Factors influencing the onset of SW monsoons:

1. Intense low-pressure formation over the Tibetan Plateau

2. The permanent high-pressure cell in the South of the Indian Ocean
3. Subtropical jet stream
4. African Easterly jet (Tropical easterly jet)
5. Inter-Tropical Convergence Zone (ITCZ)
Factors influencing intensity of SW monsoons:

1. Strengths of the low pressure over Tibetan plateau and the high pressure over the south
Indian Ocean
2. Somali Jet
3. Somali Current
4. Indian Ocean dipole
5. Indian Ocean branch of the Walker Cell

Branches of Indian Monsoon

Arabian Sea branch

o It is sometimes referred to as the South-West monsoon. This monsoon trough travels

around the Arabian Sea.
o It moves quickly and arrives in the country’s northwestern and north-eastern regions in the
first week of June.
o The Inter-Tropical Convergence Zone is a low-pressure zone in the equator where trade
winds converge, and it is found about 20-25 degrees latitude in June (known as monsoon
trough).
o As a result of this change, southern hemisphere trade winds blow from southwest to
northwest and northeast. In the plains, these trade winds bring south west monsoon.

Bay of Bengal Branch

o It is sometimes referred to as the South-East Monsoon. This branch of monsoon flows

along the Bay of Bengal.
o It moves quickly and arrives in the country’s north-eastern region in the first week of June.
o The heating of the continent creates low pressure, which attracts southeast trade winds from
the high-pressure area of the seas.
o Monsoon winds move from a high-pressure area to a low-pressure area. A south-east
monsoon from the Bay of Bengal is expected.
o In the Northern Plains, this combines with southwest trade winds.
What Causes the Monsoon Season?

o Differential Heating: The monsoon season is primarily caused by differential heating

between land and sea.
o Summer Heating: During summer, land surfaces heat up more quickly than oceans, creating
a temperature difference.
o Low-Pressure Area: As land heats up, low-pressure areas form, drawing moist air from
oceans towards the land.
o Wind Shift: The pressure difference causes a shift in wind patterns, with moist air being
pulled from the Indian Ocean.
o Moisture Laden Air: The onshore winds bring moisture-laden air, leading to abundant
rainfall over the affected regions.
o Monsoon Trough: The shift in wind patterns establishes a monsoon trough, a region of low
pressure supporting sustained rainfall.
o Seasonal Reversal: The monsoon season exhibits a seasonal reversal of wind direction,
impacting various parts of the world.
o Importance for Agriculture: The monsoon is crucial for agriculture, providing essential
water for crops in affected regions.

Characteristics of Indian Monsoon

o The Indian Monsoon is marked by a dramatic change in wind patterns, with moist winds
from the southwest in summer and dry winds from the northeast in winter.
o The southwest monsoon brings significant rainfall, crucial for agriculture, filling rivers,
and replenishing groundwater.
o There is substantial variation in monsoon onset and intensity across different regions of
India, with the Western Ghats and northeastern states receiving the highest rainfall.
o The monsoon is driven by the temperature differential between the Indian Ocean and the
Indian landmass, leading to the movement of air masses.
o The monsoon typically arrives in early June in Kerala and retreats from northern India by
September.
o The Indian economy, particularly agriculture, is heavily dependent on the monsoon rains
for crop production.
o Periods of reduced rainfall, known as breaks in the monsoon, can significantly impact water
availability and agricultural productivity.
o Events like El Niño and La Niña can alter the monsoon's pattern and intensity, causing
either droughts or excessive rainfall.

Factors affecting Indian Monsoon

Coriolis Force

o The Coriolis Force is a fictitious force that exists only due to the spinning action of the
Earth.
o This force is also responsible for the rotational motion observed in tropical cyclones.
 o As a result, the monsoon winds are diverted eastwards and now blow from the south-west
to the north-east.
o Because the rotating motion of the Earth is constant, the Coriolis Force experienced by a
particular parcel of air with a specific velocity at a certain latitude is likewise constant.

Mascarene High

o The Mascarene High is regarded as the driving force behind the southwest monsoon winds
in the Indian subcontinent.
o This high-pressure area often forms by mid-April, and its strength is an essential element
in determining the severity of the monsoon in India.
o A stronger high pressure will result in greater winds or a stronger monsoon current.
o If the development of Mascarene High is delayed, it is possible that the onset of monsoon
in India may be delayed as well.

