Conventional wisdom has commonly associated La Nina episodes with strong severe weather seasons in the middle of the country. A new study recently released in the Journal of the American Meteorological Society suggests it's not quite so simple as having a La Nina to get a better severe weather pattern.
La Nina is one ingredient in the setup but the Nina needs to be in a relaxing state, transitioning towards neutral or Nino conditions. This occurrence, known as Trans-Nino, helps create northward surface and low level winds that transport additional moisture into the middle of the country. This then provides for stronger storm systems with more thunderstorm activity. The tornado part comes from the variation in wind trajectory from southerly at the surface to the predominant westerly wind in the jet stream. Trans-Ninos work best when water temperatures in the Eastern Pacific are warming, while remaining cooler in the Western Pacific.
Trans-Ninos on their own aren't the be-all and end-all for severe weather development but they are a factor of sorts. "It's dangerous to try to explain all the tornado outbreak events by the impact of climate," University of Miami researcher Sang-Ki Lee said. "Much of it is due to normal atmospheric variability."
Lee's research looked back to the ten most prevalent tornadic seasons since the 1950's and found that seven of the ten years featured Trans-Nino state. Going back further, Lee's research indicates that Trans-Nino conditions may have contributed to severe weather outbreaks that were common in 1917 and 1925 -- years that featured deadly outbreaks of severe weather.
2011 was a Trans-Nino episode as well as the MEI (one of the key Nino/Nina indicators) rose from -1.492 for the March/April period to -0.087 in June/July, effectively neutral. While we did ultimately transition back to a weak La Nina this past winter, the rapid rise in ENSO from Nina to neutral may have contributed to the additional severe weather episodes we saw around the country last year. On the flip side, other big tornado years in our history such as 1954 (solid Nina throughout) would not fit the criteria. Nonetheless, having a Nina state in the Pacific tends to help...and if it's weakening, it may be an added bonus.