Monsoons

The word “monsoon” is derived from the Arabic word ‘mausim’ for
season, and the distinguishing attribute of the monsoonal regions of
the world is considered to be the seasonal reversal in the direction of
the wind. The monsoonal region delineated on the basis of significant
change in the wind direction between winter and summer (with the
direction of the prevailing wind within each season being reasonably
steady) extends over a large part of the tropics, namely, 25◦S to 35◦N,
30◦W to 170◦E.
Dr. Priyanka Puri, Associate Professor, Miranda House
 Monsoons

• Monsoonal Rainfall Regions

• Onset of South West monsoon
• Factors favouring advancement
• Characteristics of monsoon rainfall
• Significance of monsoons
• Retreat of monsoon- North East
Monsoon
• Monsoon climate is generally related to those areas which register
complete seasonal reversal of wind direction and are associated with
tropical deciduous forests. However, there are some departures from
this close relationship also.
• The area of monsoon climate are divided into the following categories:
• True monsoon regions- These include India, Burma, Pakistan,
Bangladesh, Thailand, Cambodia, Laos, North and South Vietnam,
southern China, Phillippines, and north coastal area of Australia.
• Areas of monsoonal tendencies or psuedo monsoon areas- These are
found along the south west coast of Africa including the coasts of
Guinea, Sierra Leone, Liberia and Ivory Coast; eastern Africa and
western Madagascar.
• Area of monsoon effects- These include north east coast of Latin
America;, eg.
Monsoonal Rainfall Regions
East Venezuela, Guyana, Surinam. French Guyana, and north east Brazil. Besides,
Puerto Rico and Dominican Republic also have mild monsoon.
• Area of modified monsoon- These are found in parts of central America and south
east USA.
• The major monsoon systems of the world consist of the West African and Asia-
Australian monsoons. The inclusion of the North and South American monsoons with
incomplete wind reversal has, however, been debated. There are generally two
versions of a tropical monsoon climate:

• The first version of the tropical monsoon climate features wet and dry seasons, with
less pronounced dry seasons. Regions with this variation of the tropical monsoon
climate typically sees copious amount of rain during the wet season(s), usually in the
form of frequent thunderstorms. However, unlike most tropical savanna climates, a
sizeable amount of precipitation also falls during the dry season(s). In essence, this
version of the tropical monsoon climate generally has less pronounced dry seasons
than tropical savanna climates.
• The second version of the tropical monsoon climate features wet
and dry seasons, with extraordinarily rainy wet seasons and
pronounced dry seasons. This variation features pronounced dry
seasons similar in length and character to dry seasons observed in
tropical savanna climates. However, this is followed by a sustained
period (or sustained periods) of extraordinary rainfall. In some
instances, up to (and sometimes in excess of) 1,000 mm of
precipitation is observed per month for two or more consecutive
months. Tropical savanna climates generally do not see this level of
sustained rainfall.
Monsoonal Rainfall- Origin and Mechanism
• ‘Monsoons' is derived from the Arabic word “mausim” which means season
and it is a Weather Phenomenon which is common to Asia. Monsoons are
defined as winds that change their direction with the change of seasons.

• Asian Monsoons
• The prevalence of monsoons in this region can be
accounted by the fact that the northern hemisphere has
more land surfaces than the southern hemisphere.
• The air above the land gets heated faster than water and
rises so that moisture-laden cold air from the ocean
rushes in to take its place.
• Monsoons which are quite common and the strongest
are the ones which appear in South-east Asia between
the months of June and September. But it is always not
possible to predict the exact time of its arrival. The
countries which are most affected by the monsoons are
the countries of the Indian peninsula.
 Concept of the Origin of Monsoon
• The origin of monsoons is attributed to the thermal and dynamic factors. The
concepts are hence divided as thermal and dynamic and there are recent concepts
which have forwarded an explanation of monsoon circulation.
• Thermal concept- The contribution of propounding this concept goes to Edmund
Halley in 1686. According to this concept, the monsoons are a result of
heterogeneous character of the globe with regards to the unequal distribution of land
and sea. This causes a differential heating and cooling pattern which is particularly
observed during the seasons. Following observations can be made:
• During northern winter or winter solstice, the sun is vertical over the Tropic of
Capricorn in the southern hemisphere, high pressure areas are developed over Asia
due to very low temperature. Two main high pressure areas that are observed are
near Baykal Lake and near Peshawar. On the other hand, a low pressure centre is
developed in the southern Indian Ocean due to summer season and related high
temperature in the Ocean in the southern hemisphere.
• As a result, winds blow from high pressure land areas to the low pressure ocean
areas. These are called north east monsoons or winter monsoons. These are dry
winds as they are form the land.
• A reversal is observed in these conditions at the time of summer
solstice when the sun is vertical over the Tropic of Cancer in the
northern hemisphere. Due to high temperature low pressure centres
are developed over the land and wind starts to flow from the oceans
to the land. High pressure is particularly observed over the Indian
Ocean and to the north of Australia and south of Japan.

