El Niño, La Niña, and Indian Monsoon

3.1 El Niño (PYQ 2023 — Q38)

El Niño (Spanish: "the child" — named for its occurrence around Christmas) refers to the anomalous warming of the central and eastern equatorial Pacific Ocean (surface temperatures 0.5–2°C above normal) that occurs irregularly every 3–7 years.

Mechanism:

• Normally, Walker Circulation moves warm surface water westward (from South America toward Indonesia/Australia) due to strong trade winds; cold upwelling occurs off Peru
• During El Niño: Trade winds weaken or reverse; warm water spreads eastward to central Pacific; Walker Circulation weakens/reverses; warm water "pools" off South America

Impact on India:

• Weakens the temperature gradient between Indian subcontinent and Indian Ocean (which drives the monsoon)
• Results in below-normal monsoon rainfall — drought risk, especially in central and peninsular India
• Indian droughts are strongly correlated with El Niño: 1965, 1972, 1982, 1987, 2002, 2009, 2014, 2023 El Niño years saw reduced monsoon in India
• However, correlation is not perfect — some El Niño years (2015 was weak, 2023 moderate) still had near-normal monsoon in India due to positive Indian Ocean Dipole (IOD) offsetting the effect

3.2 La Niña

La Niña is the opposite — anomalous cooling of central/eastern Pacific:

• Stronger Walker Circulation, stronger trade winds, warmer Bay of Bengal
• Generally strengthens Indian SW monsoon → excess rainfall, flood risk
• Some years of above-normal monsoon (1994, 1998, 2019, 2020) correspond to La Niña conditions

3.3 Indian Ocean Dipole (IOD)

IOD is the temperature difference between the western and eastern Indian Ocean:

• Positive IOD: Western Indian Ocean warmer than eastern → enhances Indian monsoon (counteracts El Niño effect)
• Negative IOD: Weakens Indian monsoon
• 2019 saw El Niño but monsoon was normal due to strong positive IOD

3.4 Western Disturbances (WDs) (PYQ 2023 — Q38)

Western Disturbances are extra-tropical cyclonic weather systems that originate over the Mediterranean Sea, Caspian Sea, or Atlantic Ocean and travel eastward along the Subtropical Westerly Jet Stream at 25°N–35°N latitude.

Key characteristics:

• Occur mainly October–March (winter months); frequency peaks December–February
• Bring winter rain and snowfall to J&K, Himachal Pradesh, Uttarakhand, Punjab, Haryana, and western UP
• The associated snowfall at high altitudes (Himadri) stores water for release as glacial meltwater in summer — feeding perennial rivers
• Sometimes cause unseasonal rainfall in NW India during pre-monsoon season (April–May) — critical for rabi crops (wheat)

Impact on agriculture:

• Beneficial: Winter wheat in Punjab/Haryana needs 2–3 WD events for adequate soil moisture; apple orchards in Himachal need winter snowfall
• Damaging: Late WDs in March can damage standing rabi crops; intense WDs cause blizzards, landslides in Himalayan regions

Climate change impact on WDs: Studies (IIT Delhi, 2023) show WDs are intensifying — bringing more rainfall per event, more erratic timing, and affecting monsoon variability as the jet stream shifts.

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