The Indian Monsoon

The term “monsoon” originates from the Arabic word Mausim, meaning season. It refers to a period when the direction of prevailing winds shifts completely. The monsoon season in India is regarded as a vital rainy season that has a big impact on the climate of the nation.

Monsoon Winds in India

• Cross the equator in the Indian Ocean, blowing from the south-west.
• Draw moisture-laden winds towards low-pressure areas over North-West India and Central Myanmar.
• Transition from hot, dry trade winds to moisture-laden rain-bringing winds.
• Replaces hot, dry tropical continental air with moist equatorial maritime air at altitudes between 3 to 5 kilometers.
• Influenced by the Himalayan mountain range in the Indian subcontinent.
• Lasts 2 to 5 months, with rainfall between June and September accounting for 75 to 90% of India’s annual precipitation.

Monsoon Nature and Mechanism:

• Differential heating of the land and water was proposed as the origin of monsoons in early 19th-century hypotheses.
• Land heats up more rapidly than sea, creating low-pressure and high-pressure areas.
• This pressure difference causes winds to blow from high-pressure to low-pressure areas, resulting in monsoon winds.
• This theory was overly simplistic and did not fully explain the complexities of the monsoon system.

A more comprehensive understanding came from studying the global atmospheric circulation. Several scientists have since proposed alternative theories to explain the origin of the monsoons.

Theories of the Origin of Monsoons:

Classical Theory: Monsoons were first mentioned in ancient texts like the Rigveda, but these texts did not offer an explanation of the monsoon mechanism.Arab Explorer Al Masudi’s First Scientific Study
Described reversal of ocean currents and winds in North Indian Ocean.
Sir Edmund Halley proposed monsoon caused by thermal contrasts due to differential heating.

Halley’s theory involved the land-sea breeze mechanism, but extended it to seasonal variations. He explained that during the summer, the large landmass of Asia heats up rapidly, creating a low-pressure center, while the surrounding oceans remain cooler with higher pressure. This results in air flowing from the high-pressure areas over the oceans to the low-pressure areas on land, forming the monsoon winds. However, this theory has limitations in explaining the monsoon’s full complexity.

Dynamic Concept or Shifting of the Inter-Tropical Convergence Zone (ITCZ):

In 1951, H. Flohn, a scientist from the German Weather Bureau, proposed a dynamic explanation for the monsoon system. According to Flohn, the monsoon results from the seasonal shifts of the planetary wind system due to temperature and pressure changes caused by the Sun’s seasonal movement. The Inter-Tropical Convergence Zone (ITCZ), also known as the monsoon trough, is a low-pressure zone where trade winds from the northern and southern hemispheres converge. The ITCZ shifts its position northward or southward based on the apparent position of the Sun, influencing the monsoon winds.

Sun heats up tropical land, forming low-pressure system over northern India.
South-East trade winds from Southern Hemisphere draw across equator.
South-West Monsoon deflects winds from southwest to northeast.
Sun shifts to Tropic of Capricorn in winter, reversing wind direction in India.

Phenomenon of the Indian Monsoon:

Systematic studies have helped to understand key aspects of the monsoon phenomenon, including the onset, landward advance, rain-bearing systems, and the break or retreat of the monsoon.

1. Monsoon Onset and Landward Advance
   • The Indian subcontinent heats up in April and May, creating a low-pressure area in the north-west.
   • This leads to the onset of the South-West Monsoon, which advances landward, bringing rainfall.
2. Rain-Bearing Systems and Distribution of Rainfall
   • Monsoon rainfall is in spells, often interrupted by dry spells or breaks.
   • The intensity and frequency of these breaks are influenced by tropical depressions in the Bay of Bengal or South China Sea.
3. Withdrawal or Retreat of the Monsoon
   • The South-West Monsoon retreats gradually, starting in September and leaving by mid-October.
   • As it retreats, it establishes the North-East Monsoon, affecting the Tamil Nadu coast and the East Coast of Sri Lanka.

Characteristics of the Indian Monsoon:

The South-West Monsoon arrives in June and lasts until mid-September, contributing one-fourth of India’s annual rainfall. This season is crucial for the Indian economy, especially for the production of kharif crops like rice and pulses. The main characteristics of the Indian monsoon are:

1. Seasonal Rainfall: Monsoon rainfall occurs seasonally between June and September.
2. Orographic Rainfall: Rainfall is mainly influenced by the Western Ghats and the Himalayas, with the windward sides receiving over 250 cm of rainfall. The Indus-Ganga-Brahmaputra plains and North-East India also experience significant rainfall.
3. Declining Rainfall Inland: Rainfall decreases as one moves from the coastal regions to the interior of the country.
4. Spatial Variability: There is significant variation in the distribution of rainfall across the country.
5. Uncertainty: The exact timing of the monsoon’s onset and withdrawal can vary, making it unpredictable in some regions.

The monsoon’s significance for India’s agriculture, economy, and daily life makes it one of the most important climatic events in the country, influencing everything from crop yields to water supply and flood risks.

Distribution of Rainfall in India

India experiences significant regional variations in rainfall distribution. The country’s average annual rainfall is about 125 cm, but it is unevenly spread across different regions. The highest rainfall occurs along the West coast, the Western Ghats, and the sub-Himalayan regions, including the North-East and the Khasi hills of Meghalaya.

