Mechanism of Indian Monsoons & Rainfall Patterns | Geography Optional for UPSC (Notes) PDF Download

Indian Monsoon

• The term 'monsoon' is thought to have originated from the Arabic word 'mawsim,' which means season. Monsoons are essentially seasonal winds that change direction based on the shift in seasons, making them periodic winds. These winds travel from the sea to the land during the summer and from the land to the sea during the winter, creating a dual system of seasonal winds. Some experts even consider monsoon winds as large-scale land and sea breezes.
• Historically, monsoons have played a crucial role in facilitating trade and navigation for seafarers. While monsoons occur in various regions, including the Indian subcontinent, central-western Africa, and Southeast Asia, they are most prominent in the Indian subcontinent. India experiences southwest monsoon winds during the summer and northeast monsoons during the winter. The former is caused by the formation of a strong low-pressure system over the Tibetan Plateau, while the latter is due to high-pressure cells forming over the Siberian and Tibetan plateaus.
• Southwest monsoons bring heavy rainfall to most parts of India, while northeast monsoons primarily affect the southeastern coast of the country, including the southern coast of Seemandhra and the coast of Tamil Nadu. Countries such as India, Indonesia, Bangladesh, and Myanmar receive most of their annual rainfall during the southwest monsoon season, whereas Southeast China and Japan experience significant rainfall during the northeast monsoon season.
  

Mechanism of Indian Monsoon

The monsoons are a complex meteorological phenomenon experienced in tropical areas, approximately between 20° N and 20° S. The origin of monsoons is not entirely understood, and several theories attempt to explain their mechanism. To comprehend the monsoon mechanism, it is essential to consider the following factors:

Thermal Concept:

• Noted astronomer Halley hypothesized that the primary cause of the Indian monsoon circulation was due to the differential heating effects of land and sea. According to this concept, monsoons can be seen as an extensive land breeze and sea breeze on a larger scale. During winter, the vast Asian landmass cools more rapidly than the surrounding oceans, resulting in a strong high-pressure center developing over the continent. In contrast, the pressure over adjacent oceans is relatively lower. Consequently, a pressure gradient is directed from land to sea, causing an outflow of air from the continental landmass towards the adjacent oceans, bringing cold, dry air towards the low latitudes.
• In summer, the temperature and pressure conditions reverse. The vast Asian landmass heats up rapidly, creating a strong low-pressure center. Furthermore, the Intertropical Convergence Zone (ITCZ) shifts poleward to a position over Southern Asia, reinforcing the thermally induced low-pressure center. With higher pressure over the adjacent oceans, a sea-to-land pressure gradient is established. The surface airflow is directed from the high-pressure areas over the oceans towards the low-pressure areas over the heated land. The air attracted into the low-pressure centers from the oceans is warm and moist.
• However, Halley's concept has been criticized for its inability to explain the intricacies of monsoons, such as the sudden burst of monsoon, breaks in monsoon, and the spatial and temporal distribution of monsoon. The low-pressure areas are not stationary, and rainfall is not solely convectional; it is a combination of orographic, cyclonic, and convectional rainfall.

Recent Concept about the Origin of Indian Monsoon
After world war second, the upper atmospheric circulation has been studied significantly. It is now believed that the differential heating of sea and land alone can’t produce monsoon circulation. Apart from it, the recent concept of monsoon rely heavily on the role of

• Himalayas and Tibetan plateau as a physical barrier and a source of high-level heat.
• Circulation of upper air jet streams in the troposphere.
• Existence of upper air circum-polar whirl over north and south poles in the troposphere.
• The occurrence of ENSO (El-Nino and Southern Oscillation) in the South Pacific ocean
• Walker cell in Indian Ocean.
• Indian Ocean Dipole

Role of the Himalayas and Tibetan Plateau

• In the 1970s, it was found that the Tibet plateau plays a crucial role in initiating the monsoon circulation. The plateau of Tibet extends over an area of about 4.5 million sq. km. The average height of these highlands is 4000 m. Due to its enormous height, it receives 2-3^oC more insolation than the neighboring areas. The heating of these areas leads to clockwise air circulation in the middle troposphere and two-wind streams originate from this area. One of these wind streams blows southward and develops into the tropical easterly jet stream (TEJ). The other stream blows in an opposite direction towards the North Pole and becomes the westerly jet stream over Central Asia.
  
