For short-term climate prediction, which one of the following events, detected in the last decade, is associated with occasional weak monsoon rains in the India subcontinent?

1. Mechanisms of the Indian Monsoon: Thermal and ITCZ Concepts (basic)

To understand the Indian Monsoon, we must first look at it through the lens of Differential Heating, a concept first proposed by the astronomer Edmond Halley in 1686. At its simplest level, the monsoon is often described as a large-scale sea breeze. During the summer, the vast landmass of Asia (specifically the Indian subcontinent) heats up much faster than the surrounding Indian Ocean. This intense heating creates a Thermal Low Pressure zone over North-West India and Pakistan. To fill this void, moisture-laden winds blow from the high-pressure areas over the cooler ocean toward the land, bringing the much-awaited rains Geography of India, Majid Husain, Climate of India, p.1.

However, the thermal concept alone is insufficient because it treats the earth as a stationary object and ignores the complex dance of the atmosphere. Modern meteorology relies on the Dynamic Concept, introduced by H. Flohn in 1951. This theory focuses on the Inter-Tropical Convergence Zone (ITCZ)—a low-pressure belt near the equator where the Northern Hemisphere's Northeast Trade Winds and the Southern Hemisphere's Southeast Trade Winds meet and rise Geography of India, Majid Husain, Climate of India, p.3.

In the summer, as the sun moves toward the Tropic of Cancer, the ITCZ also shifts northward, reaching as far as 20°N-25°N latitude over the Gangetic Plain. This shifted ITCZ is known as the Monsoon Trough. As the ITCZ moves north, the Southeast Trade Winds from the Southern Hemisphere are pulled across the equator. Because of the Coriolis Force (caused by Earth's rotation), these winds deflect to the right in the Northern Hemisphere, transforming into the moisture-heavy Southwest Monsoon winds INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.30.

Feature	Thermal Concept (Halley)	Dynamic Concept (Flohn)
Primary Driver	Temperature difference between land and sea.	Seasonal migration of planetary pressure belts (ITCZ).
Wind Origin	Local sea-to-land pressure gradient.	Southern Hemisphere Trade Winds crossing the Equator.
Role of Earth's Rotation	Largely ignored.	Central (Coriolis force deflects winds to the right).

Sources: Geography of India, Majid Husain, Climate of India, p.1-3; INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.30

2. Role of Jet Streams and Mascarene High (intermediate)

To understand the Indian Monsoon, we must look beyond the surface and into the upper atmosphere. The transition from winter to summer is marked by a dramatic shift in Jet Streams—high-altitude, fast-moving air currents. During winter, the Sub-tropical Westerly Jet Stream flows south of the Himalayas, acting like a lid that prevents the monsoon from entering. For the monsoon to 'burst,' this jet must withdraw to the north of the Tibetan Plateau NCERT Class XI, Climate, p.31. Once it clears, a new player emerges: the Tropical Easterly Jet (TEJ).

The TEJ is born from the intense heating of the Tibetan Plateau during summer. As the plateau heats up, air rises and spreads outwards in the upper atmosphere, eventually flowing southwards as an easterly jet roughly along the Kolkata-Bangalore axis Majid Husain, Climate of India, p.7. This jet acts as a conveyor belt, carrying air from the northern hemisphere across the equator. These jet streams are not just passive observers; they maintain the global heat balance by moving vast masses of air, which can trigger everything from droughts to floods PMF IAS, Jet streams, p.389.

The final piece of this atmospheric puzzle is the Mascarene High, a permanent high-pressure cell located in the southern Indian Ocean near the Mascarene Islands (near Madagascar). The Tropical Easterly Jet descends specifically over this region, 'pumping' more air into the cell and significantly intensifying its pressure Majid Husain, Climate of India, p.8. Because winds naturally move from High to Low pressure, a stronger Mascarene High acts as a powerful engine, pushing the moisture-laden South-West Monsoon winds with greater force toward the thermally induced low pressure over the Indian landmass.

