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Hail and Tornadoes? Blame ENSO

Paper: Allen, J. T., et al., 2015. Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the United States. Nature Geoscience, v. 8. DOI: 10.1038/NGEO2385

Tornados and hailstorms cause massive damage across the US each year. (Source: The Why Files)

Tornados and hailstorms cause massive damage across the US each year. (Source: The Why Files)

Warm and cold phases of El Niño/Southern Oscillation (ENSO) in the tropical Pacific Ocean have done their fair share of damage on coastlines in the past. In 1997, the strongest El Niño event in recent history caused Hurricane Pauline to hit Mexico, killing over 250 people and dropping over 2 feet of rainfall in 24 hours in the town of San Luis Actatlan. But did you know that ENSO also has an effect on extreme weather events thousands of miles from the ocean? Every year in the United States, severe thunderstorms that host tornadoes, hail, and extreme winds cause an average of $1.6 billion in damage, and ENSO could be partially to blame.

El Niño is only one phase of ENSO, which as a whole refers to the cycle of warm and cold sea surface temperatures (SST) and atmospheric pressure in a band of the equatorial Pacific Ocean. The warm phase of ENSO, El Niño, is recognized by a winter of warmer SST and low air pressure in the eastern Pacific Ocean, off the coast of western South America, Central America, and southwestern North America. The cool phase of ENSO, La Niña, results in colder SST and higher air pressures in the same areas (Figure 1). The phases can persist for years at a time, controlling winter conditions and precipitation.

Figure 1: A plot of the Southern Oscillation Index (SOI) derived from pressure measurements at Tahiti and Darwin. Negative (red) values show El Niño events, while positive (blue) values show La Niña events. (Source: NIWA)

Figure 1: A plot of the Southern Oscillation Index (SOI) derived from pressure measurements at Tahiti and Darwin. Negative (red) values show El Niño events, while positive (blue) values show La Niña events. (Source: NIWA)

ENSO activity does not just dictate the winter weather along the coasts; it also has large effects on global weather and climate. ENSO affects the position of the jet stream, causing temperature and rainfall anomalies. The jet stream has powerful downstream effects on weather over the Northeast US, as evidenced from the relentless winter of 2014-2015. Movement of the position of the jet stream also causes shifts in extreme weather, including the number of tropical cyclones expected in the Pacific and Atlantic Oceans, and also thunderstorms across central US.

Linking extreme weather in the central US to ENSO is not a new idea, but a new study by climate scientist John Allen and other researchers suggests that ENSO modulates tornado and hail occurrence well past winter and into spring, when the frequency of these events skyrocket. The researchers used statistics based on past ENSO and weather data to create seasonal composites for the El Niño and La Niña phases of ENSO.

The data used for analysis was sourced from the National Climatic Data Center (NCDC) Storm Data from January of 1979 to December 2012. From this, they calculated the total seasonal occurrence of hail and tornado events for winters and springs. They looked especially at years of a moderate to strong signal in the seasonal mean Oceanic Niño Index (ONI), which is used to describe the strength of the ENSO state. A value greater than 1 is considered strong for winter (termed the DJF phase for “December, January, February”), while an ONI value above 0.5 is considered strong for spring (termed MAM for “March, April, May”).

Figure 2: Composite mean anomalies of spring (MAM) hail and tornadoes conditioned on the winter ENSO state. (Allen et al., 2015)

Figure 2: Composite mean anomalies of spring (MAM) weather conditioned on the winter (DJF) ENSO state. El Niño events (top maps) seem to produce fewer hailstorms and tornados, while La Niña events produce more (Allen et al., 2015).

What they found was highly significant, both statistically and meteorologically speaking. It appears that fewer tornado and hail events occur over the central US during an El Niño event, and conversely more occur during La Niña conditions. In addition, the stronger the winter phase of ENSO is (ONI >1), the higher the frequency of hail and tornado events are in the spring phase (Figure 2). So, if there is a strong La Niña during the winter, hail and tornado-prone regions of the US should brace themselves for a rough spring. This was certainly true in 2011, a strong La Niña year, when over 550 people died in tornado-related incidents, more than the total tornado casualties of the 10 previous years combined.

Scientists are already able to predict the phase of ENSO several months before winter. Now, they can take this a step further, and use the findings from this study to provide a foundation for long-range seasonal prediction of severe thunderstorm activity. In light of the damage caused by tornadoes in recent years, this is an important tool to add to our wealth of knowledge in meteorology. Although this will still not allow us to predict when and where a tornado will strike, it can give us a good view of what the spring season has in store for us.


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