Water temperatures in the Gulf of Mexico often enter the conversation during hurricane season, but they also play a role in tornado season each spring. Concern is growing for the potential impact that abnormally warm sea surface temperatures will have in the weeks and months ahead, with experts warily keeping tabs on what could be a busy tornado season.
Friday’s tornado outbreak in Mississippi and Alabama, which killed at least 24 people, coincided with gulf waters up to several degrees warmer than normal and as high as the upper 70s.
April, May and June are historically the most active for severe weather and tornadoes, with an average of 660 twisters spinning up within the three-month window. They materialize seemingly like clockwork, creeping north across the Deep South before swimming over the Plains and the nation’s heartland.
While considerable variability can occur year to year, there may be ties between gulf sea surface temperatures and the frequency and intensity of severe weather events in the U.S. Deep South in particular.
In the months ahead, several key atmospheric processes could favor a busier-than-normal tornado season, including the warm gulf water, but the jury is still out on just how active it will be. Seasonal tornado forecasting is still in its infancy, and meteorologists are careful to remind the public that confidence remains low in what lies ahead.
A very active winter for tornadoes
The year has already been off to a historically active start, with a preliminary total of 168 tornadoes touching down across the Lower 48 states in January. That’s the second-most on record. In February, there were 55 tornadoes — double the average of 29.
Oklahoma reported 17 tornadoes in January and February. The average during that window is one.
On Jan. 16, two tornadoes spun up in eastern Iowa, the state’s first January tornadoes in 50 years.
Alabama also logged 29 tornadoes during January, smashing state records, and New Jersey even faced an EF2 tornado, on the 0-to-5 scale for twister intensity, on Feb. 21. That marked the Garden State’s first February twister in nearly a quarter-century.
Warm Gulf of Mexico water temperatures probably boosted the winter activity, consistently running several degrees above average. In addition, temperatures were exceptionally high over land, especially in the eastern United States. January was 5.1 degrees above normal in the Lower 48. February was 2.7 degrees above average.
March tornado activity had been quiet until Friday’s outbreak in the South, which may have kick-started the spring season.
What do tornadoes require?
Tornadoes require two simple ingredients: fuel and spin. Enough fuel for storms — known as convective available potential energy or CAPE — must be present to build thunderstorms and brew storm clouds. CAPE is generated by warm and humid air. Changing winds with altitude, known as wind shear, can help those clouds to rotate.
Both ingredients need to overlap to yield a tornado threat. Without shear, thunderstorms tend to take the form of disorganized clusters, squall lines or clumps. That reduces the tornado threat. Too much shear, however, and a thunderstorm can be torn apart before it can mature and deliver a tornado.
During the winter, shear abounds — it’s largely ever-present thanks to the jet stream, which lurks over the United States throughout the cold season. But CAPE supplied is usually limited in the winter because of the lack of heat and humidity. During summer, CAPE is plentiful as it heats up, but shear is often lacking as the jet stream retreats. Spring marks the juxtaposition of both.
The drivers for tornado season
While scientists generally understand the ingredients necessary for tornadoes, seasonal tornado outlooks are still considered experimental and low-confidence.
“I think the current ability to make seasonal tornado forecasts is at best quite modest,” Michael Tippett, a professor of applied physics at Columbia University, said in an email.
He said the only reliable indicator of how a tornado season may shape up is ENSO, or the El Niño Southern Oscillation. ENSO is a chain-reaction process that first manifests in warmer or cooler sea surface temperatures in the eastern equatorial Pacific before shuffling global weather patterns. El Niño is the warm phase, and La Niña is the cool phase. La Niña has been linked to extra-active Atlantic hurricane seasons and busy U.S. tornado seasons.
“It would be nice to have sources of seasonal forecast skill other than ENSO, but to date, even the ability to make seasonal forecasts of temperature and precipitation is mostly the ability to predict ENSO and its impacts,” Tippett said.
Complicating matters is the lack of an obvious ENSO signal heading into this spring’s peak tornado period. The National Oceanic and Atmospheric Administration announced the demise of the La Niña pattern that had persisted for nearly three years, leaving the planet in a “neutral” zone before an anticipated shift toward El Niño in the fall.
Some experts point to the transition from La Niña to El Niño as a potential cause for concern, with at least some link between the transition period and a flare-up in activity.
However, it might not matter much — Tippett explained that any ENSO-related impact to tornado season is most prevalent in March before the signal fades and is “very modest” in April and May.
“And seasonal climate forecasts of any kind are especially difficult at a time like this when ENSO is shifting to neutral,” noted Tippett.
That opens the door for other factors to potentially have a greater impact this year. Warmer Gulf of Mexico sea surface temperatures can help moisten and heat the air, increasing CAPE to fuel nasty thunderstorms.
Is the warm gulf cause for concern?
Research into the specific role of gulf water temperatures is still just beginning to emerge.
A 2019 paper in the journal ScienceAdvances noted that Gulf of Mexico sea surface temperatures in April have a “robust dynamical linkage” to annual tornado accounts in the southern Great Plains. The authors noted that, by May, other factors dominate, and the role of sea surface temperatures is less important.
Forecasters note that they have spotted a trend, too.
“As a perspective, working the forecast desk day in and day out, we do notice that when sea surface temperatures are anomalously high, the atmosphere has a capacity to hold more moisture,” said Jonathan Howell, science and operations officer at the National Weather Service in Mobile, Ala.
Currently, water temperatures in the Gulf of Mexico are running about 2.7 degrees above average. In parallel, air temperatures over the past month across the Southeast and South have hovered 3 to 6 degrees above average, and many cities — like Atlanta; Montgomery, Ala.; Mobile, Ala.; Hattiesburg, Miss.; Tallahassee, Jacksonville, Savannah and countless others — have logged their warmest mid-February to mid-March on record. Several locations set record highs in the 80s ahead of Friday’s tornado outbreak.
Kevin Laws, the science and operations officer at the National Weather Service in Birmingham, Ala., described Gulf of Mexico sea surface temperatures as a “proxy” for how much moisture could be available for storms — and how quickly that moisture can spread over the Deep South.
“In general, warmer [sea surface temperatures] that are anomalously high [mean] we can get a return flow of deeper moisture inland more easily,” he said.
In other words, warmer sea surface temperatures more effectively draw moisture into the air, meaning the atmosphere can recover from arid to soupy more quickly.
NOAA has not issued an outlook on what this season could hold, but oceanographer Sang-Ki Lee said the warm gulf waters could well have an influence. Lee has published research on the relationship between ocean temperatures and tornado outbreaks.
“Warm [sea surface temperatures] anomalies in the Gulf of Mexico may enhance the supply of warm and moist low-level air to the U.S., increasing the chance of developing tornado outbreaks in the South and Ohio Valley,” Lee wrote in an email.
At the end of March 2020, Gulf of Mexico sea surface temperatures were running 3 degrees above average, not unlike this year. Two weeks later, a swarm of tornadoes tore across the South in one of the most prolific outbreaks ever observed. It was the second most active April on record for tornadoes.
Every tornado season is different, and a host of factors influences the number of storms in addition to water temperatures.
“My group makes a statistical March through May forecast of tornado activity … and this year it shows very modest shifts from equal odds (33 percent) to more likely above-normal activity,” Tippett said.
In other words, a slightly more-active-than-normal tornado season is predicted.