Indian Monsoon

Let’s say you are standing at the seashore, experiencing cool, dry breezes in winter but, come summer, facing strong, moisture-laden winds that bring heavy rains.. This reversal of winds, bringing torrential rains to India, is what we call the monsoon—derived from the Arabic word Mausim, meaning “season.” Despite its profound impact, the origin of monsoons remains partially mysterious, with several theories attempting to explain its mechanism.

Initially, it was thought that monsoons occurred due to the differential heating of land and sea. However, modern meteorology has revealed a more complex interplay of factors.

Classical Theory: The Early Understanding

Monsoons in Ancient Texts & Early Observations

References to monsoon-like winds can be found in ancient Indian scriptures such as the Rig Veda, though they do not explain their mechanism. The first practical understanding came from Arab traders who depended on monsoonal winds for maritime trade with India.

In the 10th century, Arab explorer Al Masudi noted the reversal of ocean currents and monsoon winds over the North Indian Ocean. Later, in the 17th century, Sir Edmund Halley proposed that monsoons resulted from the differential heating of land and sea—where land heats up faster than the ocean, creating a low-pressure area over India that pulls in moisture-laden winds. While this was a significant step, it was only part of the puzzle.

Modern Theory: A Scientific Approach

Today, we understand that monsoons are a product of multiple interconnected factors, including:

• Position of the Himalayas & Tibetan Plateau – Acting as a barrier, they influence air pressure and wind patterns.
• Jet Streams – Powerful air currents in the upper atmosphere play a role in monsoon onset and withdrawal.
• Inter-Tropical Convergence Zone (ITCZ) – The shifting equatorial low-pressure belt plays a crucial role.
• Differential Heating of Land & Sea – The vast Indian subcontinent heats up differently than the surrounding oceans, driving seasonal wind shifts.
• Apparent Motion of the Sun – As the Sun moves north and south, it shifts the ITCZ, impacting monsoon dynamics.

Local Storms: Monsoon’s Preludes & Side Effects

Before the full onset of monsoon, local thunderstorms occur across India. Some of these are famous by their regional names:

1. Mango Showers (Kerala & Karnataka) – Help in early mango ripening.
2. Blossom Showers (Kerala) – Essential for coffee plantations.
3. Norwesters (Kalbaisakhi) (Bengal & Assam) – Evening thunderstorms benefiting tea, jute, and rice cultivation.
4. Loo (North India) – Hot, dry winds in summer, often causing heat strokes.

Characteristics of Indian Monsoon

• Seasonal Rainfall – Most rain is received in the Southwest Monsoon (June-September).
• Relief & Topography Influence – Mountains and hills shape rainfall patterns (e.g., Western Ghats receive heavy rain, while Deccan Plateau remains drier).
• Declining Rainfall with Distance from Coast – Coastal areas receive more rain than inland regions.
• Monsoon Dependency in Agriculture – Around 75% of India’s total rainfall is from monsoons, making it crucial for farming.
• Arabian Sea vs. Bay of Bengal Branch –
  • Arabian Sea Branch (stronger) contributes 65% of rainfall.
  • Bay of Bengal Branch contributes 35% of rainfall.

Breaks in the Monsoon

Despite being known for torrential rain, the monsoon isn’t continuous. Dry spells occur, leading to breaks in rainfall. These occur due to:

• Northern India – Weak monsoon trough or shifting ITCZ.
• West Coast – Winds blowing parallel to the coast prevent moisture influx.
• Mid-August Breaks – Most common and longest, affecting rainfall patterns across the country.

Mechanism of Indian Monsoon Explained

Till now, in this chapter you have read bits and pieces about the Indian monsoon, but have you ever visualized how this grand phenomenon actually unfolds? Let’s dive deep into the mechanism of the Indian monsoon, breaking it down step by step. By the end of this section, you won’t just understand it—you’ll feel how the forces of nature come together to shape the monsoonal winds over India.

The mechanism of the Indian monsoon during the winter season

Step 1: Role of Sub-Tropical Westerly Jet Stream (STWJ)

• The Sub-Tropical Westerly Jet Stream (STWJ) flows from west to east at an altitude of around 6 km and above.
• The Himalayas (6–7 km in height) obstruct the flow of these jet streams, leading to their division into two branches:
  1. Northern Branch: Moves north of the Himalayas.
  2. Southern Branch: Much stronger due to the apparent motion of the Sun in the Southern Hemisphere (SH) during winter.

