El-Nino and La Nina

La Nina and El Nino are the two extremes of the El Nino Southern Oscillation Cycle (ENSO). These phenomena involve variations in the temperature of water in the central and eastern Pacific Ocean. They affect global climate patterns, including the Indian monsoon. During La Nina, there is an accumulation of colder water than normal along the coast of Peru due to the strengthening of trade winds. Generally, La Nina has a positive effect on the Indian monsoon; that is, there is higher than normal rainfall in a La Nina year.

El Nino Southern Oscillation Cycle (ENSO): It refers to the wide ranging temperature variation in the sea surface temperature in the central and eastern tropical Pacific ocean. It is a recurring phenomenon that impacts the Indian climate as well as global climate patterns. El Nino and La Nina are two extremes of the ENSO cycle.

• El Nino:
  • It means ‘the boy’ or ‘christ child’ in spanish. When the sea surface temperature of the central and eastern Pacific ocean becomes warmer than the normal condition then the El Nino conditions are said to be emerging.
  • In a normal year, trade winds push warm water from the Americas to Asia. However, in an El Nino condition, these trade winds weaken, and there is a reversal of trade winds that push warm water towards the Americas. Hence, there is an accumulation of warm water along the coast of Central and South America.
  • There is an atmospheric Low Pressure condition in the eastern Pacific Ocean and atmospheric High Pressure condition over the western Pacific Ocean.

Impact of El Nino:

• Since there are low pressure conditions over the equatorial eastern pacific there is increased rainfall and flooding in the US Gulf coast and Southeastern US. Whereas there are High Pressure conditions over the equatorial western pacific which is associated with drought like conditions in Australia and the Indonesian archipelago
• The El Nino phenomenon has an impact on the Indian monsoon as well. In an El Nino year, in India, extreme heat conditions and suppressed or below normal rainfall during the monsoon are witnessed. For example, in 2014, which was an El Nino year, rainfall was 12% less than normal during the June to September months.
• El Nino also suppresses upwelling of deeper water off the coast of South America. In a normal year, cold water from the deep ocean comes to the surface to replace the warm water that is being transported towards Asia. This cold water brings phytoplankton from the deep ocean to the surface and that is consumed by fish. However, in an El Nino year upwelling reduces so does the fish stock.

• La Nina:
  • In Spanish, it literally means ‘little girl’. It is also known as ‘El Viejo’ or the cold event.
  • It is the other extreme of the ENSO cycle. In this event, trade winds are stronger than in a normal year.
  • Because of this, an even larger amount of warm water is transported from the west coast of the western equatorial Americas towards Asia. This results in the accumulation of cold water off the western coast of the Americas
  • There is greater upwelling of nutrient rich water during the La Nina event.
  • There is an atmospheric High Pressure condition in the eastern pacific ocean and atmospheric Low Pressure condition over the western pacific ocean.

• The La Nina event is associated with dry conditions in the Southern USA, droughts in Peru and Ecuador, and heavy rainfall in Australia.
• La Nina has a positive effect on the Indian monsoon. In the La Nina year, there is higher than normal rainfall in India.
• It has been witnessed that there is an increased formation of cyclones and hurricanes in the Bay of Bengal during the La Nina event.
• Due to the significantly higher upwelling of nutrient rich water off the coast of the Americas, there are healthy fish stocks available in the Pacific Ocean.

El Nino Southern Oscillation (ENSO) cycle has a major impact not only on India but also on the global weather pattern. The effects of El Nino and La Nina on the Indian monsoon are significant. The monsoon has wide ranging effects on the Indian economy particularly in the agriculture sector. Hence, it is imperative to study and monitor major global circulations, such as ENSO, so that mitigation strategies can be developed to minimise the hazardous effects.