Indian Monsoon

Table of Contents

The word ‘monsoon’ has been derived from the Arabic word ‘Mausim’ which means ‘season’. Originally, this word was used by Arab traders to describe a system of seasonal reversal of winds along the shores of the Indian Ocean.
Monsoons are especially prominent within the tropics on the eastern sides of the great landmass, but in Asia, it occurs outside the tropics in China, Korea and Japan.

Thermal Concept

Halley, a noted astronomer, hypothesized that the primary cause of the annual cycle of the Indian monsoon circulation was the differential heating effects of the land and the sea.
According to this concept, monsoons are the extended land breeze and sea breeze on a large scale.
During winter the huge landmass of Asia cools more rapidly than the surrounding oceans with the result that a strong high pressure center develops over the continent.
On the other hand, the pressure over adjacent oceans is relatively lower. As a consequence the pressure- gradient is directed from land to sea.
Therefore there is an outflow of air from the continental landmass towards the adjacent oceans so that it brings cold, dry air towards the low latitudes.
In summer the temperature and pressure conditions are reversed. Now, the huge landmass of Asia heats quickly and develops a strong low pressure center.
Moreover, the pole-ward shift of the Intertropical Convergence Zone (ITCZ) to a position over Southern Asia reinforces the thermally induced low pressure center.
The pressure over the adjacent oceans being high, a sea-to-land pressure gradient is established. The surface air flow is, therefore, from the highs over the oceans towards the lows over the heated land. The air that is attracted into the center’s Low pressure from over the oceans is warm and moist.

Halley’s conc₹ept is criticized on following lines:

It fails to explain the intricacies of monsoon such as sudden burst of monsoon, breaks in monsoon, spatial and temporal distribution of monsoon.
The low pressure areas are not stationary. The rainfall is not only conventional but a mix of orographic, cyclonic and convectional rainfall.

Recent Concept about the Origin of Indian Monsoon: 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 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 2-3oC more insolation than the neighboring areas.
The TEJ rapidly forms after the Somali Jet has shifted northward of the Himalayas in early June.
Flowing from east to west at altitudes of 6-9 km, the TEJ influences peninsular India and the Northern African region.
The formation of the TEJ causes a reversal in upper air circulation patterns, switching from high pressure to low pressure, which triggers the onset of monsoons.
Recent observations indicate that the heating intensity and duration of the Tibetan Plateau directly impact India’s monsoon rainfall.
When the temperature over Tibet remains consistently high, it strengthens the easterly jet, leading to heavy rainfall in India.
The easterly jet does not form if the snow on the Tibetan Plateau does not melt, thereby hindering rainfall in India.
Therefore, years with thick and widespread snow over Tibet result in weak monsoons and less rainfall in subsequent years.

**Fig: – Tibet anticyclone and Easterly Jet stream**

Role of Jet Stream

The subtropical westerly jet stream is bifurcated by the high-land Tibet in winters.
Northward branch extends up to 20oN-35oN .
Tropical easterly jet stream (TEJ), that branches 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 south-westerly summer monsoon.
There are significant low-level winds called low-level jets (LLJs) in the lower troposphere.
In the tropical regions, the prominent LLJs are the Somali Jet and the African Easterly Jet (Tropical Easterly Jet).
The Tropical Easterly Jet (TEJ) is a dominant feature during the northern hemisphere summer, mainly over southern Asia and northern Africa, located between 5° and 20°N.
The TEJ exhibits persistence in its direction and intensity from June to early October, with its position fluctuating between 5° and 20°N.
The TEJ rapidly forms after the Somali Jet has shifted northward of the Himalayas in early June.
Flowing from east to west at altitudes of 6-9 km, the TEJ influences peninsular India and the Northern African region.

Role of ENSO

The Indian monsoon is also influenced by EL-Nino, southern oscillation and Somali 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, 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 Ocean.
The oscillation has a period varying from 2-7 years. It is measured with the 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.