Indian Summer

o Winds move from places of high pressure to areas of low pressure.

o The Himalayan range plays an important role in summer heating by preventing cold air
from the north from entering and allowing warmth to occur.
o During this season, the country of India becomes extremely hot. Temperatures in the oceans
surrounding the nation are also rising.
o We notice a gradient between air pressure over the sea and that over India as a result of the
various heating rates and capacities of air over the sea and that over the land (especially
Rajasthan).
o The air pressure over India is lower than the air pressure over the southern Arabian Sea and
the Bay of Bengal. The monsoon winds are drawn in by this method.

Indian Ocean Dipole

o N H Saji of Japan’s University of Aizu and colleagues found an ENSO-like phenomena in

the Indian Ocean in 1999 and dubbed it the Indian Ocean Dipole (IOD).
o IOD, like ENSO, has three phases: positive, negative, and neutral. During the positive
phase of IOD, sea surface temperatures in the western Indian Ocean are warmer than those
in the eastern Indian Ocean.
o The opposite occurs during the IOD negative phase, and no gradient is noticed during the
IOD neutral period.
o It has been found that during the positive IOD phase, the Indian summer monsoon rainfall
is significantly higher than during the negative IOD period.

El Nino

o This is the term used to describe the occasional emergence of a warm ocean current along
Peru’s coast as a temporary substitute for the cold Peruvian current.
o ‘El Nino’ is a Spanish term that means ‘the kid,’ and it relates to the infant Christ, as this
stream begins to flow during the Christmas season.
 o El Nino causes a rise in sea-surface temperatures and a decrease of the trade winds in the
region.

El Nino Southern Oscillation

o The oscillatory phase of sea surface temperatures around the equatorial Pacific Ocean is
referred to as the El Nino Southern Oscillation (ENSO).
o For a long time, forecasters have struggled with ENSO because of its mainly unexpected
character.
o Because it impacts air circulation, ENSO has been a driver of worldwide weather
(especially in nations bordering the Pacific Ocean).

Intertropical Convergence Zone(ITCZ)

o In equatorial latitudes, the Inter Tropical Convergence Zone (ITCZ) is a large trough of
low pressure.
o This is where the trade winds from the northeast and southeast meet.
o This convergence zone is roughly parallel to the equator, although it travels north and south
with the apparent movement of the sun.
o The strength and location of this high-pressure system have an impact on the Indian
Monsoon.

Tropical Easterly Jet

o The TEJ is critical in kicking off the south west monsoon.

o This jet falls over the Indian Ocean (around Madagascar) and deepens its high pressure
cell, causing a south-west monsoon to form.
o When the summer temperature of the air over Tibet remains high for an extended period of
time, it aids in the development of the easterly jet, resulting in heavy rainfall in India.
o If the snow on the Tibet Plateau does not melt, the easterly jet does not form. This has a
negative impact on the incidence of rainfall in India.
o As a result, every year with heavy and widespread snow in Tibet will be followed by a year
with a weak monsoon and less rainfall.

Mechanism of Indian Monsoon

Onset of the South-West Monsoon

Source: IMD

o With the apparent movement of the Sun, the position of the ITCZ varies north and south of
the equator.
o The sun shines vertically over the Tropic of Cancer in June, while the ITCZ moves
northward.
o Under the influence of Coriolis force, the southern hemisphere’s southeast trade winds
cross the equator and begin blowing in a southwest to northeast direction.
o As they pass over the warm Indian Ocean, these winds pick up moisture.
o The ITCZ changes to 20°-25° N latitude in July and is positioned in the Indo-Gangetic
Plain, while the south-west monsoons blow from the Arabian Sea and the Bay of Bengal.
The ITCZ at this position is commonly referred to as the Monsoon Trough.
 o The phenomena of the withdrawal of the westerly jet stream from its location over the north
Indian plain, south of the Himalayas, is also connected to the shift in the position of the
ITCZ.
o The relief and thermal low pressure over northwest India change the direction of these
winds as they reach the land.
o The monsoon enters the Indian mainland through two channels: the Arabian Sea Branch
and the Bay of Bengal branch.

