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El Niño Southern Oscillation
El Niño Southern Oscillation (ENSO) is an irregularly periodical climate change caused by variations in sea surface temperatures over the tropical eastern Pacific Ocean, affecting much of the tropics and subtropics. The warming phase is known as ''El Niño'' and the cooling phase as ''La Niña''. The two variations are coupled: ''El Niño'' is accompanied with high, and ''La Niña'' with low air surface pressure in the tropical western Pacific. The two periods last several years each (typically three to four) and their effects vary in intensity.〔(【引用サイトリンク】title=El Niño, La Niña and the Southern Oscillation )〕
The two phases relate to the Walker circulation, 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 system over the eastern Pacific ocean, and a low pressure system over Indonesia. When the Walker circulation weakens or reverses, an ''El Niño'' results, causing the ocean surface to be warmer than average, as upwelling of cold water occurs less or not at all. An especially strong Walker circulation causes a ''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.
==Gilbert Walker==
(詳細はUniversity of Cambridge when he became director-general of observatories in India in 1904. While there, he studied the characteristics of the Indian Ocean monsoon, the failure of whose rains had brought severe famine to the country in 1899. Analyzing vast amounts of weather data from India and lands beyond, over the next fifteen years he published the first descriptions of the great seesaw oscillation of atmospheric pressure between the Indian and Pacific Ocean, and its correlation to temperature and rainfall patterns across much of the Earth's tropical regions, including India. He also worked with the Indian Meteorological Department especially in linking the monsoon with Southern Oscillation phenomenon. He was made a Companion of the Order of the Star of India in 1911.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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