Sea surface temperature data showing El Nino and La Nina conditions in the tropical eastern Pacific Ocean. El Nino is a period of warmer than usual sea temperatures, shown by the red region extending westwards into the ocean. The converse, La Nina, is a period of cooler than normal temperatures. These two conditions form the extremes of the El Nino-Southern Oscillation, a long and irregular-period phenomenon that has profound effects on the weather in the region and further afield. This animation shows monthly temperature patterns in the tropical Pacific during the exceptionally strong 1997-98 El Nino (warm phase) and the following years, as the climate pattern reversed to its cool, La Nina, phase.