14 Oct

El Nino and Monsoon

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Contents

• El-Nino Southern Oscillation
• The La Nina Phase
• Reason behind Oscillation
• How is El-Nino situation predicted?
• Impact of El Nino in India

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Introduction

Understanding El-Nino Southern Oscillation (ENSO)

• EL-Nino Southern oscillation, also known as ENSO is a periodic fluctuation in the sea surface temperature (El-Nino) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial pacific ocean. It is a recuring climatic pattern which has impact on temperature and precipitation across the globe.
• This scientific phenomenon occurs in 3 phases: El-Nino Phase, the La Nina phase, and the Neutral Phase.
  • The El-Nino Phase
    •  During El-Nino, the trade winds weaken or even reverse:
      • Instead of blowing form the east (South America) to West (Indonesia),
        they could turn into westerlies. As the wind blows from West to East, they cause the masses of warm water to move into the central and eastern equatorial pacific ocean. This leads to increased rainfall along the Western Latin America, the Caribbean and the US Gulf Coast, while depriving SE Asia, Australia and India of rainfall.
      • The strength of trade wind depletes and it is not able to take warm water to the Australian Coast.
      •  This reduces the pressure difference between Eastern Pacific and Western Pacific.
      • This phase is characterized by severe drought in Indonesia, Australia (Western Pacific) and heavy rainfalls in the Eastern Pacific (i.e. the west coast of the Equatorial South America)
  • The neutral phase (Normal Condition) of ENSO involves sea surface temperature in the tropical pacific ocean that are closer to average.
    •  The trade winds blow easterly across the surface near the equator.
    •  Warm water accumulates in the Western Pacific (East Coast of Australia) creating low pressure there and high pressure in the Eastern Pacific (near the Peruvian Coast).
  • This phase is characterized by heavy rains on the East Coast of Australia and lack of rainfall on the Peruvian coast.

• The La-Nina Phase

  • It is opposing phase to El-Nino.
  • It basically refers to abnormal cooling of the central and eastern pacific ocean waters off the coast of Ecuador and Peru. Such cooling (SSTs falling 0.5 degree Celsius or more below a 30-year average for at least five successive three months period) is a result of strong trade winds blowing west along the equator (strong easterly trade winds), taking warm water from South America towards Asia. The warming of western equatorial pacific, then, leads to increased evaporation and concentrated cloud formation activity around that region, whose affect percolate to India as well. It also leads to decreased rainfall in tropical pacific.
  • For e.g. the bountiful rainfall during 2019-22 has been significantly attributed to La Nina.
    • The latest La-Nina event was one of the longest ever, lasting from July September 2020 to Dec-Feb 2022-23. And it brought copious rainfall to India.
    • This was also the case with two previous strong La Ninas in 2007-08 and 2010-11, followed by one moderate episode in 2011-12.

Discovery of El-Nino and La-Nina

• El Nino Phenomenon was first noticed by the scientists in the 1920s, though local population in Peru and Ecuador were aware of the periodic warming much earlier.
• The La Nina phenomenon, on the other hand, was discovered only in the 1980s.

Reasons behind Oscillation (i.e. El-Nino and La-Nina)

• Not fully understood.
• But the two components of ENSO – Sea Surface Temperature (SST) and Atmospheric Pressure are strongly related.
  • The strengthening and weakening of the trade winds is a function of changes in the pressure gradient of the atmosphere over the tropical Pacific. Ironically, the warming of the sea surface works to decrease the atmospheric pressure above it by transferring more heat to the atmosphere and making it more buoyant. So, in summary, the pressure gradient affects the sea surface temperatures, and the sea surface temperatures affect the pressure gradient.

How is El-Nino situation predicted?

A. The Ocean Part of ENSO is measured by Oceanic Nino Index (ONI).

• Monitoring of ENSO primarily focuses on Sea Surface Temperature (SST) anomalies in the 4 geographical regions of the equatorial pacific.
• The Nino 3.4 region refers to central and equatorial pacific while Nino 3 is the adjoining far-east pacific.
•  During an El-Nino, the Nino 3.4 region gets relatively warmer (by atleast 0.5 degree Celsius).
• These are averaged over five, three-month sessions on a trot to arrive at the Oceanic
  Nino Index (ONI).
• During a La-Nina it gets relatively cooler (by at least 0.5 degree Celsius).

Thermal expansion of warming water in the eastern part of the basin measurably raises sea level in these regions, and this change in sea level can be measured by satellite sensors. Thereby, variations in sea level are good indicators of the presence of an El-Nino.

B. The Atmospheric part is monitored through Southern Oscillation Index, or SOI.

• Southern Oscillation Index (SOI) is calculated on the basis of the atmospheric pressure
  
  

              El Nino Years

  difference between Tahiti (Southern Pacific Ocean) and Darwin (Australia).

• Negative phase of SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. It corresponds to warm waters across the eastern tropical pacific typical of El-Nino.
• Positive phase of SOI coincides with cold ocean waters across the eastern tropical pacific typical of La Nina episodes.

ENSO and Climate

• In general, El Nino has warming effect on the planet, while La Nina tends to cool it down. The warmest years in a decade are usually
  the El Nino years.
• The warmest ever year on record, 2016, was part of one of the longest and strongest El Nino episodes ever, dubbed the Godzilla El Nino.

Accurate Prediction of El-Nino provide valuable information for managing its impacts on vulnerable regions:
A. Agriculture and Food Security: Based on the expected changes in rainfall due to El-Nino farmers can adjust their crop choices, planting schedule and irrigation options.
B. Water Resource Management: Early prediction of El-Nino enable water resource managers to optimize reservoir storage and allocations.
C. Disaster Preparedness: Steps can be taken to deal with disasters like drought or heatwaves in vulnerable regions.

Impact of El-Nino in India

• El-Nino has been generally known to suppress monsoon rainfall in India.
  • Practically, all drought years in India since Independence – marked by large declines in food grains production or monsoon failures – have witnessed El-Nino events of varying intensity. The sole exception was 1966-67, where drought took place without El-Nino.
  • Please note that this doesn’t mean the all El-Nino years have been drought years.

Conclusion1

• Though El-Nino and La-Nina are naturally occurring climate events, there impacts in recent years have been exacerbated by extreme weather events.
• With a good disaster management strategy, India needs to remain prepared for these adverse weather phenomena.