  Role of Jet stream

Role of Jet Stream

• As already discussed, the sub-tropical westerly jet stream is bifurcated by the high-land Tibet in winters. The northward branch extends up to 20N-35N. Tropical easterly jet stream (TEJ), that branch off from anticyclone developed over Tibet, sometimes reaches to the tip of Peninsular India. Apart from this, Jet speed winds are also reported over other parts of Peninsular. This jet descends over the Indian Ocean and intensifies its high-pressure cell known as Mascarene High. It is from this high-pressure cell that the onshore winds start blowing towards the thermally induced low-pressure area, developed in the northern part of the Indian subcontinent. After crossing the equator such winds become south-westerly and are known as the southwesterly summer monsoon.

Role of ENSO

• The Indian monsoon is also influenced by EL-Nino, southern oscillation, and Somalian current. We know that El Nino is the reversal of normal conditions in the Pacific Ocean’s sea surface temperature. Though there is no direct correlation between bad monsoon and El Nino, but both are generally associated. There are years when India faced severe drought and those are not El -Nino years and vice-versa. Southern Oscillation is the see-saw pattern of atmospheric pressure between the eastern and western Pacific oceans. The oscillation has a period varying from 2-7 years. It is measured with Southern Oscillation Index (SOI) by measuring the pressure difference between two points in the Pacific Ocean (Tahiti and Darwin). A negative value of SOI implies high pressure over the north Indian Ocean during the winter season and a poor monsoon.
• The Somalian current changes its direction of flow after every six months. During the North-East Monsoon, the Somali Current flows to the south-west, while during the South-West Monsoon it is a major western boundary current, comparable with the Gulf Stream. Normally, there remains a low-pressure area along the eastern coast of Somalia. In exceptional years, after every six or seven years, the low-pressure area in the western Arabian Sea becomes a high-pressure area. Such a pressure reversal results into a weaker monsoon in India.
  
  Role of ENSO

Walker Cell

• It is observed that there is an east-west atmospheric circulation over the tropical oceanic regions. Such circulation in the Pacific Ocean is generally called walker cell. However, many scientists use the term ‘walker cell’ for all east-west circulations in different oceans. Walker cell is associated with southern oscillation and its strength fluctuates with that of the Southern Oscillation Index (SOI). With a high positive SOI, there would be a zone of low atmospheric pressure over Australia and the Indonesian archipelago. The rising air from this region deflects in the upper atmosphere in both directions towards Africa and South America. In the Indian Ocean, the air descends down at a high-pressure zone from where surface winds blow as Southwest monsoon towards the Indian sub-continent in summers. During La-Nina Indian ocean branch of the walker cell gets strengthened and surface winds are more intense. La-Nina condition is generally associated with good monsoon.
• During the appearance of El-Nino or negative SOI, the ascending branch of the Walker cell shifts to the central regions of the Pacific Ocean from the western pacific region (Figure 8). In a result, the Indian Ocean cell shifts towards the east. The surface winds or Southwest monsoon winds are weaker than normal conditions.

Indian Ocean Dipole

• The Indian Ocean Dipole (IOD) also known as the Indian Nino is a coupled Ocean-atmosphere phenomenon in the Indian Ocean. It is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia. The IOD Involves a periodic oscillation of sea-surface temperatures (SST), between “positive”, “neutral” and “negative” phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia. The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
• The IOD is one aspect of the general cycle of global climate, interacting with similar phenomena like the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean. Positive and negative IOD both have been seen coupled with La Nina. Thus, there is no direct correlation between IOD and ENSO.
• The IOD also affects the strength of monsoons over the Indian subcontinent. Positive IOD which is associated with warm sea-surface temperatures of the western Indian Ocean is favourable for monsoon.
  