Feature	Sub-tropical Westerly Jet	Tropical Easterly Jet (TEJ)
Season	Winter (dominates India)	Summer (dominates India)
Role in Monsoon	Must withdraw north for monsoon to start	Intensifies the Mascarene High to drive the monsoon

Sources: Geography of India (Majid Husain), Climate of India, p.7-8; INDIA PHYSICAL ENVIRONMENT (NCERT Class XI), Climate, p.31; Physical Geography by PMF IAS, Jet streams, p.389

3. Indian Ocean Dipole (IOD): The 'Indian Nino' (intermediate)

While the El Niño-Southern Oscillation (ENSO) is a global powerhouse, the Indian Ocean Dipole (IOD) is our very own local regulator of the monsoon. Often called the 'Indian Niño', the IOD refers to the difference in sea surface temperatures (SST) between two specific 'poles' in the Indian Ocean: a western pole in the Arabian Sea and an eastern pole in the eastern Indian Ocean, south of Indonesia Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. This phenomenon typically begins to develop around April and reaches its peak intensity in October.

The IOD operates in three phases, but for your UPSC preparation, the Positive (+IOD) and Negative (-IOD) phases are the most critical to understand. When the IOD is positive, the Arabian Sea becomes unusually warm while the waters near Indonesia cool down. This temperature gradient sets up a convection cell where warm air rises over the western Indian Ocean, leading to increased rainfall over India and East Africa, but causing droughts and bushfires in Australia. Conversely, a Negative IOD sees warmer waters shift toward Indonesia, suppressing rainfall over the Indian subcontinent Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.416.

Feature	Positive IOD (+IOD)	Negative IOD (-IOD)
SST Pattern	Western Indian Ocean is Warmer than Eastern.	Eastern Indian Ocean is Warmer than Western.
Impact on Monsoon	Benefits the Indian Monsoon (Higher rainfall).	Weakens the Indian Monsoon (Deficient rainfall).
Cyclones	More cyclones in the Arabian Sea.	More cyclones in the Bay of Bengal.

One of the most fascinating aspects of the IOD is its ability to moderate or amplify ENSO. You might recall that El Niño typically brings drought to India. However, if a strong Positive IOD occurs simultaneously, it can counteract El Niño's drying effect. A classic example is the year 1997: despite a powerful El Niño that should have caused a drought, India received normal rainfall because a very strong Positive IOD 'saved' the monsoon Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. This demonstrates that the IOD is a vital independent variable in our climate system.

Sources: Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415-416

4. Madden-Julian Oscillation (MJO): Intraseasonal Variation (exam-level)

Imagine the earth's tropical atmosphere as a giant, slow-moving wave of weather that circles the globe every 30 to 60 days. This is the Madden-Julian Oscillation (MJO). Unlike El Niño, which persists for many months or years (interannual), the MJO is an intraseasonal variation—meaning it happens within a single season. It is essentially a 'pulse' of clouds, rainfall, and winds that travels eastward along the equator, starting in the Indian Ocean and moving toward the Pacific Ocean.

The MJO consists of two distinct halves: an enhanced convective phase and a suppressed convective phase. In the enhanced phase, rising air leads to cloud formation and heavy precipitation. Conversely, in the suppressed phase, sinking air brings clear skies and dry conditions. This movement is a critical driver of the 'Active' and 'Break' spells of the Indian monsoon. When the enhanced phase of the MJO is positioned over the Indian Ocean, it provides the moisture and energy needed for a 'burst' in monsoon rainfall. Physical Geography by PMF IAS, Tropical Cyclones, p.381.

The MJO doesn't just bring rain; it acts as a massive energy distributor. It can trigger 'twin cyclones'—where tropical storms form simultaneously on both sides of the equator (Northern and Southern Hemispheres). This happens because the MJO promotes deep convection and creates surface westerly wind bursts that help fuel these storms. For instance, in May 2022, the MJO helped generate the twin cyclones Asani and Karim. Physical Geography by PMF IAS, Tropical Cyclones, p.379. Beyond the tropics, it even influences the jet streams, which can lead to extreme heat or cold air outbreaks in higher latitudes. Physical Geography by PMF IAS, Tropical Cyclones, p.381.

