El Nino and La Nina are caused by the sea-surface temperatures in the tropical part of the Pacific Ocean interacting with the atmosphere.
The cycle of the water temperature changing from warm to cold usually occurs every three to four years.
This is known as oscillation.
In the Northern Hemisphere, the peak season for El Nino and La Nina is during the winter.
Characteristics of an El Nino Cycle...
A large pool of warm water is located in the central and western part of the Pacific Ocean.
Atmospheric conditions can increase the temperature of the surface water.
As a result, this pool of warm water enlarges and shifts eastward to cover the tropics.
El Nino also causes a decrease in the eastward flow of the trade winds.
On the eastern side of the Pacific, parts of Australia, New Zealand, the Philippines and Indonesia experience drought conditions.
The effect can spread as far as India.
The opposite happens in North and South America where the general climatic conditions are wetter and warmer temperatures.
Characteristics of a La Nina Cycle...
The large pool of warm water in the Pacific shrinks because the eastbound trade winds strengthen and carry the colder surface water from the east to the west.
This reduces the overall temperature of the surface water.
On the western side of the Pacific, rainfall is generally heavier than normal but on the eastern side, the rainfall is reduced with cooler temperatures.