  Indian Ocean Dipole

Nature of Indian Monsoon

Systematic studies of the causes of rainfall in the South Asian region help to understand the salient features of the monsoon, particularly some of its important aspects, such as:

• Onset and advance of monsoon
• Rain-bearing systems and the relationship between their frequency and distribution of
• monsoon rainfall.
• Break in the monsoon
• Retreat of the monsoon

Onset and Advance of Monsoon

• The primary cause of the monsoon is believed to be the differential heating of land and sea by many meteorologists. In May, the low pressure at the Intertropical Convergence Zone (ITCZ), located over northern India, becomes so strong that it pulls the trade winds from the southern hemisphere northwards. These southeast trade winds then cross the equator and enter the Bay of Bengal and the Arabian Sea, getting caught up in the air circulation over India. As they pass over the warm equatorial currents, the winds bring with them large amounts of moisture.
• With the ITCZ shifting northwards, an easterly jet stream develops over 15°N latitude. The rain during the southwest monsoon season starts quite abruptly, leading to a significant drop in temperature. This sudden onset of moisture-laden winds, accompanied by intense thunder and lightning, is often referred to as the "break" or "burst" of the monsoons.
• The southwest monsoon first reaches the Andaman-Nicobar Islands around May 15th, followed by the Kerala coast on June 1st. It then arrives in Mumbai and Kolkata between June 10th and 13th. By July 15th, the southwest monsoon has covered the entire Indian subcontinent.

Rain Bearing Systems and Distribution of Rainfall

• The southwest monsoon splits into two branches, the Arabian Sea Branch and the Bay of Bengal Branch near the southernmost end of the Indian Peninsula. Hence, it arrives in India in two branches: the Bay of Bengal branch and the Arabian Sea branch. First originated in the Bay of Bengal causing rainfall over the plains of north India. Second is the Arabian Sea current of the southwest monsoon which brings rain to the west coast of India. The latter extends toward a low-pressure area over the Thar Desert and is roughly three times stronger than the Bay of Bengal branch.
• The monsoon winds originating over the Arabian Sea further split into three branches:
  • One branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats and as a result of the orographic rainfall phenomenon, the windward side of Ghats receives very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.
  • Another branch of the Arabian Sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm of rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
  • A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kutch. It then passes over west Rajasthan and along the Aravallis, causing only a scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.
• The intensity of rainfall over the west coast of India is, however, related to two factors:
  • The offshore meteorological conditions.
  • The position of the equatorial jet stream along the eastern coast of Africa.
• The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from the south and southeast instead of from the south-westerly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India.
• One branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. Its sub-branch strikes the Garo and Khasi Hills of Meghalaya. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.
• The Tamil Nadu coast remains dry during this season because it is situated in the rainshadow area of the Arabian Sea branch of the southwest monsoon and lies parallel to the Bay of Bengal branch of the southwest monsoon.
• The frequency of tropical depressions originating over the Bay of Bengal varies from year to year. The path of these depressions also keeps changing with the position of the ITCZ, also known as the monsoon trough (Figure – position of Inter-tropical Convergence Zone (ITCZ) in the month of January and July). As the axis of the monsoon trough oscillates with the apparent movement of the sun between Tropic of Cancer and Tropic of Capricorn, there are fluctuations in the track and direction of these depressions, and the intensity and the amount of rainfall vary from year to year. The amount of rainfall in north India varies with the frequency of the tropical depressions. On average, one to three depressions are observed every month and the life span of one depression is about one week.
• The rain which comes in spells displays a declining trend from west to east over the west coast, and from the southeast towards the northwest over the North Indian Plain and the northern part of the Peninsula. Rajasthan desert receives low rainfall in spite of being in the path of the Arabian Sea branch of the monsoon. This branch blows parallel to the Aravalis mountain chain without obstruction and thus, does not release moisture here.