Feature	Madden-Julian Oscillation (MJO)	ENSO (El Niño/La Niña)
Time Scale	Intraseasonal (30–60 days)	Interannual (2–7 years)
Movement	Eastward moving 'pulse'	Stationary/Shifting pressure centers
Primary Impact	Active/Break spells of monsoon	Overall seasonal drought or surplus

Sources: Physical Geography by PMF IAS, Tropical Cyclones, p.379; Physical Geography by PMF IAS, Tropical Cyclones, p.381

5. The Walker Circulation and Normal Pacific Conditions (basic)

To understand the global climate, we must look at the Pacific Ocean as a massive heat engine. The Walker Circulation is an atmospheric loop that spans the equatorial Pacific, driven by the difference in sea surface temperatures between the East and the West. In a "Normal Year," this circulation acts like a conveyor belt, moving air and water in a predictable cycle that keeps the tropical weather balanced.

Under these normal conditions, the Western Pacific (near Indonesia and Northern Australia) is home to a massive pool of warm water. This heat causes the air to expand and rise, creating a Low-Pressure system and intense convective storms (thunderstorms) Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.412. Conversely, the Eastern Pacific (near the coast of Peru and Ecuador) has much cooler water. Cold air is denser and heavier, so it sinks, creating a High-Pressure system and dry, clear skies.

This pressure difference (High in the East, Low in the West) drives the Trade Winds, which blow strongly from East to West across the ocean surface. As these winds push the warm surface water toward Asia, they trigger a vital process called upwelling off the South American coast. Cold, nutrient-rich water from the deep ocean rises to replace the water pushed away, supporting massive fish populations Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.412. The boundary between this warm surface water and the deep cold water is called the thermocline, which sits deep in the West but very shallow in the East.

Feature	Eastern Pacific (Peru)	Western Pacific (Indonesia)
Water Temp	Cold (Upwelling)	Warm (Pool)
Air Pressure	High Pressure (Sinking Air)	Low Pressure (Rising Air)
Weather	Dry and Calm	Heavy Rainfall/Storms

When these normal conditions are especially strong, we call it La Niña. During La Niña, the South Pacific subtropical high becomes even stronger, the Trade Winds become more forceful, and even more warm water is dragged westward, often leading to enhanced monsoon rains in India Geography of India, Majid Husain, Climate of India, p.11.

Sources: Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.412; Geography of India, Climate of India, p.11

6. Understanding ENSO: El Nino and Southern Oscillation (intermediate)

To understand the **El Niño–Southern Oscillation (ENSO)**, we must first look at the 'normal' state of the Pacific Ocean. Typically, strong **trade winds** blow from east to west, pushing warm surface water toward Indonesia and Australia. This creates a 'warm pool' in the western Pacific, leading to low pressure and heavy rainfall, while the eastern Pacific (near Peru) remains cool due to the **upwelling** of deep, cold water. This atmospheric loop—rising air in the west and sinking air in the east—is known as the **Walker Circulation**. As noted in Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p. 413, El Niño represents a breakdown of this normal cycle.

El Niño is the oceanic phase where these trade winds weaken, allowing the warm water to 'slosh' back toward the coast of South America. Simultaneously, we see the Southern Oscillation, which is the atmospheric counterpart. This involves a see-saw shift in air pressure between the central Pacific (Tahiti) and the western Pacific (Darwin, Australia). When pressure rises over Darwin and falls over Tahiti, it is a 'negative' **Southern Oscillation Index (SOI)**, signaling an El Niño event. This pressure shift effectively weakens or even reverses the Walker Cell, shifting the zone of rising air (and rain) away from the Indian Ocean toward the central Pacific Geography of India by Majid Husain, Climate of India, p. 11.

