Warming and cooling trends in the equatorial Pacific Ocean affect the frequency of tornadoes (pictured) in parts of the United States.
John Allen of Columbia University in New York and his colleagues focused on environmental indices (such as wind shear) that are linked to extreme US storms, and analysed their relationship with periodic warming and cooling events in the tropical Pacific. These events are known as El Niño and La Niña, respectively. The team found that fewer tornadoes and hailstorms occur in winter and spring in the central and southern plains during El Niño, and more occur during La Niña, than in years when temperatures in the Pacific are relatively stable.
Although El Niño and La Niña events tend to be strongest during winter, the authors suggest that ocean conditions could be used to forecast extreme spring storm events several months in advance.
Nature Geosci. http://doi.org/243 (2015)