ENSO phase transition in spring and its potential impact on tornado outbreaks in the U.S

The record-breaking U.S. tornado outbreaks in the spring of 2011 prompt the need to identify long-term climate signals that could potentially provide seasonal predictability for U.S. tornado outbreaks. This study uses both observations and model experiments to show that a positive phase TransNino may be one such climate signal. Among the top 10 extreme outbreak years during 1950-2010, seven years including the top three are identified with a strongly positive phase TransNino. The number of intense tornadoes in April- May is nearly doubled during the top 10 positive TransNino years from that during 10 neutral years. TransNino represents the evolution of tropical Pacific sea surface temperatures (SSTs) during the onset or decay phase of the El Nino–Southern Oscillation. A positive phase TransNino is characterized by colder than normal SSTs in the central tropical Pacific and warmer than normal SSTs in the eastern tropical Pacific. Modeling experiments suggest that warmer than normal SSTs in the eastern tropical Pacific work constructively with colder than normal SSTs in the central tropical Pacific to force a strong and persistent teleconnection pattern that increases both the upper-level westerly and lower-level southwesterly over the central and eastern United States. These anomalous winds advect more cold and dry upper-level air from the high latitudes and more warm and moist lower-level air from the Gulf of Mexico converging into the east of the Rockies, and also increase both the lower-tropospheric (0-6 km) and lower-level (0-1 km) vertical wind shear values therein, thus providing large-scale atmospheric conditions conducive to intense tornado outbreaks over the United States.