El Niño Southern Oscillation (ENSO) is a climate pattern that occurs across the tropical Pacific Ocean approximately every 3 to 7 years, characterized by the warming of eastern Pacific Ocean waters and low atmospheric pressure in that region. El Niño events can have widespread global impacts, including severe droughts in some areas like the Philippines as well as floods in other regions such as California during the strong 1997-1998 El Niño when abnormally high sea levels caused hundreds of millions in flood damage in the San Francisco Bay area.