El Nino UPSC – La Nina, ENSO and Walker Circulation

El Nino UPSC Preparation

El Nino UPSC preparation must be done by keeping in mind that the concept is a bit complicated. So while doing El Nino UPSC preparation the concept must be dealt with in totality. For this, other related topics such as ENSO, Walker circulation and La Nina must also be taken care of in the El Nino UPSC studies. Questions are regularly asked on El Nino in UPSC CSE prelims as well as mains. At the end of this article some El Nino UPSC PYQs for prelims and mains are given. So do take note of that. With these basic things in mind about El Nino UPSC preparation, lets now understand the concept in a simple and systematic manner.

What is ENSO

El Nino UPSC
3 Phases of ENSO

ENSO stands for El Nino Southern Oscillations. ENSO happens to be one of the most important climate phenomena on Earth as it is instrumental in changing the global atmospheric circulation, which in turn, impacts temperature and precipitation across the globe.  Moreover ENSO gives an indication of arrival of many seasons in advance with far reaching impacts on weather and climate. Though it is a single climatic phenomenon, but it has basically 3 phases, 2 of which are opposing phases namely El Nino and La Nina. The third phase is the neutral phase. The word ‘oscillations’ in ENSO means that there is a repeating pattern of the 3 phases. Let us probe these 3 phases individually.

Neutral Phase

El Nino UPSC
Neutral Phase

For a proper understanding of the ENSO cycle, we must first understand how the Pacific is like in its neutral state. As the state of the ocean and atmosphere depend on each other, the equatorial Pacific climate acts as a “coupled system”. Thus if the conditions of the ocean change, the atmosphere responds accordingly and vice versa. The primary indicators of this are pressure and temperature. Lets see how:

Air Pressure

During neutral years on an average, atmospheric pressure is low in the warmer western tropical Pacific ocean region i.e. close to eastern Australia and relatively higher in the cooler central/eastern tropical Pacific ocean region near Peru. Air has a natural tendency to move from areas of high pressure to low pressure. Thus this difference in pressure moves the equatorial air, known as the trade winds, from the coast of South America toward the western Pacific Ocean.

Ocean Temperatures

During normal times the sun heats water at the surface of the ocean, but the easterly trade winds push surface water by friction from east to west in the Pacific ocean. When warm water is moved from its original location, cold water from deep ocean moves up to replace the shifted surface water in a process referred to as upwelling.  During neutral conditions, waters in the eastern Pacific are upwelled along the equator and the coast of South America.

Since the upwelled equatorial and coastal waters near the Andean coast originate deep below the surface, they are cold and rich with nutrients. When the surface water arrives in the western Pacific, land mass of Australia and Papua New Guinea restrict its further movement. As a result the water “piles up”, making sea level approximately half a meter higher around Indonesia than it is off the coast of Peru and Ecuador. As the water accumulates and stagnates, it continues to warm. The warm water eventually evaporates, travels eastwards, condenses and eventually bring rains on the Peruvian coast. This phenomenon of neutral phase is known as Walker Circulation.

Summary of Neutral Phase

  • In this phase the Central Pacific Ocean is warm. 
  • Trade winds are those winds that blows in the tropical region from eastern side to the western side they are also known as tropical easterlies. 
  • The equator receives great amount of sun rays and that’s what warms the Pacific Ocean. 
  • The trade winds push the warm ocean water towards the Asian side. Since the trade winds blow from east to west, the western Pacific Ocean gets warmed up. 
  • The region around New Zealand, Australia and Indonesia is called the western Pacific pool. 
  • Here the ocean temperature is warm. 
  • The warm ocean current affects the surrounding atmosphere by increasing the temperature as well as the moisture content. 
  • We know that warm air rises high into the atmosphere through the convection process and that’s how clouds are formed and then it rains. 
  • The warm air then travels east towards eastern Pacific Ocean. 
  • The region near South America especially countries like Ecuador and Peru, the warmer air when it goes up it reaches the end of troposphere. 
  • The top of the troposphere is cold. When warm air meets cool air it slowly loses its moisture content and the air becomes dry. 
  • The dry air travels towards the eastern Pacific side and comes down over the Peruvian coastal region making the region cold.

What is meant by Walker Circulation?

Also known as the Walker cell, it is circulation of air caused by as a result of contrasting temperatures in along eastern and western margins of pacific ocean in the southern hemisphere. The Walker circulation happens in the latitudinal direction. But there is a considerable influence of the Hadley cell along the longitudinal direction in the tropical region. Changes in the Walker Circulation with time occur in tune with changes in surface temperature.

What changes during El Niño years?

El Nino UPSC
El Nino Phase2 Loops Created

During El Nino episodes, the walker circulation gets disturbed as the tropical easterlies or the trade winds get weakened. El Nino is a Spanish word meaning little boy. Since this phenomenon was first observed in the 17th century during Christmas time so this name was given signifying baby Christ.

