upper level winds are occurring. The sharp turning of the winds at higher altitude can favor more rotating thunderstorms. Even at this writing in a soaking wet pattern, take a look at this very wavy pattern aloft as of Friday morning.
The jet stream typically becomes flatter, slower and shifts to the north as we move toward summer. When that flatter amplitude is reached, the incidence of severe weather tends to diminish during the summer.
For the more adventurous among you, here is a learning tutorial about the jet stream from the National Weather Service learning website, Jetstream.
The higher rate of tornadoes through most of April does not mean this trend will continue through the rest of the spring and the year ahead.
Many nonscientists assume a warming climate is going to be associated with an increase in tornadoes, since it is known to be linked with more heavy-precipitation events, and many of those events are tied to thunderstorms. About 18 months ago, I participated in an online webinar conducted by Dr. Harold Brooks of the National Severe Storms Laboratory in Norman, Okla. Brooks is one of the most accomplished and active severe convection experts in the world and I had the feeling he was going to tell us just such a link between our warming climate and an increase in tornadoes had been established.
Like so many things in science and in life, it isn’t that simple.
Brooks’ and others' research shows the suspected link is tenuous to some extent. Some of the uncertainty in research is tied to the small scale of tornadoes and the difficulty in simulating their development in larger scale computer models. There is a higher certainty of the link between a warming climate and more numerous intense thunderstorms with torrential downpours. The extra heating adds more buoyancy to moist low level air by taking advantage of the potential energy already present in that air. A warming climate is linked with more episodes of rapid upward motion of moist parcels of air, lifted to where it cools and must condense. With enough strong lift comes violent, heavy thunderstorms.
Since tornadoes come from thunderstorms, and usually violent storms, it must follow the increase in those storms would have an almost linear relationship with an increase in tornadoes. The complexity and uncertainty enters the picture when models show a warming climate will probably lessen the favorable change in winds with increased altitude which feeds the spin in the atmosphere leading to tornadoes.
That change is called wind shear, and there are two basic types. There is speed shear, in which the horizontal winds aloft blow faster and faster at greater heights, and there is directional shear. That’s the more complex type of shear. Winds that veer in a clockwise direction with increased altitude produce a property called helicity. That is, if surface winds are from the south or the southeast and winds at 10,000 feet are from the southwest, and winds at 18,000 feet are from the west, that veering from helicity makes conditions more favorable for rotating, spinning columns of air to develop with thunderstorms.
That is nowhere near a complete picture of how tornadoes form. However, computer simulations have predicted more buoyancy but less wind shear and helicity with a warming climate. If that hypothesis were proven true, it would probably mean more thunderstorms, more severe thunderstorms and more flooding, but fewer tornadoes. The first three of those four appear to be correct. But the latter — fewer tornadoes — is another story. More refined research seems to show the days in which necessary wind shear is lower have been days in which tornado formation wasn’t likely to begin with.
At the time of our seminar, Brooks told us there was suggestive but inconclusive evidence a warming climate may lead to fewer days with tornadoes but more frequent violent and long-lasting tornadoes in a few bad outbreaks. In 2014, Florida State University Professor James Elsner stated, “the risk of violent tornadoes appears to be increasing” and damage paths in such storms appear to be lengthening.
Most scholars such as Brooks and Elsner feel we have much to learn about any link between a warming climate and tornadoes. The relationship between warming and tornado frequency and intensity appears to be taking a different shape than might have first been suspected.