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ALL ABOUT ENSO AND MJO

3rd September, 2021 Prelims

ENSO Cycle

  • El Niño–Southern Oscillation (ENSO) is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean.
  • Every three to seven years, the surface waters across tropical Pacific Ocean warm or cool by 1°C to 3°C, compared to normal.
  • The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña.
  • Thus, El Niño and La Niña are opposite phases of what is known as the El Niño-Southern Oscillation (ENSO) cycle.
  • These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate.

Note: El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.

Concepts we need to know before we delve deeper into the ENSO mechanism

A thermocline is the transition layer between the warmer water at the surface and the cooler deep water below.

The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region (between 30°N and 30°S latitudes).

Convection is the process generally associated with warm rising air and the formation of cloud.

ENSO Mechanism

The 'Neutral phase'

  • Under ‘normal’ conditions, we have a situation referred to as a Neutral phase.
  • At first, Easterly Trade Winds (blowing from east to west) push the warm surface water away from South America towards tropical South-East Asia.
  • Conversely, off the west coast of South America the depth of the warm equatorial top layer of the ocean has reduced, and the thermocline is nearer the ocean surface. This means that cold, nutrient-rich water upwells into the upper parts of the ocean here.
  • Around tropical South-East Asia, warm surface water evaporates.
  • Thiscauses increased rainfall around Indonesia and neighbouring lands such as the Top end of Australia.
  • In contrast, over the eastern tropical Pacific and off the coast of Peru, dry air accompanied with cloud dissipation sinks, resulting in much drier weather there.

 Note: The pattern of air rising in the west and falling in the east with westward moving air at the surface is referred to as the Walker Circulation.

El Niño phase

  • The Trade Winds loses most of their strength in an El Niño phase, and may even reverse into a westerly wind (west to east) direction.
  • As a result, warm surface water from tropical South-East Asiaflows towards the coast of South America.
  • Off the coast of South America, the thermocline sinks. The cold deep water no longer upwells into the surface layer of the ocean.
  • The source of nutrients is cut off, which has a subsequent effect on fishing stocks off South America.
  • Nearby, over the eastern tropical Pacific, warm surface water evaporates. This results in more rainfall than usual there.
  • Conversely, on the other side of the Pacific, Indonesia and neighbouring countries are drier than usual during an El Niño.

La Niña Phase

  • In this pattern, trade winds blow warm water at the ocean’s surface from South America to Indonesia.
  • As the warm water moves west, cold water from the deep rises to the surface near the coast of South America.
  • Since, La Niña blows all of this warm water to the western Pacific places like Indonesia and Australia get much more convectionalrain than usual.
  • However, the cold water in the eastern Pacific causes less rain clouds to form there. So, places like the southwestern United States are much drier than usual.
  • La Nina can also lead to more lightning activity within the Gulf of Mexico and along the Gulf Coast.
  • And more tropical cyclones—which include hurricanes—forming in the deep tropics (near the islands in the Caribbean, for example).
  • In a nutshell, La Nina causes drought in the South American countries of Peru and Ecuador, heavy floods in Australia, high temperatures in Western Pacific, Indian Ocean, off the Somalian coast and a comparatively better monsoon rains in India.

Impact of La Niña

ENSO and India

El Nino and La Nina impact India greatly.
El Nino during winter causes warm conditions over the Indian subcontinent and during summer, it leads to dry conditions and deficient monsoon. Whereas, La Nina results in better than normal monsoons in India.
India experienced deficient rainfall during El Nino years 2002 and 2009 whereas monsoon was normal during El Nino years 1994 and 1997. It means that in about 50 per cent of the year with El Nino during summer, India experienced droughts during monsoon.
Crops like Paddy, Maize, Groundnut, Guar, Castor, Tur, Moong and Bajra suffer due to El Nino.


No cyclones in India in October 2020

  • October to December period is among the favourable months for the development of cyclones in the Bay of Bengal and the Arabian Sea.
  • This year, however, October passed without witnessing a cyclonic storm.
  • This is due to the weak La Nina conditions along the equatorial Pacific Ocean.
  • Cooler than normal sea surface temperatures over this region—termed as La Nina— has been prevailing since August this year.
  • Also, because Madden Julian Oscillation (MJO) was positioned in a favourable phase, the low-pressure systems intensified maximum only up to a deep depression.
  • Also, there was the high wind shear noted between the different atmospheric levels.
  • The high vertical wind shear—— created due to huge wind speed difference observed between higher and lowers atmospheric levels prevented the low-pressure systems and depression from strengthening into a cyclone. For cyclones to develop a low vertical wind shear is a must. (**Refer to previous article to know about wind shear).

 

What is MJO?

  • The Madden-Julian Oscillation (MJO) is the major fluctuation in tropical weather on weekly to monthly timescales.
  • The MJO can be characterized as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days.
  • The MJO consists of two parts, or phases: one is the enhanced rainfall (or convective) phase and the other is the suppressed rainfall phase.
  • Strong MJO activity often dissects the planet into halves: one half within the enhanced convective phase and the other half in the suppressed convective phase.
  • These two phases produce opposite changes in clouds and rainfall and this entire dipole (i.e., having two main opposing centers of action) propagates eastward.

 

Digging deeper

  • In the enhanced convective phase, winds at the surface converge, and air is pushed up throughout the atmosphere.
  • At the top of the atmosphere, the winds diverge. Such rising air motion in the atmosphere tends to increase condensation and rainfall.
  • In the suppressed convective phase, winds converge at the top of the atmosphere, forcing air to sink and, later, to diverge at the surface
  • As air sinks from high altitudes, it warms and dries, which suppresses rainfall.
  • It is this entire dipole structure, illustrated in the Figure above that moves west to east with time in the Tropics, causing more cloudiness, rainfall, and even storminess in the enhanced convective phase, and more sunshine and dryness in the suppressed convective phase.

How does MJO affect Indian Monsoon?

  • The journey of MJO goes through eight phases, as the oscillation propagates from the Indian Ocean through the Pacific Ocean and into the Western Hemisphere.
  • When it is over the Indian Ocean during the Monsoon season, it brings good rainfall over the Indian subcontinent.
  • On the other hand, when it witnesses a longer cycle and stays over the Pacific Ocean, MJO brings bad news for the Indian Monsoon.
  • Basically, it is linked with enhanced and suppressed rainfall activity in the tropics and is very important for the Indian monsoonal rainfall.
  • If the periodicity of MJO is nearly 30 days then it brings good rainfall during the Monsoon season.
  • If it is above 40 days in Pacific then MJO doesn't give good showers and could even lead to a dry Monsoon.
  • Shorter the cycle of MJO, better the Indian Monsoon. Simply because, it then visits the Indian Ocean more often during the four month-long period.
  • Presence of MJO over the Pacific Ocean along with an El Nino is detrimental for Monsoon rains.

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