Effect of global Warming
El Niño-Southern Oscillation is a periodic change in the atmosphere and ocean of the tropical Pacific region. It is defined in the atmosphere by changes in the pressure difference between Tahiti and Darwin, Australia, and in the ocean by warming or cooling of surface waters of the tropical central and eastern Pacific Ocean. El Niño is the name given to its warm phase of the oscillation -- the period when water in that region is warmer than average. La Niña is the name given to the cold phase of the oscillation, or the period when the water in the tropical Eastern Pacific is colder than average. The oscillation has no well-defined period, but instead occurs every three to eight years. Mechanisms that sustain the El Niño-La Niña cycle remain a matter of research.