El Niño and La Niña are opposite phases of what is known as the El Niño-Southern Oscillation (ENSO) cycle. The ENSO is a recurring climatic pattern involving temperature changes in the waters of the eastern and central tropical Pacific Ocean, and changes in the patterns of upper and lower level winds, sea level pressure, and tropical rainfall across the Pacific Basin. El Nino is often called the warm phase and La Nina is called the cold phase of ENSO. These deviations from the normal surface temperatures can have a large-scale impact on the global weather conditions and overall climate.
El Nino means ‘little boy’ or ‘Christ child’ in Spanish. The phenomenon was thus named because it was first recognised by South American fishermen in the early part of the 17th century. The events, i.e., warm waters in the Pacific Ocean, tended to occur in December, hence, the name was chosen.
El Nino refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific. It is associated with high pressure in the western Pacific. El Nino adversely impacts the Indian monsoons and hence, agriculture in India.
Let us see how El Nino affects India.
In a normal monsoon year (without El Nino), the pressure distribution is as follows:
- The coast of Peru in South America has a higher pressure than the region near northern Australia and South East Asia.
- The Indian Ocean is warmer than the adjoining oceans and so, has relatively lower pressure. Hence, moisture-laden winds move from near the western Pacific to the Indian Ocean.
- The pressure on the landmass of India is lower than on the Indian Ocean, and so, the moisture-laden winds move further from the ocean to the lands.
If this normal pressure distribution is affected for some reason, the monsoons are affected.
What happens because of El Nino?
The cool surface water off the Peruvian coast goes warm because of El Nino. When the water is warm, the normal trade winds get lost or reverse their direction. Hence, the flow of moisture-laden winds is directed towards the coast of Peru from the western Pacific (the region near northern Australia and South East Asia). This causes heavy rains in Peru during the El Nino years robbing the Indian subcontinent of its normal monsoon rains. The larger the temperature and pressure difference, the larger the rainfall shortage in India.
La Nina means ‘little girl’ in Spanish and is also known as El Viejo or ‘cold event’. Here, the water temperature in the Eastern Pacific gets colder than normal. As a result of this, there is a strong high pressure over the eastern equatorial Pacific. Now, there is low pressure in the Western Pacific and off Asia. La Nina causes drought in Peru and Ecuador, heavy floods in Australia, high temperatures in Western Pacific, Indian Ocean, off the Somalian coast and good monsoon rains in India. A La Nina is actually beneficial for the Indian monsoon.
Generally, El Nino and La Nina occur every 4 – 5 years. El Nino is more frequent than La Nina. Typically, the episodes last for nine to twelve months.
Since 1950, out of the 13 droughts that India faced, 10 have been during El Nino years and one in a La Nina year. This is because in general, an El Nino means lesser than average rains for India. Indian agriculture is heavily dependent on the monsoons and because of this, lesser rainfall during the monsoons generally translates to below-average crop yields.
El Nino and La Nina or the ENSO is a very important concept in climatology in Geography for the UPSC exam. Since the phenomenon affects India and global weather conditions, it is to be studied in detail for the IAS exam. Adding to the topic’s importance is the impact it can have on Indian agriculture. Therefore, students should study El Nino and La Nina thoroughly for the IAS. It is a part of General Studies I in the UPSC syllabus.
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|El Nino||General circulation of the atmosphere|
|Heating and Cooling of the Atmosphere||Tropical cyclones|