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

Changes in sea surface temperature (SST) and atmospheric convection in the tropical Pacific characterises the El Niño/Southern Oscillation (ENSO) and modulates the global weather and climate (1,2,3,4). The temperature and precipitation in the USA is influenced by the phase ENSO and it has long been hypothesised that ENSO also influences severe thunderstorm activity over the US (5,6,7,8,9,10,11). Limitations (12) of the severe thunderstorm record combined with large variability (12,13) from year to year (12,13), have made the demonstration of an influence by ENSO during the spring season difficult. In this paper Allen, Tippett & Sobel¹ show by using environmental indices (14, 15, 16) that correlate with tornado and hail activity that ENSO modulates tornado and hail activity during the winter and spring by altering the large-scale environment. Their data show that during El Nino events there are fewer tornados and hail events over the central US and conversely there are more at time of La Niña conditions. Also, winter ENSO conditions often persist into early spring, and consequently changes in tornado and hail activity during the following spring can be predicted by the use of winter ENSO state. These findings form a basis for long-range seasonal prediction of severe thunderstorm activity when combined with current ability ENSO months in advance (17).

Sources & Further reading

1. Allen, J. T., M. K. Tippett and A. H. Sobel (2015). "Influence of the El Nino/Southern Oscillation on tornado and hail frequency in the United States." Nature Geosci 8(4): 278-283.