During the Indian monsoon season

1. Foundations of the Indian Monsoon (basic)

To understand the Indian Monsoon, we must start with the word itself: Mausim, an Arabic term meaning 'season.' At its most fundamental level, the monsoon is a seasonal reversal of wind direction. This phenomenon was first explained scientifically in 1686 by the astronomer Edmund Halley through the Thermal Theory. He proposed that monsoons are essentially large-scale versions of land and sea breezes, driven by the differential heating of land and water Geography of India, Majid Husain, Chapter 4, p.1. Because land heats up and cools down much faster than the ocean, a massive pressure gradient is created between the hot Indian landmass and the relatively cooler Indian Ocean during summer INDIA PHYSICAL ENVIRONMENT, Geography Class XI, Chapter 4, p.29. During the summer months (June to September), the sun's rays fall vertically over the Tropic of Cancer. This intense solar radiation creates a zone of intense low pressure over North-West India and Central Asia. Meanwhile, the surrounding seas remain cooler, maintaining higher pressure. This pressure difference forces moisture-laden winds to rush from the high-pressure zones over the ocean toward the low-pressure center on land, bringing life-giving rain Certificate Physical and Human Geography, GC Leong, p.157. However, thermal heating alone doesn't explain everything—for instance, why does the heat peak in April, but the rains only 'burst' in June? The answer lies in the upper-level atmospheric circulation. For the monsoon to truly 'burst' over India, a dramatic shift must occur in the upper atmosphere. The Subtropical Westerly Jet Stream, which flows over the Indian plains during winter, must withdraw and shift north of the Himalayas. Once it moves, the Tropical Easterly Jet (TEJ) sets in over the southern peninsula (typically around 15°N latitude). This easterly jet is a unique summer feature and is considered a primary driver for the sudden onset or 'burst' of the monsoon rains Physical Geography by PMF IAS, Chapter 27, p.389.

Feature	Summer Monsoon (June-Sept)	Winter Monsoon (Dec-Feb)
Land Temperature	Extremely High (Low Pressure)	Cool/Cold (High Pressure)
Wind Direction	Sea to Land (South-West)	Land to Sea (North-East)
Upper Air Jet	Tropical Easterly Jet (TEJ)	Subtropical Westerly Jet (STWJ)

Sources: Geography of India ,Majid Husain, (McGrawHill 9th ed.), Chapter 4: Climate of India, p.1-2; Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), The Tropical Monsoon and Tropical Marine Climate, p.157; INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Chapter 4: Climate, p.29-31; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Chapter 27: Jet streams, p.389

2. Understanding Jet Streams & Upper Air Circulation (intermediate)

To understand why the Indian monsoon is so dynamic, we first need to look up—way up. Jet Streams are narrow, high-velocity ribbons of air flowing in the upper troposphere (usually between 9 to 14 km high). Think of them as "atmospheric superhighways" that steer weather systems below. They are geostrophic, meaning they are the result of a delicate balance between the pressure gradient force (moving air from warm to cold areas) and the Coriolis force (caused by the Earth's rotation) Physical Geography by PMF IAS, Jet streams, p.383.

In the context of India, the most important player for most of the year is the Subtropical Westerly Jet Stream (STWJ). During winter, this jet is physically split into two branches by the massive wall of the Himalayas. The southern branch flows right across the Indo-Gangetic plains. As long as this westerly jet sits over northern India, it maintains high-pressure conditions that keep the monsoon at bay. It also brings in "Western Disturbances" from the Mediterranean, which give North India its winter rain Geography of India, Majid Husain, Climate of India, p.8.

The magic happens in the summer. As the sun moves north toward the Tropic of Cancer, the Tibetan Plateau begins to bake, acting as a massive high-altitude heat source. This intense heating causes the STWJ to "jump" north of the Himalayas. This sudden withdrawal of the westerly jet from the Indian plains acts like opening a floodgate, allowing the low-level monsoon winds to rush in. Simultaneously, a unique Tropical Easterly Jet (TEJ) forms in the upper atmosphere over peninsular India (around 15°N), which is often credited with the spectacular "burst" of the monsoon India Physical Environment NCERT Class XI, Chapter 4, p.31.

Feature	Subtropical Westerly Jet (STWJ)	Tropical Easterly Jet (TEJ)
Season	Dominant in Winter (over India)	Exists only in Summer
Direction	West to East	East to West
Impact	Brings Western Disturbances; delays monsoon	Triggers the Monsoon 'Burst'

Sources: Physical Geography by PMF IAS, Jet streams, p.383-385; Geography of India by Majid Husain, Climate of India, p.5-8; India Physical Environment NCERT Class XI, Climate, p.31

3. The Winter Pattern: Western Disturbances (intermediate)

While we often associate Indian rainfall with the summer monsoon, the winter months see a fascinating atmospheric phenomenon known as Western Disturbances (WDs). These are extratropical cyclones—low-pressure systems that originate far away over the Mediterranean Sea. Despite their distant origin, they travel thousands of kilometers across Western and Central Asia to reach the Indian subcontinent, primarily between November and May Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p. 52.

