Short-Term Climate Extremes over the Continental United States and ENSO. Part I: Seasonal Temperatures

Abstract

This study documents statistical relationships between the El Niño?Southern Oscillation (ENSO) phenomenon and extreme seasonal temperature anomalies over the continental United States. Relationships are examined for El Niño and La Niña conditions for each of the four standard seasons. Two complementary approaches are used. In the first approach, seasonal temperature anomalies are ranked from coldest to warmest over a 100-yr climate division dataset. Mean Southern Oscillation index (SOI) values are then computed for times preceding or concurrent with extreme seasonal temperature anomalies to define regions where relationships between the SOI and seasonal temperature extremes are statistically significant. In the second approach, seasonal extremes in the SOI, which are generally related to El Niño or La Niña, are first identified, and then the numbers of extreme temperature seasons occurring in association with these events are determined. Comparison of the observed number of extreme seasons with the climatologically expected values provides quantitative estimates of how ENSO alters the conditional probability, or risk, of large seasonal temperature anomalies in a given region. The results show that the greatest geographical coverage of statistically significant relationships between ENSO and seasonal temperature extremes occurs in winter and spring, especially with the SOI leading by one season. Certain well-recognized relationships for seasonal temperature anomalies are also confirmed for extreme seasons, such as the association of El Niño conditions with very warm winters over the Pacific Northwest and very cold winters along the Gulf Coast. Other less-discussed relationships also appear, including possible nonlinearities in relationships between El Niño and La Niña events and extremes in autumn temperatures. Some relationships show evidence of secular changes, especially in summer. In some regions and times of year, El Niño and La Niña conditions substantially alter the probabilities of very warm or very cold seasons. For example, over Texas, El Niño conditions in winter almost triple the risk that the subsequent spring will be very cold, while significantly reducing the risk of a very warm spring. In the same region, wintertime La Niña conditions double the risk that the following spring will be very warm, while significantly reducing the likelihood of a very cold spring. Therefore, given the proper ENSO phase, skillful forecasts of regional risks of seasonal temperature extremes appear feasible.