Mawsynram, located on the southern slope of the Khasi hills, holds the record for the highest average annual rainfall globally, with remarkable stalagmite and stalactite caves. Nearby Cherapunji also experiences very high rainfall, exceeding 1200 cm. The Khasi and Jaintia hills experience rainfall of up to 1100 cm. However, as we move towards the Brahmaputra valley and surrounding hills, rainfall decreases significantly, dropping to around 110 cm. The isohyets (lines of equal rainfall) of 100 cm extend southward from Gujarat’s coast, running almost parallel to the Western Ghats and continuing to Kanyakumari.

In the peninsular regions to the east of this 100 cm isohyet, rainfall declines drastically, often falling below 60 cm. In areas like the southern parts of Gujarat, East Tamil Nadu, and parts of Odisha, Jharkhand, Bihar, and Eastern Madhya Pradesh, rainfall ranges between 100 to 200 cm. The 100 cm isohyet moves eastward, passing through Jammu and Kashmir, Himachal Pradesh, and Northern Uttar Pradesh.

Monsoon rainfall is often inconsistent, influenced by changes in tropical depressions and the path of the Inter-Tropical Convergence Zone (ITCZ). Dry spells are common along the West Coast, especially when winds blow parallel to the coastline, leading to droughts. These dry conditions are more frequent in regions with low rainfall.

Drought-prone areas in India include Rajasthan, Haryana, Gujarat, and parts of the leeward side of the Western Ghats. Nearly 1 million square kilometers of land is considered drought-prone, experiencing inadequate rainfall and frequent droughts.

Jet Streams and the Indian Monsoon

Jet streams are fast-moving ribbons of air that help guide weather systems across the globe. These winds flow from west to east in the upper troposphere, around 9-16 km above the Earth’s surface. In winter, the Westerly Jet streams extend up to latitudes of 20°N-35°N, influencing weather patterns in India, especially in the north-western regions. The Himalayas and Tibetan Plateau cause a bifurcation of these Westerly Jet streams, creating high-pressure systems over Afghanistan and Pakistan, which lead to dry weather conditions.

In addition to the Westerly Jet, the Tropical Easterly Jet (TEJ) influences the Indian monsoon. During summer, the Tibetan Plateau heats up, causing the air to rise, and it generates anti-cyclonic conditions. The Earth’s rotation deflects the rising air to the right, causing it to travel in an anti-clockwise direction, forming the southwest monsoon. The development of the TEJ and the warming of Tibet are key factors for the intensity of the Indian monsoon.

El Niño and its Impact on the Indian Monsoon

El Niño is a weather phenomenon characterized by the unusual warming of the equatorial Pacific Ocean due to weakened trade winds. This typically occurs every 2 to 5 years and can last for about a year. The warming of the Pacific Ocean disrupts the trade winds, which weakens the monsoon winds over India, reducing rainfall and causing uneven distribution.

El Niño has been linked to some of the most severe droughts in India, such as those in 2002 and 2009. However, not all El Niño years cause droughts. For instance, the 1997-98 El Niño event did not result in a drought in India.

El Niño-Southern Oscillation (ENSO) and the Walker Circulation

El Niño is closely related to the Southern Oscillation, also known as ENSO. It is driven by the warming and cooling cycles of the surface ocean in the central and eastern Pacific. This creates shifts in atmospheric pressure, which can weaken the temperature differences between the Indian Ocean and the Indian subcontinent. These changes disrupt the regular monsoon patterns and can cause variations in rainfall.

Indian Ocean Dipole (IOD)

The Indian Ocean Dipole (IOD) refers to the temperature difference between the western and eastern parts of the Indian Ocean. The IOD can either be positive or negative, and it has a significant impact on the Indian monsoon. A positive IOD occurs when sea surface temperatures in the Arabian Sea are higher than usual, resulting in increased monsoon rainfall in India. Conversely, a negative IOD leads to reduced rainfall and more frequent monsoon breaks.

Monsoons and Economic Life in India

The monsoon is crucial to India’s agricultural economy, as almost 50% of the population relies on agriculture, which is highly dependent on the southwest monsoon. Variability in rainfall can cause droughts or floods, affecting crop yields, especially in areas without irrigation systems.

Excessive rainfall during the monsoon can lead to soil erosion, while winter rains from temperate cyclones benefit Rabi crops. Regional climatic variations also impact food, clothing, and housing across India.

The Himalayan Cryosphere

The Himalayan Cryosphere, which consists of glaciers, snow, and ice, plays a critical role in the Earth’s climate system. The region holds a significant portion of the world’s freshwater, contributing to major river systems like the Ganges, Indus, and Brahmaputra. Glacial meltwater is crucial for the people living in the surrounding lowlands, with glacial discharge accounting for a significant portion of the water in these rivers.

The Himalayas’ glaciers are categorized into different zones based on their interaction with the Westerlies and the monsoon. In some areas, glaciers are growing due to snow accumulation from the winter rains, while in others, they are retreating rapidly due to increasing temperatures.

The melting of glaciers due to global warming is a cause for concern, as it will lead to a reduction in freshwater availability and increase the likelihood of Glacial Lake Outburst Floods (GLOF). These floods have already caused significant damage, such as in the 2013 Uttarakhand disaster.

Importance of Cryosphere Conservation

The Himalayan glaciers are vital to the livelihoods of around 1.2 billion people. As glaciers retreat faster due to rising temperatures, the availability of water for irrigation and agriculture will be severely impacted. Moreover, the risk of GLOFs will increase, leading to further disasters. Projects like HICOM (Himalayan Cryosphere Observations and Modeling) are essential for monitoring the glaciers’ health and understanding their impact on hydrology and the broader climate system.