1. What is El Nino?
2. What is it like during the normal non-El Nino conditions?
3. What is La Nina and what does it have to do with El Nino?

El Nino is a natural weather phenomenon that disrupts the climate of the tropical Pacific Ocean. However, it has great consequences on the weather around the world, such as more rain in the southern United States of America and in Peru, and droughts in Indonesia, Australia and much of Asia.

Sea surface temperature anomaly chart of the Pacific Ocean on December 1997. Redder areas represent warmer than normal sea surface temperatures.

Credit: NOAA

El Nino is characterised by the weakening or reversal of trade winds blowing across the tropical Pacific Ocean. During an El Nino, these trade winds blow across the surface of the water from east, off the coast of Indonesia and Australia, to the west, to the coast of Peru. This blowing of the trade winds across the surface of the ocean brings warm surface water, which is heated by the sun, to the western coasts of South America.

Regions of low pressure are formed over the warm surface waters, causing moisture-rich air from the high-pressure regions over the other parts of the ocean to rush into the low-pressure region over the western coast off South America (if you remember, there is a body of warm surface water off the coasts of South America). This moisture rich air brings rains to North and South America, but brings drought to Asia.

Usually, the heating of the surface of the ocean waters off the western coast of South America during an El Nino begins in summer and gradually builds till the end of the year, when the ocean waters are the warmest. Then, the El Nino will be over by the next summer.

During non-El Nino conditions, the trade winds blow across the equatorial Pacific Ocean from the west, off the coast of South America, eastwards, towards Indonesia and Australia. This causes the warm surface waters of the ocean to be held back against the coasts of Indonesia and Australia, resulting in rains and monsoon seasons across Asia and Australia, and droughts in South America.

On the tail of El Nino comes the opposite weather phenomenon La Nina. As the opposite of El Nino, La Nina brings unusually cold waters to the western coast of South America and causes a lot of rain in Asia. La Nina also brings a severe and soggy winter to the northern states of the United States of America, while leaving the Southeast rather mild and dry and the Southwest with high temperatures and low precipitation.

La Nina is caused by colder-than-average sea surface temperatures in the same area as the warmer-than-average temperatures of El Nino. During a La Nina year, temperatures along a section of the equator drop fifteen degrees in a month (May-June 1998).

La Nina is known for its unaccountability. El Nino is now able to be predicted and is expected about every two to seven years, while La Nina doesn't always follow an El Nino or is especially varying in the strength. For example, the intense El Nino of 1982-83 brought a weak La Nina, while the modest 1986-87 El Nino was followed by a strong La Nina, and the moderate El Nino of 1991-95 was followed by a weak La Nina. Even the conditions within the La Nina year vary, with one month mild and the next extreme.

Wherever you are, get ready for a La Nina because it looks like one is on the way. The effects will probably be the opposite of what you felt during El Nino, but you never know! Be sure to tell us how La Nina is affecting you (or how El Nino affected you) by joining chats, discussions, or posting a story in "Interactive."