The journey of these disturbances is made possible by the Subtropical Westerly Jet Stream (STWJ). In winter, as the sun moves south, the high-altitude circumpolar whirl shifts equator-ward, and the STWJ positions itself over the north Indian plains Geography of India, Majid Husain, Chapter 4, p. 8. A crucial geographical detail here is the Tibetan Plateau and the Himalayas. This massive physical barrier acts like a pier in a stream, forcing the westerly jet to bifurcate (split) into two branches. The southern branch flows south of the Himalayas, acting as a high-speed "conveyor belt" that drags these Mediterranean low-pressure waves into North-West India Geography of India, Majid Husain, Chapter 4, p. 8.

When these disturbances arrive, they bring a distinct change in weather. While the surface is dominated by dry North-East trade winds, these upper-air storms cause cloudiness and light to moderate rainfall in the plains and heavy snowfall in the Western Himalayas. For an agrarian economy like India, these disturbances are a blessing in disguise as they provide vital moisture for Rabi crops, especially wheat Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p. 52. However, their passage is often followed by a sharp arrival of cold waves as the clear skies after the storm allow for rapid terrestrial radiation and the influx of cold air from the north.

Sources: Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.52; Geography of India, Majid Husain, Climate of India, p.8; Physical Geography by PMF IAS, Jet streams, p.383

4. External Drivers: ENSO and Indian Ocean Dipole (intermediate)

While the internal dynamics of the Indian landmass (like the heating of the Tibetan Plateau) set the stage for the monsoon, the performance of the rains is often dictated by teleconnections—climatic links between distant parts of the globe. The two most significant "external drivers" are the El Niño-Southern Oscillation (ENSO) in the Pacific and the Indian Ocean Dipole (IOD) in our own backyard.

ENSO refers to the periodic fluctuation in sea surface temperatures and atmospheric pressure across the equatorial Pacific Ocean. Under normal conditions, low pressure over the Western Pacific (near Australia/Indonesia) helps pull moisture toward Asia. However, during an El Niño year, this low-pressure zone shifts eastward toward the central and eastern Pacific. This shift weakens the trade winds and diverts moisture away from India, often leading to droughts Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. Conversely, La Niña is the "cool phase" that typically strengthens the monsoon, bringing surplus rainfall.

Closer to home, the Indian Ocean Dipole (IOD) acts as the Indian Ocean's version of ENSO. It is defined by the temperature difference between the western pole (Arabian Sea) and the eastern pole (south of Indonesia) Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. The atmospheric component of this oscillation is known as EQUINOO (Equatorial Indian Ocean Oscillation). The state of the IOD can significantly alter the monsoon's impact, as shown in the table below:

Feature	Positive IOD (+IOD)	Negative IOD (-IOD)
Western Indian Ocean (Arabian Sea)	Warmer than normal	Colder than normal
Eastern Indian Ocean (Indonesia)	Colder than normal	Warmer than normal
Impact on Indian Monsoon	Stronger Monsoon (increases rainfall)	Weaker Monsoon (reduces rainfall)

The most fascinating aspect of these drivers is their interplay. A strong Positive IOD can actually "save" the Indian monsoon even during a severe El Niño year. For example, in 1997, despite a record-breaking El Niño that should have caused a devastating drought, India received normal rainfall because a powerful Positive IOD neutralized the negative Pacific influence Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.416.

Sources: 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.416

5. The Tibetan Plateau: A Thermal Engine (exam-level)

To understand the Indian monsoon, we must look at the Tibetan Plateau not just as a mountain range, but as a massive "atmospheric heater" sitting 4,000 to 5,000 meters above sea level. Because of its immense altitude and vast area, the plateau receives 2°C to 3°C more insolation (solar radiation) than the surrounding lowlands. This intense summer heating transforms the plateau into a high-level heat source, often referred to as a "thermal engine" Geography of India, Chapter 4, p.5.

