Climatic Phenomena affecting the Indian Monsoon:

1. ENSO (El Niño–Southern Oscillation):

• ENSO originates in the equatorial Pacific Ocean and strongly influences the Pacific Walker Circulation (PWC).
  1. El Niño years: The eastern and central Pacific waters warm up. This weakens the Walker circulation and reduces the flow of moisture-laden winds from the Mascarene Highs towards India. As a result, the monsoon becomes weak or deficient, often leading to droughts.
  2. La Niña years: The opposite happens; Pacific waters cool, the Walker circulation strengthens, and strong cross-equatorial winds from the Mascarene Highs bring more moisture into India. Monsoon rainfall is usually above normal, sometimes leading to floods.
• Key point: ENSO acts like a “remote controller” sitting in the Pacific but directly influencing the strength of the Indian monsoon winds.

2. Indian Ocean Dipole (IOD):

• The Indian Ocean itself has its own seesaw pattern of sea surface temperatures.
  • Positive IOD: Western Indian Ocean (near Africa) is warmer, and eastern Indian Ocean (near Indonesia) is cooler. This strengthens cross-equatorial winds from the Mascarene Highs, feeding more moisture into India. Result: Good rainfall, strong monsoon, even if El Niño is present.
  • Negative IOD: Western Indian Ocean is cooler, eastern side is warmer. This pulls away monsoon winds from India and weakens the rainfall.
• Key point: IOD is a “local driver” sitting in the Indian Ocean, which can either amplify or cancel out ENSO’s effect.

3. ENSO–IOD Interaction:

• Monsoon outcome is not decided by ENSO or IOD alone, but by how they combine:
  • El Niño + Positive IOD: IOD can cancel El Niño’s bad effect (1997 monsoon was near normal).
  • El Niño + Negative IOD: Worst-case combo, often brings severe droughts.
  • La Niña + Positive IOD: Both reinforce each other, leading to very heavy rainfall and flood risk.
  • ENSO Neutral + Positive/Negative IOD: IOD becomes the deciding factor.

The Big Picture:

• Pacific Walker Circulation is the “conveyor belt” moving rising and sinking air across the Pacific and Indian Oceans.
  • When it shifts east (El Niño): India gets less rain.
  • When it strengthens west (La Niña): India gets more rain.
• IOD modifies this system locally in the Indian Ocean-  it can either buffer or worsen ENSO’s impact.
• The Mascarene Highs act as the main “engine room” for cross-equatorial winds, but the Walker circulation and IOD decide how strong that engine runs.
• Madden–Julian Oscillation (MJO): An eastward-moving pulse of cloud and rainfall that travels around the equator every 30–60 days.
  • MJO decides the intra-seasonal variability: when it rains heavily (active phase) and when dry breaks occur.

1. The duration of the monsoon decreases from southern India to northern India.

2. The amount of annual rainfall in the northern plains of India decreases from east to west.

Which of the statements given above is/are correct?

Options: (a) 1 Only (b) 2 Only (c) Both 1 and 2* (d) Neither 1 nor 2

[UPSC 2017] With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?

1. The IOD phenomenon is characterized by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.

2. An IOD phenomenon can influence an El Nino’s impact on the monsoon.

Select the correct answer using the code given below:

(a) 1 only  (b) 2 only * (c) Both 1 and 2  (d) Neither 1 nor 2