Conversely, La Niña is often described as the 'extreme normal' phase. During La Niña, the trade winds become exceptionally strong, pushing even more warm water to the west and causing intense cooling in the eastern Pacific Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p. 417. For the Indian Monsoon, these phases act like a remote control: El Niño generally acts as a 'brake,' diverting moisture-laden winds away and often leading to droughts, while La Niña acts as an 'accelerator,' often resulting in above-average rainfall and occasional flooding.

Feature	El Niño (Warm Phase)	La Niña (Cold Phase)
Eastern Pacific SST	Unusually Warm	Unusually Cold
Trade Winds	Weak or Reversed	Extremely Strong
SOI (Tahiti - Darwin)	Negative (Low pressure at Tahiti)	Positive (High pressure at Tahiti)
Impact on Indian Monsoon	Weakened; high drought risk	Strengthened; high flood risk

Sources: Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.413, 415, 417; Geography of India by Majid Husain, Climate of India, p.11

7. ENSO Teleconnections and Indian Monsoon Failure (exam-level)

To understand why the Indian monsoon sometimes fails, we must look thousands of miles away at the Pacific Ocean. This 'action at a distance' is what meteorologists call a teleconnection. The most influential teleconnection is the El Niño–Southern Oscillation (ENSO), a coupled phenomenon where changes in sea surface temperatures (ocean) and air pressure (atmosphere) in the Pacific dictate global weather patterns. Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.413. In a 'normal' year, warm water and low pressure are concentrated over the Western Pacific (near Indonesia/Australia), creating a rising limb of air that fuels moisture-heavy winds toward India. However, during an El Niño phase, this warm pool and the corresponding low-pressure zone shift eastward toward the central and eastern Pacific (near Peru). Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415.

This eastward shift disrupts the Walker Circulation—the giant loop of air rising over the West Pacific and sinking elsewhere. When the rising limb moves east, the descending limb (high pressure) often settles over the Indian Ocean and the Indian subcontinent. High pressure acts like a lid, suppressing the upward movement of air needed for cloud formation and rain, which leads to monsoon failure or weak rainfall. Historically, most of India's major droughts have coincided with strong El Niño years. Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.414. However, the relationship is inverse but not perfect. A classic example is 1997, a record-breaking El Niño year that did not result in a drought in India. This was because a positive Indian Ocean Dipole (IOD) acted as a counter-balance, providing enough moisture to offset the Pacific's negativity. Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.417.

Phase	Walker Cell Rising Limb Position	Impact on Indian Monsoon
Normal / La Niña	Western Pacific (Indonesia/N. Australia)	Good to Heavy Rainfall
El Niño	Central/Eastern Pacific (near South America)	Weak Rainfall / Potential Drought

Sources: Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.413; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.414; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.417

8. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of Walker Circulation and the atmospheric coupling of the Southern Oscillation, this question demonstrates how those concepts intersect. The Indian Summer Monsoon (ISM) is a massive thermal engine, but its strength is heavily influenced by global teleconnections. Specifically, the El Niño–Southern Oscillation (ENSO) acts as a primary driver of interannual variability. When you see abnormal warming in the eastern Pacific (El Niño), the rising limb of the Walker cell shifts eastward, creating high pressure and sinking air over the Western Pacific and the Indian subcontinent—conditions that inherently suppress monsoon rainfall. As noted in Physical Geography by PMF IAS, many of India's most prominent droughts have historically coincided with these El Niño phases.

To arrive at (C) El Nino and Southern Oscillations as the correct answer, you must think of the ocean and atmosphere as a coupled system. While the question asks for a predictor of "weak" monsoon rains, it points toward the specific decade-long observations of Pacific SST anomalies. Option (A) La Niña is a common trap; it is the "cool" phase that typically brings above-average rainfall to India, the opposite of what the question requires. Option (B) Movement of Jet Streams is a permanent seasonal driver of the monsoon's onset and withdrawal, rather than a predictor of occasional interannual weakness. Finally, Option (D) Greenhouse effects represents a long-term climatic trend, whereas the question specifically targets the short-term variability caused by coupled oscillations. By isolating the coupled ocean-atmosphere mechanism, you can see why ENSO remains the gold standard for short-term monsoon forecasting.