Step 2: Anticyclonic Circulation Around Himalayas

• The stronger Southern Branch of STWJ leads to a circular motion of the jet stream around the Himalayas in an anticlockwise fashion.
• In the upper part of the atmosphere, this creates low pressure.
• As a result, high pressure (HP) forms on the surface of the Earth.

Step 3: Development of High Pressure over Land and Low Pressure over Ocean

• During winter, the Mascarene region (South Indian Ocean) warms up due to the Sun’s apparent movement in the Southern Hemisphere (SH).
• This warming creates low pressure (LP) over the ocean.
• Meanwhile, high pressure (HP) develops over the Indian subcontinent due to cooling of land in winter.

Step 4: Formation of North-East Monsoon Winds

• Since winds always move from high pressure to low pressure, air flows from HP over the Indian subcontinent to LP over the ocean.
• This results in the formation of North-East (NE) Monsoonal Winds, moving from NE to SW.

Step 5: Characteristics of Winter Monsoon Winds

• These NE Monsoon winds are offshore winds, meaning they move from land to sea.
• As a result, they do not cause rainfall in most parts of India.
• However, due to the coastline orientation of India, the NE Monsoon winds pick up moisture from the Bay of Bengal and bring rainfall to Tamil Nadu and parts of the Coromandel Coast.

Thus, the winter monsoon mechanism results in dry weather over most of India except for rainfall in Tamil Nadu due to the North-East Monsoon winds.

The mechanism of the Indian monsoon during the summer season

Step 1: Northward Shift of the Sun and STWJ

• In summer, the apparent movement of the Sun shifts towards the Northern Hemisphere (NH).
• This leads to:
  • Increased heating effect over the Indian subcontinent.
  • Northward movement of trade winds, westerlies, and the Sub-Tropical Westerly Jet Stream (STWJ).
  • The northern branch of STWJ becomes stronger.

Step 2: Formation of Low Pressure Over Land

• The northward shift of the Sun leads to clockwise motion of winds around the Himalayas.
• This creates:
  • High pressure (HP) in the upper atmosphere.
  • Low pressure (LP) on the surface of the Earth over the Indian subcontinent.
• Meanwhile, the Mascarene region (South Indian Ocean) experiences a cooling effect, leading to the formation of high pressure (HP) over the ocean.

Step 3: Formation of South-West Monsoon Winds

• Since winds flow from high pressure to low pressure, air moves from HP over the Mascarene region to LP over India.
• This results in the formation of South-West (SW) monsoonal winds, which bring heavy rainfall to the Indian subcontinent.
• The SW monsoon bifurcates into two branches:
  1. Arabian Sea Branch (60%) – Stronger, bringing more rainfall.
  2. Bay of Bengal Branch (40%) – Also significant but more influenced by the terrain.

Step 4: Bay of Bengal Branch and Heavy Rainfall in North-East India

• The Bay of Bengal Branch enters the North-Eastern region of India, hitting the Meghalaya Plateau.
• The Northeastern mountains are much higher than the Northern Himalayas, forcing the monsoon winds to rise steeply.
• As the winds rise:
  • Clouds form continuously, leading to heavy rainfall.
  • This results in Cherrapunji becoming the wettest place on Earth.
• Reason for heavy rainfall: The mountains have a funnel-shaped structure, leaving the winds no option but to rise high.

Step 5: Some Monsoonal Winds Move Inland

• Some monsoon winds escape the North-East region and move into the interior of India.
• Certain regions such as UP, Bihar, and Jharkhand have:
  • Continental effect (far from large water bodies).
  • High temperatures, causing low pressure (LP) to form.
• Due to this LP formation, air rises and rainfall occurs due to convective action.

Step 6: Arabian Sea Branch and Rainfall in Western India

• The Arabian Sea Branch moves towards Western India, bringing heavy rainfall to:
  • Kerala, Karnataka, Goa, Maharashtra (Western Ghats), and Gujarat.
• The Aravali Range in Rajasthan is oriented parallel to the direction of monsoon winds.
  • Because of this, it fails to obstruct the monsoon winds, leading to no rainfall in Rajasthan.
  • This is why the Aravali region remains a desert-prone area.

Let’s summarise:

• The South-West monsoon, originating from the Mascarene region, is the primary source of rainfall in India.
• It bifurcates into the Arabian Sea and Bay of Bengal branches, with the Arabian Sea Branch contributing 60% of the total rainfall.
• The Northeastern states receive extremely high rainfall due to their mountain structure.
• Some interior regions receive rainfall due to convective action.
• Rajasthan remains a desert region due to the orientation of the Aravali range.

Thus, the summer monsoon mechanism explains how differential heating, atmospheric circulation, and geographical features shape the rainfall distribution in India during this season.

