Indians have become familiar with the “El Nino” term as it was all over the news in recent months leading up to the monsoon. I have tried to explain this phenomenon so that everyone can understand what it really is and what are its effects on the Indian subcontinent:-
The term “El Nino” is Spanish in origin which refers to the Christ child or also “the boy”. This is due to the fact that this phenomenon is noticed in the Pacific near South America usually around Christmas. The counterpart of El Nino which occurs after this phenomenon is called “La Nina”, Spanish for “the girl”.
Uptil now, there has been no definite scientific understanding on why this phenomenon occurs. Only the geographic events that occur during the phenomenon have been explained by scientists. Also, although the phenomenon is periodic in nature occurring once in 2-3 years, nothing can be predicted definitely on when it will happen next. It has been known to occur even after a gap of 5-6 years and as short as a gap of 1 year.
This effect was first discovered by Peruvian fishermen who found that after every 3-7 years a sudden change in weather conditions occurred and there would be an abnormal scarcity of fish in the Peruvian coastal waters in the year which El Nino occurred.
To explain El Nino, we will first have to learn what are the normal prevalent conditions in the Pacific Ocean most of the time.
The warmest part of the Pacific Ocean is the region near the equator. Due to the spinning of the earth (Coriolis Force), the prevailing winds flow from east to west. This pushes the warm waters westwards, towards Southeast Asia. This typical east-west circulation of zonal winds which push the Pacific waters towards the western Pacific i.e. eastern coast of Southeast Asia and results in the upwelling of cold water near the eastern Pacific i.e. western coast of South America is known as “Walker Circulation”. In fact, Walker circulation is a convective cell of air circulation., which is formed due to the development of pressure gradient from from east to west in the equatorial Pacific ocean.
After an interval of 2-3 years, this circulation is reversed i.e. pressure gradient becomes west to east. Thus, there are oscillations in the pressure gradient and air circulation after every 2-3 years. Walker called such oscillation as “Southern Oscillation”.
In normal conditions, high pressure develops on the sea surface the equatorial eastern Pacific oceanand the western coasts of South America due to subsidence of air from above and upwelling of cold ocean water. On the other hand, low pressure is formed in the equatorial western Pacific ocean due to rise of air from the warm sea surface, This pressure gradient from east to west generates east-west circulation of trade winds on the surface while there is reverse upper air circulation i.e. from west to east which completes the convective cell. This east-west air circulation drives the ocean water mass from the western coast of South America towards the west. This phenomenon facilitates further upwelling of cold sea water near the coasts of Peru and Ecuador resulting in further cooling of air, high air pressure, atmospheric stability and dry weather conditions.
Contrary to this, this normal east-west circulation becomes warm north-east trades in the equatorial west PAcific ocean where it, after being heated, rises upward, becomes unstable and causes precipitation. After rising to a certain height it turns eastward and descends in the equatorial eastern PAcific to complete the convective cell.
Therefore, tropical eastern and western Pacific is characterized by dry and wet weather conditions respectively.
During the El Nino phenomenon, all the above conditions are reversed for the eastern and western Pacific ocean waters. Phe reversal in pressure conditions facilitates the return of warm sea water which was driven from the coasts of South America westward, towards the tropical east Pacific. Consequently, low air pressure is formed in the south-east Pacific i.e. coasts of Peru and Ecuador, upwelling of cold sea water is stopped, warm air rises upward, becomes unstable and yields rainfall. It is evident that the general normal condition has got reversed. This event is called the El Nino phenomenon. The convective cell fromed during the El Nino is called the El-Nino Southern Oscillation (ENSO).
When the Hadley circulation is activated after the weakening of the normal Walker circulation during El Nino, the phenomenon is again reversed leading to the occurrence of normal conditions. This accompanying normal event is known as “La Nina”.
Impact of El Nino
1. Due to El Nino, the waters near the coast of South America are warmer than usual resulting in large scale fish kill as the upwelling cold water only contains the nutrients for the fish.
2. There is widespread precipitation in the western coasts of the Americas. Sometimes, floods also occur.
3. As there are dry conditions in Asia during El Nino, the intensity of monsoon is reduced resulting in food security problem in India.