This heating initiates a process called anti-cyclogenesis. As the air over the plateau warms up, it rises and creates a warm-core anticyclone (a high-pressure system) in the middle of the troposphere, typically around the 500 mb level Geography of India, Chapter 4, p.7. This anticyclone acts as a massive "pump" in the upper atmosphere. It performs two critical functions that pave the way for the monsoon:

• Weakening the Westerlies: The high pressure over Tibet helps push the Subtropical Westerly Jet Stream north of the Himalayas. This is essential because, as long as the westerly jet sits over the Indian plains, the monsoon cannot advance Geography of India, Chapter 4, p.6.
• Creating the Tropical Easterly Jet (TEJ): The air flowing out from the southern flank of this Tibetan anticyclone moves toward the equator. Due to the Earth's rotation, it turns westward, forming the Tropical Easterly Jet (TEJ) at approximately 15°N latitude Geography of India, Chapter 4, p.14.

According to experts like P. Koteswaram, it is this upper-level setup—specifically the development of the TEJ—that triggers the 'burst' of the monsoon. The jet stream acts like an exhaust fan, pulling air from the surface of the Indian Ocean toward the subcontinent, thereby sustaining the low-level South-West monsoon winds Geography of India, Chapter 4, p.14. In essence, the Tibetan Plateau dictates the timing and strength of the entire system by rearranging the wind patterns of the upper atmosphere.

Sources: Geography of India, Climate of India, p.5; Geography of India, Climate of India, p.6; Geography of India, Climate of India, p.7; Geography of India, Climate of India, p.14

6. Seasonal Shift: STWJ Withdrawal and TEJ Onset (exam-level)

Think of the upper-level atmosphere over India as a stage that undergoes a complete change of scenery just before the monsoon arrives. During the winter and spring, the Subtropical Westerly Jet (STWJ) dominates the North Indian plains, flowing south of the Himalayas. However, as the sun moves north and the Tibetan Plateau begins to heat up intensely, this westerly jet acts like a "lid" that prevents the moist monsoon winds from advancing. For the monsoon to "burst," this lid must be removed. This happens through a sudden, northward shift where the STWJ withdraws from the Indian plains and repositioned itself north of the Tibetan Plateau Majid Husain, Climate of India, p.8.

The withdrawal of the STWJ is not a slow retreat; it is often described as an abrupt jump Majid Husain, Climate of India, p.14. Once the STWJ clears the region south of the Himalayas, it makes way for a brand-new player: the Tropical Easterly Jet (TEJ). The TEJ forms due to the intense heating of the Tibetan Plateau, which acts as an elevated heat source. Warm air rises from Tibet, hits the top of the troposphere, and spreads outwards. Due to the Coriolis force, this southward-moving air is deflected to become an Easterly Jet, typically establishing itself along the 15°N latitude (passing near Kolkata and Bengaluru) NCERT Class XI, Climate, p.31.

The relationship between these two jets is one of alternation. During the peak monsoon months (June to September), the STWJ is entirely absent from the Indian subcontinent, remaining far to the north. In its place, the TEJ becomes the dominant upper-air feature. This easterly jet is the "engine" of the monsoon; it descends over the South Indian Ocean near the Mascarene High, intensifying the high-pressure cell there and pushing the moisture-laden Southwest Monsoon winds toward the Indian coast with greater force Majid Husain, Climate of India, p.7.

Feature	Subtropical Westerly Jet (STWJ)	Tropical Easterly Jet (TEJ)
Season over India	Winter & Spring	Summer (Monsoon)
Direction	West to East	East to West
Position during Monsoon	North of the Himalayas/Tibet	Over Peninsular India (~15°N)
Role	Keeps monsoon out when south of Himalayas	Triggers and sustains the Monsoon 'Burst'

Sources: Geography of India, Majid Husain, Climate of India, p.7, 8, 14; INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.31

7. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize your knowledge of the thermal engine theory of the monsoon with the concept of upper-air circulation. During the transition from winter to summer, the Subtropical Westerly Jet (STWJ), which flows over the Indian plains in winter, must undergo a northward migration. As the Tibetan Plateau heats up, it creates an upper-level high-pressure zone that pushes the STWJ completely north of the Himalayas. Simultaneously, this intense heating generates the Tropical Easterly Jet (TEJ), which establishes itself over the southern peninsula. As explained in INDIA PHYSICAL ENVIRONMENT (NCERT Class XI), the sudden appearance of the TEJ is what facilitates the 'burst' of the monsoon by creating the necessary divergence to pull moist air from the surface.

The reasoning to arrive at (B) requires a precise understanding of the geography involved. Because the STWJ has retreated beyond the Himalayan barrier, it is no longer present within the 'Indian region' (defined as the area south of the Himalayas). Therefore, only the easterly jet stream alone exists in the Indian atmosphere during this period. UPSC often uses Option (C) as a geographical trap to test if you realize the STWJ has moved entirely out of the region, rather than just co-existing. Option (A) represents the pre-monsoon or winter condition, while Option (D) is physically impossible given that the monsoon is a period of dynamic energy exchange and high-speed wind currents.