El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña. The Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied by high air surface pressure in the tropical western Pacific and La Niña with low air surface pressure there. The two periods last several months each and typically occur every few years with varying intensity per period. The two phases relate to the Walker circulation, which was discovered by Gilbert Walker during the early twentieth century. The Walker circulation is caused by the pressure gradient force that results from a high-pressure area over the eastern Pacific Ocean, and a low-pressure system over Indonesia. Weakening or reversal of the Walker circulation (which includes the trade winds) decreases or eliminates the upwelling of cold deep sea water, thus creating El Niño by causing the ocean surface to reach above average temperatures. An especially strong Walker circulation causes La Niña, resulting in cooler ocean temperatures due to increased upwelling. Mechanisms that cause the oscillation remain under study. The extremes of this climate pattern's oscillations cause extreme weather (such as floods and droughts) in many regions of the world. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected. Outline The El Niño–Southern Oscillation is a single climate phenomenon that periodically fluctuates between three phases: Neutral, La Niña or El Niño. La Niña and El Niño are opposite phases which require certain changes to take place in both the ocean and the atmosphere before an event is declared. Normally the northward flowing Humboldt Current brings relatively cold water from the Southern Ocean northwards along South America's west coast to the tropics, where it is enhanced by up-welling taking place along the coast of Peru. Along the equator, trade winds cause the ocean currents in the eastern Pacific to draw water from the deeper ocean to the surface, thus cooling the ocean surface. Under the influence of the equatorial trade winds, this cold water flows westwards along the equator where it is slowly heated by the sun. As a direct result sea surface temperatures in the western Pacific are generally