The El Nino Southern Oscillation ("ENSO") is a period change in the ocean currents and atmospheric conditions in the South Pacific and Indian Ocean, which impacts the climate of the Americas, South East Asia and as far afield as Africa.
During normal conditions, an area of high atmospheric pressure exists over South America relative to lower atmospheric pressure over Northern Australia and South East Asia. This results in the East to West blowing 'trade winds' caused by air moving across the Pacific from the areas of higher pressure in the East to lower pressures in the West. Higher pressure brings drier weather for the American sub-continent, whilst lower pressure results in higher levels of rainfall over the Western Pacific. During an ENSO event, this pressure difference weakens, resulting in lower Trade Winds and changes to the weather conditions across the Pacific. Higher levels of rainfall is experienced in the Americas, whilst drier conditions prevail across the South-East Asian sub-continent, Australia, India and parts of Africa. Sea level also changes in response to the changing pressure differential. An ENSO event is also associated with a weakening of the Humbolt current, a upwelling of cooler, nutrient rich water along the Pacific South American coast from Antartica. This weakening is caused by currents of warmer, less saline water typically present around equatorial regions migrating South. ENSO events occur once every three to four years, with particularly strong occurrences every twenty years or so.
At the end of an ENSO event, the adjustment back to typical conditions often involves an over-compensation, with stronger normal conditions (e.g. trade winds) experienced for a period of time. This phenomenon has been termed 'La Nina'.
Source: NOAA (https://www.climate.gov/enso)