Now one should have a question here: why rainfall in India does not occur immediately after March 31st (onset of summer) and why April and May remain dry months, despite the rising temperatures.?

Why Doesn’t Rainfall Occur in April & May?

1. Sun Moves to the Northern Hemisphere (NH)
   • During winter, the Sun is in the Southern Hemisphere (SH).
   • As summer approaches, it shifts northward into the NH.
2. Sub-Tropical Westerly Jet (STWJ) Resists Northward Movement
   • The STWJ (Sub-Tropical Westerly Jet) is a strong wind system in the upper atmosphere.
   • It does not shift northward easily, even as the Sun moves into the NH.
   • Due to this resistance, low pressure (LP) remains at upper levels, and high pressure (HP) persists on the surface.
3. Descending Air Current Prevents Convection
   • The air current comes downward, reinforcing surface high pressure (HP).
   • Although land starts heating up, the warm air is unable to rise freely due to this subsiding air.
   • This results in strong heating without rainfall, as moisture-laden winds are unable to rise and condense.

What Happens After May?

1. STWJ Moves Northward → Entire Pressure System Shifts
   • As temperatures rise further, the STWJ finally shifts north.
   • This alters the entire pressure system, allowing low pressure to form over India.
2. Tibetan Plateau Heats Up → Formation of Tropical Easterly Jet Stream (TEJ)
   • The Tibetan Plateau absorbs heat rapidly, warming the surrounding air.
   • This creates a strong upward air current, which carries warm air high into the troposphere.
   • This rising air moves horizontally, forming the Tropical Easterly Jet Stream (TEJ).
3. Tropical Easterly Jet Causes Monsoon Burst
   • The TEJ descends over the Mascarene region, reinforcing the high-pressure system there.
   • A branch of this jet stream moves towards India, bringing the first burst of monsoon rainfall.
   • This process is explained by the Koteswaram Theory.

Role of ITCZ (Inter-Tropical Convergence Zone)

1. ITCZ is the Zone Where Trade Winds Converge
   • The ITCZ shifts depending on the apparent movement of the Sun.
   • Normally, it remains in the equatorial region.

2. STWJ’s Influence Delays ITCZ Movement
   • Since STWJ does not move northward in April & May, the ITCZ also remains stationary.
   • This prevents early monsoon rains.
3. ITCZ Moves North After May
   • Once STWJ shifts northward, the ITCZ also bulges northward due to the large Eurasian landmass.
   • This shift allows the monsoon winds to act as onshore winds, bringing rainfall to India.

So, Let’s summarise:

• April & May remain dry despite high temperatures because the STWJ prevents air from rising, delaying the monsoon.
• After May, the STWJ moves northward, the Tibetan Plateau heats up, and the Tropical Easterly Jet (TEJ) forms.
• The first monsoon burst in India is caused by TEJ, as explained by the Koteswaram Theory.
• ITCZ also shifts north only after STWJ moves, allowing rain-bearing monsoon winds to reach India.

This explains why summer monsoon rains in India are delayed until June instead of occurring immediately after the onset of summer in March.

Retreating Monsoon Explained:

1. Southwest (S-W) Monsoon is the Primary Rain-Bringer
   • India receives most of its rainfall from the Southwest Monsoon.
   • This monsoon originates from the Indian Ocean and brings moisture-laden winds to the subcontinent.
2. Northeast (N-E) Monsoon Brings Limited Rainfall
   • Unlike the S-W Monsoon, the Northeast Monsoon does not contribute much to rainfall in India.
   • Exception: Tamil Nadu (TN) receives significant rainfall during the N-E Monsoon.
3. Northeast (N-E) Monsoon Season: October to March
   • The N-E Monsoon lasts from October to March.
   • Within this period, the months of October and November are considered the Retreating Monsoon, which is a subset of the N-E Monsoon season.

Seasonal Changes Affecting Monsoon Winds

In Summer (June–September)

• The Sun moves northward, shifting the Inter-Tropical Convergence Zone (ITCZ) northward.
• This shift pulls the S-W Monsoon winds towards India, bringing heavy rains.

In Winter (Post-September)

• The Sun moves southward, causing the ITCZ to shift southward as well.
• As a result, the Southwest Monsoon retreats, and winds start blowing from the northeast.
• This retreat marks the beginning of the Northeast Monsoon.
• Since these winds originate from the land (Indian subcontinent), they are dry and do not bring rainfall—except in Tamil Nadu, where they pick up moisture from the Bay of Bengal.

