One of the most prominent aspects of our weather and climate is its variability. This variability ranges over many time and space scales, from small-scale phenomena such as wind gusts, localized thunderstorms and tornadoes, to larger-scale features such as fronts and storms, to even more prolonged features such as droughts and floods, and to fluctuations occurring on multi-seasonal, multi-year, multi-decade and even multi-century time scales. Some examples of these longer time-scale fluctuations include an abnormally hot and dry summer, an abnormally cold and snowy winter, a consecutive series of abnormally mild or exceptionally severe winters, and even a mild winter followed by a severe winter. In general, the longer time-scale phenomena are often associated with changes in the atmospheric circulation that encompass areas far larger than a particular affected region. At times, these persistent circulation features occur simultaneously over vast, and seemingly unrelated, parts of the hemisphere, or even the globe, and result in abnormal weather, temperature and rainfall patterns throughout the world. During the past several decades, scientists have discovered that important aspects of this interannual variability in global weather patterns are linked to a global-scale, naturally occurring phenomenon known as the El Niño/ Southern Oscillation (ENSO) cycle. The terms El Niño and La Niña represent opposite extremes of the ENSO cycle.