El Nino & La Nina

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El Nino

El Nino phenomenon is attributed to the abnormal movement of warm water in the eastern Pacific.

In normal years, ocean temperatures of the Amercian coast are constantly low with the continual supplement of cold water coming from the polar regions.

On the other side of the Pacific, like regions to the north of Australia, Indonesia and New Guinea where lies with the warmer sea water areas. The formation of cloud layers co-acted with the convection of the pool of hot water bring with monsoon rain there.

Trade winds blowing from the America towards the western coast of the Pacific make warm water stay in that area. Sometimes, winds can even surge up the sea level resulting that sea level over the western coast is one feet higher than the eastern coast.

In El Nino years, trade winds die down, which allows the wave of warm water to be displaced back across the Pacific towards Peru. Since warm water passes over the ocean top, cold water over the American coast cannot reach to the top. On the other hand, as warm water accumulates along the Central Pacific and the eastern coast, this favours storm clouds to develop there other than in western coast.

When El Nino occurs, very often the seasonal atmospheric circulation is disrupted on a global scale and many places would experience anomalous weather. For example, northern Australia and Indonesia would have droughts, western South America would be wetter than usual, and the Gulf of Mexico and the southeast coast of the United States would have more storms. Reports suggest that some of these conditions are currently being observed in connection with the El Nino episode now developing in the equatorial Pacific - below average rainfall in parts of Australia and Indonesia, localized flooding in southern America as well as above normal rainfall over the islands of the central tropical Pacific.

Besides, convective activity over the western Pacific and the South China Sea is reduced and the favoured location for the formation of tropical cyclones tends to shift to the central Pacific where the warm waters gather. In Hong Kong, we will have unusually wet springs.

La Nina

While El Nino refers to the anomalous and prolonged warming of the eastern and central equatorial Pacific, La Nina is the extreme opposite of El Nino, that is, the anomalous and prolonged cooling of the eastern and central equatorial Pacific.

The weather impacts associated with La Nino are usually not as pronounced as those associated with El Nino. Most often, relatively wet conditions is observed over much of Indonesia and parts of Australia, and relatively dry conditions in southeastern South America. For Hong Kong, climatology shows that in La Nina years rainfall tends to be near normal, and the average annual number of tropical cyclones affecting Hong Kong is 7, close to the normal number of 6.

Southern Oscillation

Another phenomenon closely related to El Nino is the Southern Oscillation. In essence this is the phenomenon whereby whenever pressure is high over the equatorial Pacific it will be low over the Indian Ocean and vice versa. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI) which is commonly defined as the pressure at Darwin, Australia, subtracted from that at Tahiti. Negative values of SOI are usually associated with El Nino events and this is why El Nino events are also known as El Nino-Southern Oscillation (ENSO) events. Concurrent with the warm conditions in the equatorial Pacific, SOI values have been negative.

During the occurrence of La Nina, the Southern Oscillation Index (SOI) tends to take on positive values as opposed to negative values during the occurrence of El Nino.

SOI archive can be found under this webpage, http://www.bom.gov.au/climate/current/soihtm1.shtml

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