  • In the neutral phase we saw that the trade winds played an important role in pushing the warm ocean current towards the western Pacific. 
  • Now in this phase the trade winds are weak. Well there are few months in a year when the trade winds are week. 
  • When the trade winds are weak, the warm ocean current do not get any kind of push. Therefore the warm pool of ocean water at the western Pacific slowly moves towards the central and eastern side of the Pacific Ocean. 
  • This is where the oscillation term comes in. Here warm ocean current replaces the cold ocean current that existed in the central and eastern Pacific. When we say replace it means that the cold water is dense and it settles down in deep ocean and warm water goes up and takes over the surface of the ocean.
  • When this warm ocean current moves, everything that is associated with it like the convection process and the formation of rain clouds also moves.
  • Along with this warm ocean current the Walker circulation that we saw in the neutral phase  now starts breaking into two parts (notice the loop in the diagram). 
  • As a result the ocean temperature near Australia is cool and there is no rain. 
  • Though the inland parts of Australia witness a severe drought condition, but on the other hand near the Peruvian coast the warm pool of ocean current brings heavy rains and  floods to the American continent. 

What is La Niña?

La Nina UPSC

La Niña is an enhanced version of the neutral phase. When air pressure is higher-than-average in Tahiti and lower-than-average in Australia, the easterly trade winds blow more intensely than usual. The colder-than-normal ocean water extends across the eastern and central equatorial Pacific, and the winds continue to push water westward, once again increasing the sea level around Indonesia. The water in the western Pacific warms up, which then increases precipitation in the region. 

  • La Nina this is an accelerated form of neutral phase. In this phase, the trade winds are strong since trade winds blow from east to west hence it pushes the warm ocean current from the eastern Pacific toward stay west in Pacific. 
  • This cold water is dense and it settles down in deep ocean so that means that the temperature of the ocean surface is warm. 
  • If the trade wind pushes the warm surface ocean current towards the western Pacific, the cold water from the deep ocean immediately comes up at the surface. 
  • Thermocline is the rising path of water temperature and the rest of the process is same as that in the neutral phase western Pacific region of Australia, Indonesia and New Zealand gets heavy rains. 
  • Effect of La Nina is more on these countries than El Nino. 
  • So El Nino is a warm ocean current and La Nina is a cold ocean current. 
  • Remember that if El Nino is at the eastern Pacific then La Nina will be at the opposite region i.e. western Pacific and it oscillates back and forth.

Effects of El Nino UPSC Preparation

Effects of El Nino can be discussed in terms of : Changes in the climate, economic impact, health, social and ecological consequences. El Nino does not necessarily have negative consequences. It varies from region to region. Let us briefly discuss each one of them.

Changes in the climate

El Nino has a profound impact on the global climate as some parts of the world may witness intense storms on one hand while other other parts may have droughts. The climatic impact can be seen in terms of :

  • Alteration of global weather patterns – Forest fires, droughts, heavy rains
  • Temperature changes
  • Precipitation changes
  • High Intensity storms
  • Alterarion of ocean currents
  • Changes in ocean temperature.

Economical effects

Whenever there is a continuation of the El Nino condition for a few months, far reaching economic consequences are witnessed in various parts of the world. here one thing must be kept in mind that the economic consequences are not necessarily negative. Due to reduction in the intensity of trade winds and warming of ocean waters, the upwelling of nutrient rich cold water along the Peruvian coast is compromised. As a result there are less fishes in the ocean. This creates a negative impact on the local fishing industry.  

Apart from this, there is also an impact on the commodity prices and macroeconomy of different countries. As a result there is :

  • Reduction in agricultural 
  • Commodities reduction in agricultural output 
  • Unemployment 
  • Generalized inflation
  • Social Unrest in poor countries.

But on the other hand, due to warming of the atmosphere, winters in the USA get warmer resulting in reduction in energy consumption. This saving on electricity bills may increase the US GDP by 0.5%. On the other hand the GDP of Indonesia may decrease by 1%. Some countries like India, Japan, South Africa may face short lived low economic activity.  

Health and social consequences

There is a rise in epidemic diseases. Rise in mosquito population increases the incidence of Malaria, Dengue, Rift Valley Fever in countries like India, Brazil, Venezuela and Australia. Kawasaki disease rate increases in Japan and parts of the USA. 

Ecological impact

  • Rodent outbreaks 
  • Forest fires 
  • Reduction in bird and animal population 
  • Global mass bleaching
  • Species extinctions

El Nino UPSC Prelims Questions


1. A new type of El Nino Called El Nino Modoki appeared in the news. In this context consider the following statements:
1. Normal El Nino forms in the central part of the Pacific ocean while El Nino Modoki forms in the eastern part of the Pacific ocean.
2. Normal El Nino results in diminished hurricanes in the Atlantic ocean but El Nino Modoki results in a greater number of hurricanes with greater frequency.
Which of the following statements is/are correct?

a) 1 only
b) 2 only
c) Both 1 & 2
d) Neither 1 nor 2



Correct answer (b)

With reference to the Indian Ocean Dipole (IOD) and El Nino, consider the following statements:
1. While the El Nino causes the warming of the surface of Pacific Ocean, IOD causes warming of the Indian Ocean
2. While the El Nino always brings deficient rainfall in India, IOD always brings more rains in India
Which among the above statements is / are correct?
[A] Only 1
[B] Only 2
[C] Both 1 & 2
[D] Neither 1 nor 2

Correct answer (d)

El Nino UPSC Mains Questions

  1. What are the causes and effects of El-Nino. Discuss.(250 words)
  2. Discuss the effects of El-Nino?

See Also

   Climate of India – Classification and Weather

  World Climate Zones

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