So, Let’s summarise:

• Retreating Monsoon occurs in October–November as part of the Northeast Monsoon season.
• S-W Monsoon (June–September) brings major rainfall to India, while N-E Monsoon (October–March) brings rain mainly to Tamil Nadu.
• The shift of the ITCZ southward due to the apparent motion of the Sun leads to the retreat of monsoon winds.
• The Northeast Monsoon winds are dry, as they originate from land, except when they pick up moisture over the Bay of Bengal before reaching Tamil Nadu.

This process explains why India experiences a distinct retreating monsoon phase before transitioning into winter.

Explanation of October Heat

1. What is October Heat?
   • During October, as the Sun starts moving southward, the Southwest Monsoon retreats from India.
   • However, the land is still heated, and combined with high humidity from the ocean, this creates a period of hot and humid weather—known as October Heat.
   • It is less severe than the Loo (hot, dry summer winds), but still uncomfortable due to high moisture in the air.
2. Role of Air Currents
   • A Low-Pressure (LP) zone forms over land during monsoon due to intense heating.
   • As the monsoon withdraws, High Pressure (HP) builds up over land, pushing air downwards.
   • This descending air increases the warming effect, but not as extreme as in summer.
3. Why is October Heat Not as Intense as Loo?
   • In summer, Loo (hot winds) occurs due to extreme heating of dry air over land.
   • In contrast, October Heat occurs when the retreating monsoon leaves behind moist air from the ocean, making it humid but not as dry or scorching as Loo

Conclusion

The Indian monsoon is not just a climatic phenomenon; it is the very pulse of India’s agriculture, economy, and daily life. From the ancient traders who mapped its winds to the modern meteorologists who predict its behavior, the monsoon remains a dynamic and fascinating force.

Without it, India’s vast farmlands would struggle, rivers would shrink, and even economic growth would be impacted. However, its unpredictability—whether excessive rains or prolonged droughts—reminds us that nature’s forces, no matter how well-studied, always retain an element of mystery.

Impact of El Niño on Indian Monsoon

• El Niño is an abnormal warming of sea surface temperatures in the eastern equatorial Pacific Ocean.
• It weakens the trade winds, disrupting normal monsoon patterns.

Illustration of Indian Monsoon during normal conditions:

Illustration of Indian Monsoon during El-Nino:

Impact on Indian Monsoon:

• Causes deficient rainfall and drought-like conditions in India.
• Weakens the southwesterly monsoon winds, leading to a weaker monsoon season.
• Delayed onset and uneven distribution of rainfall.
• Affects agriculture, water availability, and economy.

What is El Niño Modoki?

• Unlike conventional El Niño, El Niño Modoki involves warming in the central tropical Pacific, while the eastern and western Pacific remain cooler.
• This results in two separate Walker Circulation cells, with a wet region in the central Pacific and dry conditions in the east and west.
• Its impact on the Indian monsoon is variable, sometimes weakening the monsoon, but not always as severely as conventional El Niño.

Conclusion:

El Niño negatively impacts the Indian monsoon by reducing rainfall, leading to droughts and agricultural losses. However, factors like the Indian Ocean Dipole (IOD) and El Niño Modoki can moderate or amplify its effects. Understanding these patterns is crucial for better monsoon forecasting.

Impact of La Niña on the Indian Monsoon

What is La Niña?

La Niña is the counterpart of El Niño and represents a cooling phase of the El Niño-Southern Oscillation (ENSO). It occurs when trade winds strengthen, pushing warm surface waters westward and allowing cold, deep ocean waters to upwell in the central and eastern Pacific. This results in colder-than-normal sea surface temperatures in the tropical Pacific.

How Does La Niña Affect the Indian Monsoon?

La Niña typically enhances the Indian monsoon by strengthening trade winds and increasing moisture transport.

Effects on India and Beyond:

• Stronger and wetter Indian Monsoon: La Niña leads to above-normal rainfall in India, reducing the risk of drought and benefiting agriculture.
• Higher agricultural output: Favorable monsoons boost crop yields, improving food production and rural income.
• Increased flood risk: Excessive rainfall may lead to floods, particularly in states like Bihar, Assam, and Uttar Pradesh.
• Colder winters in northern India: The phenomenon contributes to harsher winters due to intensified western disturbances.
• Cyclone formation in the Bay of Bengal: More cyclonic storms occur during La Niña years, impacting eastern coastal regions.

Global Effects of La Niña:

• Southeast Asia & Australia: Abnormally heavy rains, leading to floods.
• Southeastern Africa: Cooler and wetter conditions.
• Northwestern U.S. & Canada: Colder and snowier winters.
• Southern U.S.: Drought conditions prevail during winter.

