Which of the following are the results of El Nino? 1. Reduction in the amount of planktons which further reduces the number of fish in the sea. 2. Irregularities in the evaporation of sea water. 3. Distortion of equatorial atmospheric circulation. Select the correct answer using the code given below :

1. General Atmospheric Circulation and Planetary Winds (basic)

Welcome to our first step in understanding how our planet breathes! To understand complex ocean-atmosphere oscillations like El Niño later on, we must first master the General Atmospheric Circulation. Think of this as the Earth’s giant air-conditioning system. Because the Sun heats the Equator more than the Poles, the atmosphere stays in constant motion to redistribute this heat, creating what we call Planetary Winds or permanent winds.

The pattern of these winds isn't random; it is dictated by five primary factors: (i) latitudinal variation of heating, (ii) the emergence of pressure belts, (iii) the migration of these belts with the sun, (iv) the distribution of land and water, and (v) the Earth’s rotation FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9, p.79. This circulation is critical because it doesn't just move air—it also sets the ocean water circulation in motion, which ultimately dictates the Earth's climate Physical Geography by PMF IAS, Pressure Systems and Wind System, p.316.

How does this look in practice? At the Inter-Tropical Convergence Zone (ITCZ) near the equator, intense heat causes air to rise through convection, creating a low-pressure zone. This air travels high into the atmosphere (up to 14 km) and moves toward the poles, eventually sinking at around 30° N and S to form the Sub-tropical High Pressure belts FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9, p.80. From these high-pressure zones, air flows back toward the equator. However, because the Earth rotates, these winds don't blow in a straight line.

This is where the Coriolis Force comes in. Due to Earth's rotation, winds are deflected to their right in the Northern Hemisphere and to their left in the Southern Hemisphere Certificate Physical and Human Geography, GC Leong, Climate, p.139. This deflection transforms simple north-south air movements into the Trade Winds and Westerlies that dominate our planet's weather systems. Understanding these "permanent" paths is the key to spotting when the system goes "off-script" during major climate events.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9: Atmospheric Circulation and Weather Systems, p.79-80; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.316; Certificate Physical and Human Geography, GC Leong, Climate, p.139

2. The Walker Circulation: The Pacific's Normal State (basic)

To understand the complex oscillations of our oceans, we must first master the Walker Circulation—the "default" or normal atmospheric state of the tropical Pacific Ocean. Think of it as a giant, horizontal loop of air that acts like a conveyor belt. This circulation is powered by the temperature difference between the Eastern and Western Pacific. Under normal conditions, the Western Pacific (near Indonesia and Australia) is a "warm pool" of water, while the Eastern Pacific (near Peru and Ecuador) is significantly cooler due to the upwelling of deep, nutrient-rich water Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.412.

This temperature gradient creates a pressure gradient. Over the cool waters of the East, the air is dense and sinks, creating a High-Pressure system. Conversely, over the warm waters of the West, the air heats up, becomes buoyant, and rises, creating a Low-Pressure system. Nature abhors a vacuum, so the surface air rushes from the High-Pressure East to the Low-Pressure West. These are our Easterly Trade Winds (the North and South Equatorial Currents). As these winds travel across the ocean, they push the sun-warmed surface water toward the West, literally "piling up" the ocean level by a few centimeters Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.491.

Feature	Western Pacific (Indonesia/Australia)	Eastern Pacific (South America)
Sea Surface Temp	Warm (Warm Pool)	Cold (Upwelling)
Air Pressure	Low Pressure (Rising Air)	High Pressure (Sinking Air)
Weather	Cloudy, Heavy Rainfall	Clear Skies, Dry/Arid

Finally, the loop closes high in the atmosphere. The air that rose over Indonesia travels eastward at high altitudes and eventually sinks over the South American coast to complete the cell. This descending branch of the Walker Cell in the East explains why places like the Atacama Desert are so dry Geography of India, Climate of India, p.13. Because the surface winds are constantly pushing water away from the South American coast, the thermocline (the layer separating warm surface water from cold deep water) stays very shallow in the East, allowing cold, fish-rich waters to rise to the surface Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.412.