Retreating Monsoon Season

o Clear sky and an increase in temperature signal the end of the southwest monsoon season.
o The ground is still wet. The weather becomes unpleasant as a result of the high temperature
and humidity. This is usually referred to as the ‘October heat.’
o The temperature begins to drop significantly in the second half of October, particularly in
northern India.
o The weather during the receding monsoon is dry in north India, but rainy in the eastern
section of the Peninsula.
o The rainiest months in this region are October and November.
o The extensive rain during this season is caused by the passage of cyclonic depressions that
start over the Andaman Sea and manage to traverse the southern Peninsula’s eastern coast.
These tropical cyclones are extremely dangerous.
o These depressions and cyclones are responsible for the majority of the rainfall on the
Coromandel Coast.
o In contrast to the rest of the country, which receives rain during the south west monsoon
season from June to September, the north east monsoon is critical for agricultural and water
security in the south.

Effects of Indian Monsoon

Positive Effects

o Agriculture provides a living for around 64% of the population in India, and agriculture is
dependent on the monsoon.
o India’s agricultural success is heavily reliant on timely and evenly distributed rainfall.
Agriculture suffers if it fails, especially in areas where irrigation systems have not been
established.
o Regional differences in monsoon climate aid in the cultivation of a variety of crops.
o In India, regional monsoon variance is expressed in a wide range of food, clothing, and
housing styles.
o Monsoon rain serves to replenish dams and reservoirs, which are then utilised to generate
hydroelectric power.
o Winter rains from temperate cyclones in north India are extremely beneficial to Rabi crops.

Negative Effects
 o Every year, the variability of rainfall causes droughts or floods in various sections of the
nation.
o Soil erosion is an issue across huge parts of India as a result of a sudden monsoon burst.
o Sudden rainfall causes landslides in steep locations, causing damage to natural and physical
infrastructure and interrupting human life economically and socially.

Impacts of El Niño on the Indian Monsoon:

1. Reduced Rainfall: El Niño typically weakens the Indian monsoon, leading to below-
average rainfall. This can result in dry spells or droughts, particularly in central, northern,
and western India.
2. Drought Conditions: Many El Niño years are associated with severe droughts in India.
For example, major droughts were recorded during strong El Niño events, such as in 1982,
1987, and 2015.
3. Delayed Monsoon Onset: The arrival of the monsoon over India can be delayed, which
affects the sowing season for crops like rice, cotton, and pulses. A late start can shorten the
growing season, reducing yields.
4. Impact on Agriculture: Since agriculture in India heavily relies on monsoon rainfall, a
weakened or delayed monsoon can lead to crop failures, particularly in rain-fed areas,
which can increase food prices and create stress on the economy.
5. Water Scarcity: Reduced rainfall can strain water resources, affecting irrigation, drinking
water supply, and hydropower generation, which depends on monsoon-fed reservoirs.
6. Economic Effects: A poor monsoon can result in lower agricultural output, which impacts
India's overall economy, as agriculture supports a significant portion of the population and
GDP.

Impacts of La Niña on the Indian Monsoon:

1. Enhanced Monsoon Rainfall: During La Niña, the sea surface temperatures in the central
and eastern Pacific Ocean are cooler than normal. This strengthens the trade winds and
enhances the flow of moist air from the Indian Ocean toward the Indian subcontinent,
resulting in more intense and prolonged monsoon rains.
2. Improved Agricultural Output: The increased rainfall can boost the agricultural sector
in India, as the monsoon is crucial for farming, especially for rain-fed crops such as rice,
pulses, and sugarcane. A stronger monsoon season typically improves water availability
for irrigation and increases crop yields.
3. Risk of Flooding: While La Niña can bring beneficial rains, it can also lead to excessive
rainfall, resulting in floods in various parts of India. Flooding can cause destruction to
infrastructure, displacement of people, and loss of crops, especially in areas like eastern
and northeastern India.
4. Regional Disparities: Although La Niña often leads to overall above-average rainfall
across the country, the distribution may not be even. Some regions may experience heavy
rainfall and floods, while others may face relatively less impact.
5. Temperature Effects: La Niña can lead to cooler-than-average temperatures during the
monsoon season due to the increased cloud cover and rainfall, helping reduce the heat stress
often experienced in the pre-monsoon months.