February was an unusually stormy month across the Southeast, with 53 tornadoes touching down across six states - the most for any February since 1950 - destroying homes and businesses and claiming several lives.
One factor fueling the spate of storms has been the strong El Niño that is altering weather patterns around the globe.
El Niño is defined by an eastward shift of warm ocean waters in the tropical Pacific. That shift alters where heat from those waters is released into the atmosphere, which in turn knocks circulation patterns out of whack, creating a cascade around the planet.
Over the United States, it tends to amp up the subtropical jet stream and shunt it southward over the southern tier of the country. This leads to greater odds for tornadoes right along the Gulf Coast, with the strongest signal in Florida.
Florida always has two of the ingredients necessary to form tornadoes - abundant moisture and unstable air masses, thanks to being surrounded by water. El Niño, through its effects on the jet stream, amps up that second ingredient and adds a third: the wind shear needed to spur the rotation that can turn into a tornado. (Wind shear is the change in direction or speed of winds at different heights in the atmosphere.)
So far this year, Florida has recorded 18 tornadoes, well above the average of seven it usually sees by this time. Nine of those tornadoes touched down in February, including two that rated an EF3 on the Enhanced Fujita scale of tornado strength, which goes from 0 to 5. Those were the first EF3 tornadoes in Florida since 1988 - the amped up ingredients that set the stage for more tornadoes can also up the odds for stronger tornadoes.
Of course, "one individual storm system or tornado outbreak cannot be blamed on El Niño," state climatologist David Zierden said in an email. "But the prevalence in Florida and the number of strong tornadoes this year can be directly tied to the strengthened subtropical jet."
For the rest of the Southeast, particularly the farther north you go, the signal from El Niño is much more muted, if it exists at all. But, it is probably fair to say that the juiced-up jet stream played a role in the recent activity, Jordan McLeod, a climatologist with the Southeast Regional Climate Center, said in an email.
Only 29 tornadoes are typically seen in the U.S. during February, and they tend to cluster in the Southeast.
Alabama saw the most activity during this February, with an outsize 20 confirmed tornadoes, compared to its average of just three. Georgia counted five, compared to an average of two, while South Carolina saw twice its usual average of one. North Carolina and Virginia, both of which typically don't see any tornadoes in February, recorded nine and eight, respectively.
Other factors likely contributed to the uptick in tornadoes for the month, McLeod and others said. McLeod cited favorable conditions from the polar jet stream as another potential influence on the stormy activity.
Sometimes, those other factors can actually overwhelm any El Niño influence. For example, while the deadliest tornado outbreak in Florida history occurred in February 1998, during the last strong El Niño, another climate pattern caused temperatures during the moderate El Niño of 2010 to be too cold to support tornado formation.
Also, El Niño actually has the opposite effect on tornado activity in the area of the central U.S. that is typically called Tornado Alley. The southward shift of the subtropical jet tends to cut off moisture from the Gulf of Mexico, which is needed to help fuel spring storms there.
John Allen, an associate research scientist at the International Research Institute for Climate and Society at Columbia University, who, with colleagues, came out with the first seasonal tornado forecast last year, expects a below-normal number of tornadoes in the south central U.S.
What climate change will mean for any of these seasonal tornado signals is still up in the air. Recent research has suggested that tornado activity might be concentrated into fewer days and the tornado season could peak earlier than it used to.
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