Role of Somali Jet

The Somali 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 is a major western boundary current, comparable with the Gulf Stream.
In exceptional years, after every six or seven years, the low pressure area in western Arabian Sea becomes a high pressure area. Such a pressure reversal results in a weaker monsoon in India.
The polar and subtropical jet streams are permanent high-speed winds that have a significant influence on the weather patterns of temperate regions.
Temporary jet streams, including the Somali Jet and the African Easterly Jet (Tropical Easterly Jet), are narrow wind currents found in the upper, middle, and sometimes lower troposphere.
These jet streams, especially the Somali Jet and the African Easterly Jet, play a crucial role in the formation and progression of the Indian Monsoons.
The onset of the Somali Jet, which passes through Kenya, Somalia, and Sahel, greatly facilitates the southwest monsoon’s progress towards India.
The Somali Jet was observed to originate from Mauritius and the northern part of Madagascar before reaching the Kenyan coast around 3º S.
It strengthens the permanent high-pressure system near Madagascar and aids in driving the southwest monsoons towards India with greater speed and intensity.
The low-level jet’s significance lies in its path around 9º N, coinciding with a zone of coastal upwelling.
The strong winds of the low-level jet drive surface coastal waters towards the east, allowing cold water from the deep sea to rise, maintaining mass continuity.
A distinctive feature of the Somali Current is its reversal in direction with the onset of the summer monsoon.
During winter, the current flows from north to south, running southwards from the Arabian coast to the East African coastline. However, with the arrival of the summer monsoon, it reverses its direction and flows from south to north.

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 the walker cell.
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 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 subcontinent in summers.
During the La-Nina Indian ocean branch of Walker cells get 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 the central regions of the Pacific Ocean from the west pacific region .
As 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.
The development of the IOD starts in the equatorial region of the Indian Ocean in April to May and peaks in October.
In a positive IOD, winds over the Indian Ocean blow from east to west, resulting in a warmer Arabian Sea and colder, drier conditions in the eastern Indian Ocean around Indonesia.
Conversely, in a negative IOD, the opposite occurs, with Indonesia experiencing warmer and rainier conditions.
A positive IOD is beneficial for Indian monsoons as it leads to increased evaporation from warm water.
The atmospheric component of the IOD is referred to as the Equatorial Indian Ocean Oscillation (EQUINOO), which involves the oscillation of pressure cells between the Bay of Bengal and Arabian Sea.
During the positive phase of EQUINOO, enhanced cloud formation and rainfall are observed in the western part of the equatorial ocean near the African coast, while such activity is suppressed near Sumatra.
Although EQUINOO and IOD often go hand in hand during strong positive IOD events, their synchronization is not always consistent.

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 western Indian Ocean is favorable for monsoon in the Indian subcontinent.

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 differential heating of land and sea is still believed to be the primary cause of the monsoon by many meteorologists.
Low pressure at ITCZ which is located over north India in the month of May becomes so intense that it pulls the trade winds of the southern hemisphere northwards.
These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea, only to be caught up in the air circulation over India.
Passing over the equatorial warm currents, they bring with them moisture in abundance. With the northwards shift of ITCZ, an easterly jet stream develops over 15oN.
The rain in the south-west monsoon season begins rather abruptly. One result of the first rain is that it brings down the temperature substantially. This sudden onset of the moisture-laden winds associated with violent thunder and lightning, is often termed as the “break” or “burst” of the monsoons.
Southwest monsoon first of all reaches the Andaman-Nicobar Islands on 15th May. Kerala coast receives it on 1st June. It reaches Mumbai and Kolkata between 10th and 13th June. By 15th of In July, the Southwest monsoon covers the whole of India

FAQs

Q. What is the duration of the monsoon season in India?

Ans. The monsoon season in India typically spans from June to September, during which the country experiences significant rainfall essential for agriculture and the overall ecosystem.

Q. How does the monsoon season impact various regions of India?

Ans. The monsoon season brings vital rainfall to different parts of India, contributing to the replenishment of water bodies, agricultural activities, and the sustenance of the country’s diverse flora and fauna.

Q. What is the current status of the Indian monsoon?

Ans. The Indian monsoon’s current status can be accessed through the India Meteorological Department (IMD) or other weather forecasting agencies, providing real-time updates on rainfall patterns, weather conditions, and any potential impact on different regions.

Q. How does the Indian monsoon current affect everyday life in the country?

Ans. Updates on the Indian monsoon current play a crucial role in enabling people to plan their activities, particularly in sectors like agriculture, tourism, and disaster management, by anticipating the impact of rainfall and potential weather-related challenges.

Q. What are the characteristics of the monsoon winds in India?

Ans. The Indian monsoon winds consist of the southwest monsoon winds, which bring the primary rainfall during the summer season, and the northeast monsoon winds, responsible for precipitation in some regions during the winter season.

Q. How do the India monsoon winds influence the country’s climate and agriculture?

Ans. The interplay of the monsoon winds in India significantly shapes the country’s climate, determining the timing, duration, and intensity of the rainy and dry seasons, thereby crucially impacting agricultural practices and crop cultivation.

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Indian Monsoon – UPSC Indian Geography Notes