|MadSci Network: Earth Sciences|
La Niņa and El Niņo both result from interaction between the ocean's surface and the atmosphere in the tropical Pacific.
The term La Niņa describes the period when the sea surface temperature in the tropical Pacific is cooler than normal, and El Niņo is the term used to describe warmer than normal sea surface temperatures in the tropical Pacific.
Both La Niņa and El Niņo impact global climate patterns. In the United States, La Niņa often results in drier than normal conditions in the Southwest in late summer through winter. Drier than normal conditions also typically occur in the Central Plains in the fall and in the Southeast in the winter. In contrast, the Pacific Northwest is more likely to be wetter than normal in the late fall and early winter. Additionally, on average, La Niņa winters are warmer than normal in the Southeast and colder than normal in the Northwest.
Now that you know a little background about La Niņa, I'll try to answer your question!
It would not be correct to say that La Niņa was the direct cause of the tornadoes that have occurred so far this year. La Niņa does, however, affect the position and intensity of the jet streams, which in turn affect the intensity and track of storms.
Since a strong jet stream is an important ingredient for severe weather, the position of the jet stream determines the regions more likely to experience tornadoes. Comparing El Niņo and La Niņa winters, the jet stream over the United States is considerably different. During El Niņo the jet stream is oriented from west to east over the northern Gulf of Mexico and northern Florida. Therefore this region is more likely to have severe weather. During La Niņa, the jet stream extends from the central Rockies east-northeastward to the eastern Great Lakes. Therefore severe weather is likely to be further north and west during La Niņa than El Niņo.
I hope this answered your question. There are plenty of websites devoted to El Niņo and La Niņa. One of those is produced by the Climate Prediction Center. Follow this link to find out more!
Try the links in the MadSci Library for more information on Earth Sciences.