Break in the Monsoon

• During the southwest monsoon period after having rained for a few days, if rain fails to occur for one or more weeks, it is known as a break in the monsoon. These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:
  • In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
  • Over the west coast the dry spells are associated with days when winds blow parallel to the coast.

Retreating/Post Monsoons (The Transition Season)

• During October-November, with the apparent movement of the sun towards the south, the monsoon trough or the low-pressure trough over the northern plains becomes weaker. This is gradually replaced by a high-pressure system. The southwest monsoon winds weaken and start withdrawing gradually. By the beginning of October, the monsoon withdraws from the Northern Plains. The months of October November form a period of transition from hot rainy season to dry winter conditions. The retreat of the monsoon is marked by clear skies and a rise in temperature. While day temperatures are high, nights are cool and pleasant. The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October heat’. In the second half of October, the mercury begins to fall rapidly in northern India.
• The low-pressure conditions, over northwestern India, get transferred to the Bay of Bengal by early November. This shift is associated with the occurrence of cyclonic depressions, which originate over the Andaman Sea. These cyclones generally cross the eastern coasts of India cause heavy and widespread rain. These tropical cyclones are often very destructive. The thickly populated deltas of the Godavari, the Krishna, and the Kaveri are frequently struck by cyclones, which cause great damage to life and property. Sometimes, these cyclones arrive at the coasts of Orissa, West Bengal, and Bangladesh. The bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones.

Features of Monsoon Rainfall

• Monsoon rainfall is a seasonal phenomenon that typically occurs between June and September. Its spatial distribution is primarily influenced by the topography or relief of the land. For example, the windward side of the Western Ghats experiences over 250 cm of rainfall, while the heavy precipitation in the northeastern states can be attributed to their hill ranges and the Eastern Himalayas. Rainfall amounts can vary significantly, ranging from 20 cm in western Rajasthan to more than 400 cm in certain parts of the Western Ghats and North-East India.
• The amount of monsoon rainfall decreases as the distance from the sea increases. As one branch of the monsoon enters from the eastern side, rainfall diminishes from east to west in the plains. For instance, Kolkata receives 119 cm, Allahabad 76 cm, and Delhi 56 cm of rainfall.
• Breaks in rainfall are connected to cyclonic depressions that mainly form at the head of the Bay of Bengal and cross into the mainland. The frequency and intensity of these depressions, along with the paths they take, determine the spatial distribution of rainfall.
• Sometimes, the monsoon rains end earlier than expected, causing significant damage to crops and making it challenging to sow winter crops.

Monsoons and the Economic Life in India

• Monsoon is that axis around which revolves the entire agricultural cycle of India. It is because about 64 percent of people of India depend on agriculture for their livelihood and agriculture itself is based on the southwest monsoon.
• Except Himalayas all the parts of the country have temperatures above the threshold level to grow the crops or plants throughout the year.
• Regional variations in monsoon climate help in growing various types of crops.
• Agricultural prosperity of India depends very much on time and adequately distributed rainfall. If it fails, agriculture is adversely affected mainly in areas where irrigation is not developed.
• Sudden monsoon burst creates the problem of soil erosion over large areas in India.

