Later, the term was scientifically used to refer to the abnormal warming of sea surface temperatures in the eastern Pacific Ocean, thousands of kilometers from Peru and Ecuador. When this happens, the temperature of large areas of seawater is three to six degrees higher than it is all year round. The rise in the water temperature of the vast waters in Pacific Ocean have changed traditional equatorial currents and southeast trade winds, causing global abnormal climate.
As the meteorological science is highly developed nowadays, it has been known to all that the central part of the Pacific Ocean is the main source of the change of summer climate in the northern hemisphere.
In most cases, there is a cold current from Peru going northwards along the west side of South America in the Pacific Ocean, part of which turns into equatorial currents moving westwards. At this point, the monsoon blowing westwards along the equatorial waters leads the warm currents to stimulate in the western part of the Pacific Ocean, while the lower cold water upwells in the east warming the western part of Pacific Ocean, south of the Philippines, and north of New Guinea. This sea area is called "the equatorial warm pool”, the temperature of the east section of the same latitude s relatively low.
There is also a temperature difference in the atmosphere over the two seas. The temperature is lower in the east and the pressure is higher. While in the west the temperature is higher and the pressure is lower.
In this way, in the middle of the Pacific, there is a sea level where cold air flows to the west, and higher hot air flows to the east, which forms the southeast trade wind near it. But sometimes the pressure difference is lower than the average for previous years and sometimes it can be higher. This phenomenon of atmospheric varieties is known as the southern oscillation.
In the 1960s, meteorologists found that El Nino was closely related to the southern oscillation, which was the trigger of El Nino when the pressure difference decreased.
After the appearance of El Nino, the monsoon of Pacific Ocean from east to west is weakened greatly by the warm currents with higher temperature, making significant changes in the atmospheric circulation with great influence on the climates of different countries along the Pacific coast. Droughts appear in humid areas whilre floods strike dry areas. When the pressure difference increases, the temperature of sea drops abnormally and here comes a phenomenon known as La Nina.
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