Why is it in News?
The Indian Meteorological Department (IMD) has forecasted normal monsoon this year due to weakening of El Nino.
What is El Nino?
- El Nino is an unusual warming of sea surface temperatures in equatorial Pacific Ocean, off the coast of Peru and Ecuador in South America.
- El Nino is a Spanish word. The term El Nino basically means ‘the child’. This is due to the fact that this current starts to flow around Christmas and hence the name referring to baby Christ.
- The presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.
- During El Nino, the trade winds weaken or even reverse sometimes, thus warming the ocean. Further, the warmer waters lower air pressure across the central and eastern Pacific, weakening the pressure gradient that would normally force the trade winds from Tahiti toward Darwin (east to west).
- Note: (Tahiti & Darwin are measuring stations for atmospheric pressure).
- The weaker trade winds reduce the amount of surface water pushed to the west and the surface water stays warm and reinforces the weakened pressure gradient.
How is it Tracked?
There are a few acronyms which one comes across while tracking El Nino:
ENSO (El Nino Southern Oscillation) refers to the oscillation between the El Nino and the La Nina. ENSO shifts irregularly back and forth between El Nino and La Nina every two to seven years.
El Nino refers to the ocean component of ENSO. The Southern Oscillation part of the term ENSO refers to the atmospheric component: the shifting of atmospheric pressure between the central/eastern Pacific and the western Pacific. As the conditions of the ocean change, the atmosphere responds, and vice versa. The main indicators of these changes are pressure and temperature.
The Southern Oscillation Index:
The Southern Oscillation Index or SOI gives an indication of the development and intensity of El Nino or La Nina. Normally when the tropical eastern South Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as the Southern Oscillation or SO. The SOI is calculated on the basis of the atmospheric pressure differences between Tahiti (South Pacific Ocean) and Darwin (Australia), separated by 8,569 km. Sustained positive SOI values are indicative of La Nina conditions while negative values suggest El Nino conditions.
What happens during an El Nino?
- El Nino has been found to impact almost half the world triggering droughts in Australia, India, southern Africa and floods in Peru, Ecuador, the United States, the Gulf of Mexico, and the Colorado River basin.
- El-Nino has also been linked to drought and harvest failures on the African continent, devastating fires in the normally moist rainforests of the Indonesian archipelago, both drought and flood in Australia, damaging floods in the Americas, and unusually mild winters in Europe.
El Nino vis-à-vis India:
- It is said to impact the south west monsoon in India, which accounts for over 70 % of the country’s annual rainfall and irrigates over half of the crop land.
- EI-Nino is used in India for forecasting long range monsoon rainfall. In 1990-91, there was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of the country ranging from five to twelve days.
- Due to recent El-Nino the monsoon ended up with a deficit of 14 per cent in 2015, the second consecutive drought year, and only the third such instance in the last 100 years.
Indian Ocean Dipole (IOD):
- Indian Ocean mimics El Nino-La Nina in which the western and eastern basins of Indian Ocean warm up relative to each other every few years with associated impact on the monsoon. This phenomenon is referred to as Indian ocean Dipole (IOD).
- Warming up of the West Indian Ocean boosts a prevailing monsoon, and vice-versa.
- Indian monsoon depends upon not only El Nino La Nina but also IOD and other such ocean phenomena. As mentioned above, an IOD can either aggravate or weaken the impact of El Nino on Indian monsoon.
- If there is a positive IOD, it can bring good rains to India despite of an El Nino year. For example, positive IOD had facilitated normal or excess rainfall over India in 1983, 1994 and 1997 despite an El Nino in those years. Similarly, during years such as 1992, a negative IOD and El Nino had cooperatively produced deficient rainfall.
Source: G. C. Leong