Monsoon as unifying factors

• The monsoon season in India plays a crucial role in the country's economic life, as it has a significant impact on the agricultural sector. Agriculture is the backbone of the Indian economy, with a majority of the population relying on it for their livelihood. The southwest monsoon is responsible for providing the much-needed rainfall for crop cultivation, which in turn influences the overall agricultural output and economic growth.
• Regional variations in monsoon climate allow for the cultivation of diverse types of crops across the country. Areas with adequate rainfall and favorable climate conditions can grow a variety of crops such as rice, wheat, sugarcane, and cotton, while others may focus on specific crops suited to the local climate.
• The timely arrival and distribution of monsoon rains are critical for a successful agricultural season. If the monsoon is delayed or rainfall is unevenly distributed, it can result in crop failure, particularly in areas where irrigation facilities are not well-developed. This can have severe consequences for the farmers, who often depend on a single crop season for their income and sustenance.
• On the other hand, sudden and excessive monsoon rainfall can lead to problems such as soil erosion and flooding, which can damage crops and infrastructure. This can also have adverse effects on the agricultural sector and consequently, the economy.
• In addition to agriculture, the monsoon season also influences other sectors of the economy. For instance, the demand for electricity tends to increase during the monsoon months due to increased agricultural activities and the need for irrigation. Industries such as fertilizers, pesticides, and seeds also see a boost in demand during the monsoon season.
• Furthermore, the monsoon season affects consumer spending patterns in India. A good monsoon season typically results in higher agricultural output, leading to increased rural income and consumer spending on various goods and services. This can boost the overall economy and contribute to economic growth.
• In conclusion, the monsoon season plays a vital role in shaping the economic life in India. Its impact on agriculture, and in turn, various other sectors and consumer spending, makes it a critical factor in determining the country's overall economic performance. Therefore, understanding and managing the effects of monsoon on the Indian economy is essential for ensuring sustainable growth and development.

Conclusion

In conclusion, the Indian monsoon is a complex meteorological phenomenon that plays a crucial role in shaping the climate, agriculture, and economy of the Indian subcontinent. Originating from seasonal wind shifts and various atmospheric circulations, the monsoon unifies the region through its cyclical pattern of seasons and rainfall distribution. Despite its unpredictability and uneven distribution, the monsoon remains essential for agricultural activities, water supply, and the overall economic growth of the country. Understanding and managing the effects of the monsoon on the Indian economy and environment is vital for ensuring sustainable development and maintaining the intricate balance of the country's diverse ecosystems.

Frequently Asked Questions (FAQs) of Mechanism of Indian Monsoons & Rainfall Patterns

What is the origin of the term 'monsoon'?

The term 'monsoon' is thought to have originated from the Arabic word 'mawsim,' which means season. Monsoons are essentially seasonal winds that change direction based on the shift in seasons, making them periodic winds.

How does the Indian Monsoon affect the country's economy?

The Indian Monsoon plays a crucial role in the country's economy as it has a significant impact on the agricultural sector. A good monsoon season results in higher agricultural output, leading to increased rural income and consumer spending on various goods and services. This can boost the overall economy and contribute to economic growth.

What is the role of the Himalayas and the Tibetan Plateau in the Indian Monsoon?

The Tibetan Plateau and the Himalayas play a critical role in shaping monsoon circulation patterns due to their unique geographical features and location, which contribute to distinct wind streams and atmospheric circulations.

How does the Indian Ocean Dipole (IOD) influence the monsoons over the Indian subcontinent?

The Indian Ocean Dipole (IOD) is a climate phenomenon that occurs due to the interaction of ocean and atmosphere. It is characterized by the variation in sea surface temperatures between two regions or poles in the Indian Ocean. A positive IOD, which corresponds to warmer sea surface temperatures in the western Indian Ocean, is favorable for monsoon development.

What are the primary features of monsoon rainfall in India?

Monsoon rainfall in India is a seasonal phenomenon that typically occurs between June and September. Its spatial distribution is primarily influenced by the topography or relief of the land. The amount of monsoon rainfall decreases as the distance from the sea increases, and breaks in rainfall are connected to cyclonic depressions that mainly form at the head of the Bay of Bengal and cross into the mainland. The frequency and intensity of these depressions, along with the paths they take, determine the spatial distribution of rainfall.