The connection between El Niño and deadly December tornadoes

History shows that Decembers during El Nino tend to be more dangerous for tornadoes in the U.S. ( <a href="http://earth.nullschool.net/#current/ocean/surface/currents/overlay=sea_surface_temp_anomaly/orthographic=-115.75,26.98,423">earth.nullschool.net</a> )

After a week of severe weather ranging from violent tornadoes to flash flooding has killed dozens of people in the South and Midwest, many are left wondering about the connection between this year’s strong El Niño and the bizarre December weather.

El Niño is a Pacific Ocean phenomenon where a vast area of exceptionally warm water stretches across the equator. In extreme El Niño events, the magnitude of this temperature anomaly disrupts the typical winter patterns, and has a ripple effect on weather across the globe. These effects can be very pronounced over North America, where high pressure and warm weather tends to settle over the Northeast, with cool troughs of low pressure digging through the West and South.

[Freak storm in North Atlantic may push temps 50 degrees above average at North Pole]

El Niño Decembers also have a history of being prolific tornado producers. Much like this month’s deadly outbreaks, two of the most significant Decembers for tornadoes also occurred during strong El Niño events — an outbreak in December 1957, which killed 19 people, and December 1982, when nearly 100 tornadoes were confirmed in the month during what was then the strongest El Niño on record.

Another shot of the Garland, TX tornado. Video soon. #txwx #TXTornadoes pic.twitter.com/s1ghOt8Bgn

— Basehunters Chasing (@Basehunters) December 27, 2015

What makes these winters so prone to tornado outbreaks? During El Niño, the subtropical jet stream, which is located across the far southern tier of the United States, tends to strengthen, leading to an increase in moist, unstable air and deep troughs. These storms tend to track across the southern states in an arc extending from southern California across the Gulf Coast and into the Southeast.

[2015 was almost the least deadly tornado year on record. Then December happened.]

Based on that trajectory, it makes sense that the Gulf Coast states experience a rise in December tornadoes during strong El Niño winters. Several studies have sought to quantify whether there is a statistically significant increase and/or a geographic shift in tornado frequency between El Niño and La Niña conditions.

One study in particular encompassed 53 years of tornado data from El Niño and La Niña years. It found the greatest enhancement of wintertime tornado activity in the Southeast over North Carolina and Florida during El Niño winters, as shown by the back shading above. Hatched states show events that are “within expected norms,” including Texas and Mississippi, both of which have experienced deadly tornado outbreaks in the past week. Gray-shaded states indicate below-average tornado frequency.

Interestingly, the study also found that in La Nina years there is a significant rise in tornado activity in the South — including Alabama, Mississippi, Arkansas and Louisiana.

[Photos: Trees and bees confused in Washington, D.C.]

So the results of this and several other studies paint a complicated picture, one in which pronounced tornado shifts across the U.S. in El Niño or La Niña yearsare not always apparent — or even contradictory.

One thing can be said about all of the studies that have looked at the relationship: the statistical imprint of El Niño’s signal — if any — can be difficult to isolate. El Niño episodes vary so greatly in intensity, evolution, duration and the effect that it has on our weather in the mid-latitudes.

Suffice it to say that there may be an intriguing connection between this year’s late-December tornado activity and the present intense El Niño, but drawing a strong connection between the two at this time would be premature.

Angela Fritz contributed to this post.