Both January and February are fickle months for tornado frequency, so a slow start doesn’t tell us much about the season to come. Tornado activity-and, more broadly, severe thunderstorm activity-for 2021 has been lower than average after a somewhat late start (5). In any one case, however, the specific atmospheric pattern on shorter timescales plays an important role (4). Like all climate influences on the weather scale, you can think of La Niña weighting the dice in favor tornado events, while on the flipside El Niño reduces the overall likelihood. For example, a similar magnitude La Niña in 1989 was accompanied by a similar near- to below-average tornado season to 2021 so far. La Niña certainly does not always increase tornado likelihood. March-May values of a blend of tornado reports with a tornado environment index (TEI) for El Nino and La Nina years. Through this, we’ve been able to show that, generally, El Niño leads to fewer tornado events in the spring, while La Niña tends toward higher-than-normal frequency. In the past decade, we’ve made great advances in understanding what causes this year-to-year variability (2), thanks to extended computer simulations of past events, new high-quality datasets that can tell us about the favorable conditions for tornado development, and the development of longer records of tornado observations. We also know that you don’t need ENSO to get a lot of tornadoes-a year when ENSO is neutral sees on average around 1,200 tornadoes across the United States, with the majority occurring in the March to June window. However, the idea of a connection between ENSO and severe thunderstorms and tornadoes is not new, with some of the most tornado-active years in history being associated with La Niña (Marzban and Schaefer 2001, Cook and Schaefer 2008). How ENSO and tornadoes are relatedĬonnecting tornadoes to the larger climate system is a hard problem, as the frequency of tornadoes typically has large year-to-year variability.
In this post, we consider the relevant factors and what this tells us about our capacity to predict seasonal tornado activity. A moderate La Niña was observed this past winter and was predicted to continue into the spring, leading to expectations and speculation in the media about a potentially active season.
Many people will remember that the last extremely active tornado year, 2011, was a strong La Niña year. While there are many contributing sources of tornado variability, there is a relationship between El Niño/Southern Oscillation ( ENSO) and the frequency of tornadoes. To begin to address what happened in 2021, we need to go back to what these seasonal predictions are based on.
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