Sources: Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.412; Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.491; Geography of India, Climate of India, p.13

3. Ocean Upwelling and Marine Productivity (intermediate)

In the vast expanse of our oceans, the most vibrant life isn't found just anywhere; it is concentrated in specific "hotspots" driven by a process called Ocean Upwelling. To understand this, we must look at the ocean as a layered cake. The top layer (photic zone) has plenty of sunlight but is often nutrient-poor because microscopic plants, called phytoplankton, quickly consume available minerals. The deep, dark layers are nutrient-rich because they act as a graveyard where organic matter decomposes, releasing nitrates and phosphates back into the water. Geography of India, Chapter 4, p.9

Upwelling occurs when steady winds (like the Trade Winds) push warm surface waters away from a coastline. Due to the Earth's rotation (the Coriolis effect), this water moves at an angle, creating a "void" at the surface. Nature abhors a vacuum, so cold, dense, and nutrient-saturated water from the deep rises to fill the gap. This brings the "fertilizer" of the deep into the sunlight, triggering massive blooms of diatoms and other phytoplankton, which form the base of the marine food web. Physical Geography by PMF IAS, Climatic Regions, p.465. Conversely, downwelling occurs when surface waters pile up and sink, carrying oxygen down but preventing nutrients from reaching the surface, resulting in low biological productivity. Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.498

Feature	Upwelling Zones	Downwelling Zones
Water Movement	Deep water rises to the surface.	Surface water sinks to the deep.
Temperature	Anomalously Cold surface water.	Relatively Warm surface water.
Nutrient Status	High (Nitrates, Phosphates, CO₂).	Low (Nutrients consumed/not replenished).
Marine Life	High (Major global fishing grounds).	Low (Biological "deserts").

A classic example is the Peru (Humboldt) Current off the South American coast. Normally, this region is a powerhouse of productivity because of consistent upwelling, supporting one of the world's largest anchovy fisheries. Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.490. However, as we will see in later steps, when this upwelling is suppressed—such as during an El Niño event—the entire marine ecosystem can collapse as the nutrient supply line is cut off.

Sources: Geography of India, Chapter 4: Climate of India, p.9; Physical Geography by PMF IAS, Climatic Regions, p.465; Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.498; Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.490

4. Southern Oscillation: The Pressure See-Saw (intermediate)

While El Niño describes the oceanic warming of the Pacific, the Southern Oscillation is its atmospheric counterpart. It is a massive, inter-annual 'see-saw' of atmospheric pressure across the tropical Indo-Pacific region. Essentially, when sea-level pressure is high in the eastern Pacific, it tends to be low in the western Pacific, and vice versa Physical Geography by PMF IAS, Chapter 29, p.413. This pressure gradient is crucial because air always flows from high to low pressure; thus, the intensity of this see-saw determines the strength of the trade winds and the health of the Indian Monsoon NCERT Class XI Fundamentals of Physical Geography, Chapter 9, p.78. To quantify this phenomenon, scientists use the Southern Oscillation Index (SOI). This index measures the pressure difference between Tahiti (French Polynesia) in the Central/Eastern Pacific and Darwin in Northern Australia (Western Pacific) Physical Geography by PMF IAS, Chapter 29, p.415.

Phase	SOI Value (Tahiti minus Darwin)	Atmospheric Condition	Impact on India/Australia
Positive Phase	Positive (High Tahiti P / Low Darwin P)	Stronger Trade Winds; Robust Walker Circulation	Good rainfall and strong Monsoon
Negative Phase	Negative (Low Tahiti P / High Darwin P)	Weakened Trade Winds; Collapsed Walker Circulation	Drought conditions and weak Monsoon

In a normal or positive SOI year, the Walker Circulation is vigorous: air rises over the low-pressure zone of Indonesia/Australia (warm, moist air) and descends over the high-pressure zone of the South American coast (cool, dry air) Geography of India by Majid Husain, Chapter 4, p.11. When this pressure see-saw tips the other way (Negative SOI), the atmospheric circulation is distorted, directly leading to the climatic anomalies we associate with El Niño.

Sources: Physical Geography by PMF IAS, Chapter 29: El Nino, La Nina & El Nino Modoki, p.413-415; Geography of India by Majid Husain, Chapter 4: Climate of India, p.11; Fundamentals of Physical Geography (NCERT Class XI), Chapter 9: Atmospheric Circulation and Weather Systems, p.78

5. Connected Concepts: La Niña and Global Weather Extremes (intermediate)

Concept: Connected Concepts: La Niña and Global Weather Extremes

6. Teleconnections: Indian Monsoon and Indian Ocean Dipole (IOD) (exam-level)

While the Pacific Ocean has ENSO, the Indian Ocean has its own "see-saw" of sea surface temperatures (SST) known as the Indian Ocean Dipole (IOD). Often called the "Indian Niño," the IOD is a climate phenomenon defined by the difference in SST between two poles: a western pole in the Arabian Sea (near Africa) and an eastern pole in the eastern Indian Ocean (south of Indonesia). This dipole usually begins developing in April, peaks in October, and can fundamentally alter the moisture supply to the Indian subcontinent Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415.

The IOD operates in two primary phases that dictate rainfall patterns across the region. During a Positive IOD, the western Indian Ocean becomes anomalously warm while the eastern side near Indonesia cools down. This setup shifts the rising limb of the atmospheric circulation toward the west, bringing heavy rains to East Africa and India. Conversely, a Negative IOD sees warmer waters move toward Indonesia, leading to increased rainfall there but causing drier conditions and potential drought in India Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.416.