Conclusion

For India, La Niña is generally beneficial as it strengthens monsoonal rainfall, supporting agriculture and water availability. However, it also brings risks of excessive flooding and harsher winters, requiring careful management of water resources and disaster preparedness.

Pacific Decadal Oscillation (PDO)

What is PDO?

The Pacific Decadal Oscillation (PDO) is a long-term fluctuation in sea surface temperatures between the northern/western tropical Pacific and the eastern tropical Pacific Ocean. Unlike El Niño or La Niña, which operate on shorter time scales (3–7 years), the PDO shifts occur roughly every 20 to 30 years.

Phases of PDO:

There are two phases of the PDO:

1. Negative (Cool) Phase:
   • Western/Northern Pacific waters become relatively warmer.
   • Eastern Pacific waters become colder.
2. Positive (Warm) Phase:
   • Western/Northern Pacific waters become relatively colder.
   • Eastern Pacific waters become warmer.

How Does PDO Impact Climate?

• The PDO influences jet streams and storm tracks across North America.
• It can amplify or weaken El Niño and La Niña events.
• Positive PDO often aligns with El Niño, strengthening its effects.
• Negative PDO is linked to La Niña, enhancing its cooling impact.

Effect on Indian Monsoon:

• A Positive PDO (associated with El Niño) weakens the Indian monsoon, leading to drought-like conditions.
• A Negative PDO (linked to La Niña) strengthens the monsoon, bringing above-normal rainfall.

Conclusion:

Although the exact causes of PDO remain unclear, its influence on global weather patterns, including monsoons and El Niño events, makes it a crucial factor in long-term climate variability. Understanding PDO helps in predicting monsoon strength and managing agricultural planning in India.

Indian Ocean Dipole (IOD)

What is IOD?

The Indian Ocean Dipole (IOD) is a climate phenomenon characterized by periodic oscillations in sea surface temperatures across the western and eastern Indian Ocean. It moves through three phases:

1. Positive IOD
2. Neutral IOD
3. Negative IOD

How Does IOD Work?

• Positive IOD:
  • The western Indian Ocean warms up (temperature rises above normal by 0.4°C).
  • The eastern Indian Ocean (near Indonesia and Australia) cools down due to the weakening of equatorial westerly winds.
  • This strengthens the Indian monsoon as southwesterly winds are redirected toward the Indian subcontinent.
  • However, it causes droughts in Indonesia and Australia.
• Negative IOD:
  • The eastern Indian Ocean warms up, while the western Indian Ocean cools down due to strong equatorial westerly winds.
  • This can weaken the Indian monsoon, leading to lower rainfall in India.
  • Australia and Indonesia receive excess rainfall.

Illustration of Positive IOD:

Look at the figure and note that:

• Sometimes, due to certain climatic conditions, the equatorial westerlies weaken.
• When this happens, trade winds dominate the region instead.

• Normally, 70% of monsoon winds move towards India, while 30% are diverted towards Southeast Asia.
• During a Positive IOD, a high-pressure (HP) system forms near Indonesia/Southeast Asia.
• This HP system redirects the winds from Southeast Asia towards India, strengthening the Indian monsoon.

• The HP-LP (High Pressure – Low Pressure) pattern formed during Positive IOD is weaker than that of El Niño.
• It is also weaker than the normal monsoonal HP-LP system, but still contributes to enhancing the monsoon.

Illustration of Negative IOD:

• Unlike Positive IOD, here the equatorial westerlies become stronger, which impacts the monsoon wind flow.
• A Low-Pressure (LP) system develops over Southeast Asia (Indonesia & nearby regions).
• This attracts more monsoonal winds towards Southeast Asia, diverting them away from India.
• With more monsoonal winds moving towards Southeast Asia, India receives only about 50-60% of its usual monsoon winds.
• This results in weaker monsoon rains over India.
• While the Indian monsoon weakens during a Negative IOD, it does not necessarily lead to drought like El Niño.
• This is because the HP-LP system during Negative IOD is much weaker than during El Niño, meaning the monsoon is reduced but not completely suppressed.

Impact on Indian Monsoon:

• A positive IOD can counteract El Niño, ensuring a strong monsoon even if El Niño is present.
• A negative IOD can weaken the monsoon, leading to below-normal rainfall in India.

Conclusion:

The IOD plays a crucial role in modulating the Southwest Monsoon, sometimes offsetting the adverse effects of El Niño. Understanding IOD helps in improving monsoon predictions and planning for droughts or floods in the Indian subcontinent.