Influence of the El Nino/Southern Oscillation on tornado and hail frequency in the United States

The El Nino/Southern Oscillation (ENSO) is characterized by changes in sea surface temperature (SST) and atmospheric convection in the tropical Pacific, and modulates global weather and climate(1-4). The phase of ENSO influences United States (US) temperature and precipitation and has long been hypothesized to influence severe thunderstorm occurrence over the US5-11. However, limitations(12) of the severe thunderstorm observational record, combined with large year-to-year variability(12,13), have made it difficult to demonstrate an ENSO influence during the peak spring season. Here we use environmental indices(14-16) that are correlated with tornado and hail activity, and show that ENSO modulates tornado and hail occurrence during the winter and spring by altering the large-scale environment. We show that fewer tornadoes and hail events occur over the central US during El Nino and conversely more occur during La Nina conditions. Moreover, winter ENSO conditions often persist into early spring, and consequently the winter ENSO state can be used to predict changes in tornado and hail frequency during the following spring. Combined with our current ability to predict ENSO several months in advance(17), our findings provide a basis for long-range seasonal prediction of severe thunderstorm activity.