Feature	Positive IOD (+ve)	Negative IOD (-ve)
Western Pole (Arabian Sea)	Warmer than normal	Cooler than normal
Eastern Pole (Indonesia)	Cooler than normal	Warmer than normal
Impact on Indian Monsoon	Favorable (Extra Rainfall)	Unfavorable (Reduced Rainfall)

The most fascinating aspect of the IOD is its teleconnection with ENSO. Historically, El Niño years are synonymous with monsoon failure in India. However, a strong Positive IOD can act as a "buffer" or a savior. Even during a powerful El Niño, if the IOD is strongly positive, it can negate the drying effects and provide India with normal or even surplus rain. A classic example is 1997—despite a "Godzilla" El Niño in the Pacific, India did not face a drought because a massive Positive IOD provided the necessary moisture Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. We also recognize an atmospheric component to this called EQUINOO (Equatorial Indian Ocean Oscillation), which involves the shifting of pressure cells between the Bay of Bengal and the Arabian Sea Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415.

Sources: Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Chapter 29: El Nino, La Nina & El Nino Modoki, p.415-416

7. The Mechanism and Consequences of El Niño (exam-level)

To understand El Niño, we must first look at the Pacific Ocean's 'normal' state. Usually, strong trade winds blow from east to west, piling up warm surface water near Indonesia and Australia. This allows cold, nutrient-rich water to rise from the depths along the South American coast—a process called upwelling. However, during an El Niño event, these trade winds weaken or even reverse. This allows the Equatorial Counter Current to carry a massive 'warm pool' of water eastward, where it replaces the cool Peruvian Current and blankets the coast of Ecuador and Peru Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p. 413. This eastward shift of warm water effectively holds the cold Peru Current in check Geography of India, Climate of India, p. 11. This physical shift in water temperature has immediate atmospheric consequences. The warming of the eastern Pacific causes the thermocline (the boundary layer between warm surface water and cold deep water) to drop significantly. This deeper thermocline cuts off the upwelling of cold, nutrient-dense water. Without these nutrients, the primary producers of the ocean—phytoplankton—cannot survive, leading to a collapse of the marine food web. The famous Peruvian anchoveta (small fish) disappear, causing economic distress for local fisheries and starvation for marine birds Geography of India, Climate of India, p. 9. Beyond the ocean, El Niño disrupts the global Walker Circulation. The rising air (low pressure) that usually sits over the Western Pacific moves toward the central and eastern Pacific. This leads to a reversal of typical weather patterns:

Feature	Normal Conditions	El Niño Conditions
Western Pacific (Australia/Indonesia)	Warm, wet, and low pressure.	Cooler water, high pressure, and drought.
Eastern Pacific (South America)	Cold water, high pressure, dry.	Warm water, low pressure, and heavy rains/flooding.
Upwelling	Strong; nutrient-rich.	Suppressed; nutrient-poor.

Sources: Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.413; Geography of India, Climate of India, p.9, 11

8. Solving the Original PYQ (exam-level)

To solve this question effectively, you must synthesize your foundational knowledge of oceanography and climatology. As you have studied in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT), El Niño is not merely a change in water temperature but a massive disruption of the ocean-atmosphere coupling. When the warm counter-current appears off the coast of Peru, it suppresses the upwelling of cold, nutrient-rich water. This directly confirms Statement 1: without these nutrients, phytoplankton (plankton) populations crash, leading to a collapse of the marine food web. Simultaneously, as explained in Geography of India by Majid Husain, the anomalous warming of Sea Surface Temperatures (SST) creates irregularities in evaporation, which fundamentally alters global precipitation patterns, making Statement 2 equally valid.

The final piece of the puzzle involves the Southern Oscillation, which is the atmospheric component of the ENSO phenomenon. Because the ocean and atmosphere are intrinsically linked, the warming of the eastern Pacific causes a see-saw shift in surface air pressure. This leads to a distortion of equatorial atmospheric circulation, specifically weakening or reversing the Walker Circulation. As detailed in Physical Geography by PMF IAS, these three statements represent the biological, physical, and atmospheric dimensions of the same event. Therefore, the correct answer is (D) 1, 2 and 3.

A common UPSC trap is to present these impacts as isolated events. Students often focus solely on the climatic results (like droughts or floods) and overlook the biological impact on fisheries, leading them to incorrectly choose Option (B). Another trap is the use of "Only" in options (A), (B), and (C), which aims to make you doubt the holistic nature of El Niño. Always remember that in Earth Systems Science, a change in one sphere (the hydrosphere) almost always triggers a cascade of effects in others (the biosphere and atmosphere).