• Consequently, winds blow form the Indian Ocean to the Asian

continent. These winds while crossing the equator turn south
westerly due to Ferrel’s law. These winds pick up moisture while
passing over the ocean and give heavy rainfall on obstruction.
These are called south west monsoons or summer monsoons.
• Dynamic concept- This concept is based on the
criticism of the thermal concept on the following
grounds:

• The ‘lows’ that are developed over the land are caused
by temperature variations, then they should remain
stationary. But this is not the case. In fact, they
suddenly shift. Thus, it was put forward that the low
pressure centres are not related to thermal conditions,
but represent cyclonic lows associated with south west
monsoon.
• It is also put forward that the rain producing capacity of
the monsoon winds is limited and this rainfall can only
be associated with tropical disturbances. Such upper
air winds change their direction seasonally and are
called ‘upper air monsoon’ or ‘aerological monsoon’.
• The dynamic concept hence rejects these proposals and puts forward that
monsoons originate due to the shifting of pressure and wind belts. This
concept suggests that tropical convergence is formed due to convergence
of north east and south east trade winds near the equator. This is called
Inter Tropical Convergence (ITC). Its northern and southern boundaries are
called NITC and SITC respectively.

• Further, there is a belt of doldrum within the ITC characterised by equatorial

westerlies. At the time of summer solstice (June 21) when the sun rays
becomes vertical over the Tropic of Cancer, NITC is extended upto 30
degree N latitude covering south and south east Asia. Hence, equatorial
westerlies are established over this area. These are termed to become
south west or summer monsoons. The NITC is also associated with
numerous atmospheric storms from July to September.
• H. Flohn of the German Weather Bureau, while rejecting the
classical theory of origin of monsoons suggested that the
tropical monsoon of tropical Asia is simply a modification of
the planetary winds of the tropics. He thinks of the thermal low
of northern India and the accompanying monsoon as simply
an unusually great northward displacement of the Northern
Inter-Tropical Convergence Zone (NITCZ).
• Similarly, the north east monsoon does not originate due to
low pressure in the southern hemisphere. according to this
concept. These are put forward as north east trade winds that
are established over south and south east Asia during
northern winter. Since they come from land, they are dry.
• In general terms, the word monsoon is used to connote a seasonal wind,
which blows from another direction for the rest of the year. Such seasonal
changes of wind are primarily the result of differences in the quantity of heat
received from the sun by different parts of the earth.

Jet Stream Theory and other concepts:

Jet stream is a band of fast moving air from west to east usually found in the middle
latitudes in the upper troposphere at a height of about 12 km. M.T. Yin (1949),
while discussing the origin of the monsoons expressed the opinion that the burst
of monsoon depends upon the upper air circulation. Some jet streams are
observed to influence the Indian monsoon. These are:
a. Tropical easterly jet stream : The south-west monsoon in southern Asia is
overlain by strong upper easterlies with a pronounced jet at 100 to 200 mb. These
easterly winds, which often record speeds exceeding 100 knot are known as the
Easterly Jet Stream of the tropics.
• Recent observations have revealed that the intensity and duration of heating of Tibet
Plateau has a direct bearing on the amount of rainfall in India by the monsoons. When
the summer temperature of air over Tibet remains high for a sufficiently long time, it
helps in strengthening the easterly jet and results in heavy rainfall in India.
• Sub tropical westerly jet stream:
• There are different jet streams and in respect of the climate and monsoons of
India it is the Subtropical Jet Stream (STJ) and the countering easterly jet that
are most important. As the summertime approaches there is increased solar
heating of the Indian subcontinent. During the summer months the STJ
deflects northwards and crosses over the Himalayan Range. With the STJ out
of the way the subcontinental monsoon cell develops very quickly indeed,
often in a matter of a few days.
• Warmth and moisture are fed into the cell by a lower level tropical jet stream
which brings with it air masses laden with moisture from the Indian Ocean.
The end of the monsoon season is brought about when the atmosphere over
the Tibetan Plateau begins to cool, this enables the STJ to transition back
across the Himalayas. This leads to the formation of a cyclonic winter
monsoon cell typified by sinking air masses over India and relatively moisture
free winds that blow seaward. This gives rise to relatively settled and dry
weather over India during the winter months.
• On reaching the southern periphery of the Himalayas, this branch is
deflected westwards. As a consequence its further progress is towards the
Gangetic Plains of India rather than towards Burma. The arrival of monsoon
in Kolkata is slightly earlier than Mumbai. By mid June, the Arabian Sea
branch spreads over Suarashtra-Kutch and central parts of the country.
After this process, both the branches merge into a single current. The
remaining parts of west UP, Haryana, eastern half of Rajasthan experience
first monsoon showers by the first week of July. By mid- July, the monsoon
extends into Kashmir and the remaining parts of the country as a feeble
current.
• Somali Jet:

The monsoon wind that is deflected to the north as it crosses the equator
is further deflected to the east by the mountains of Africa. The progress
of the southwest monsoon towards India is greatly aided by the onset of
certain jet streams including the crucial Somali jet that transits Kenya,
Somalia and Sahel and exits the African coast at 9 degrees north at low
level and very fast. J. Findlater, a British meteorologist observed this low
level jet stream was found to be most pronounced between 1.0 and 1.5
km above the ground. It is also seen as a major interaction between the
ocean currents and the
Atmosphere.
Tropical Easterly Jet (TEJ)
• The TEJ is a unique and dominant feature of the northern
hemispheric summer over southern Asia and northern Africa.
The TEJ is found near between 5° and 20°N.
• TEJ comes into existence quickly after the STJ has shifted to
the north of the Himalayas. The formation of TEJ results in
the reversal of upper air circulation patterns [High pressure
switches to low pressure] and leads to the quick onset of
monsoons.
 Tropical Easterly Jet Stream

• There are major high velocity winds in the lower

troposphere called low-level jets (LLJs). In the tropics, the
most prominent of these are the Somali Jet and the African
Easterly Jet [Tropical Easterly Jet]. The TEJ is a unique and
dominant feature of the northern hemispheric summer
over southern Asia and northern Africa. The TEJ is found
near between 5° and 20°N.
• It is fairly persistent in its direction, and intensity from June
through the beginning of October. It’s position fluctuates
between 5° and 20°N. TEJ comes into existence quickly after
the STJ has shifted to the north of the Himalayas (Early
June). TEJ flows from east to west over peninsular India at 6
– 9 km and over the Northern African region.
• The formation of TEJ results in the reversal of upper air circulation
patterns [High pressure switches to low pressure] and leads to the quick
onset of monsoons. Recent observations have revealed that the intensity
and duration of heating of Tibetan Plateau has a direct bearing on the
amount of rainfall in India by the monsoons. When the summer
temperature of air over Tibet remains high for a sufficiently long time, it
helps in strengthening the easterly jet and results in heavy rainfall in India.

• The easterly jet does not come into existence if the snow over the Tibet
Plateau does not melt. This hampers the occurrence of rainfall in India.
Therefore, any year of thick and widespread snow over Tibet will be
followed by a year of weak monsoon and less rainfall.
 Factors Affecting Monsoon
Following factors affect the advancement of
monsoonal rainfall:
1. Thermal effect
2. Effect of shifting of ITCZ
3. Jet streams

All explained earlier

• 4. Tropical Cyclones : The easterly jet stream steers the tropical
depressions into India. These depressions play a significant role in the
distribution of monsoon rainfall over the Indian subcontinent. The tracks of
these depressions are the areas of highest rainfall in India. The frequency at
which these depressions visit India, their direction and intensity, all go a
long way in determining the rainfall pattern during the southwest monsoon
period. Further, in the Arabian Sea experiences meso scale cyclonic
vortices in north south direction.
5. Indian Ocean Dipole: Indian ocean Dipole is a recently
discovered phenomena that has a significant influence on
Indian monsoons.
• Indian ocean Dipole is a SST anomaly (Sea Surface
Temperature Anomaly – different from normal) that
occurs occasionally in Northern or Equatorial Indian
Ocean Region (IOR).
• With a positive IOD winds over the Indian Ocean blow
from east to west (from Bay of Bengal towards Arabian
Sea). This results in the Arabian Sea (western Indian
Ocean near African Coast) being much warmer and
eastern Indian Ocean around Indonesia becoming colder
and dry.
• In the negative dipole year, reverse happens making
Indonesia much warmer and rainier.
6. Physiography- The physical features of the Indian subcontinent favour the advancement of
the Indian monsoon.
• First is the presence of abundant water bodies around the subcontinent - Arabian Sea, Bay
of Bengal and Indian Ocean. These help in accumulation of moisture in the winds during
the hot season.
• Second is the presence of abundant highlands like the Western Ghats and the Himalayas
right across the path of the SW Monsoon winds. These are the main cause of the
substantial orographic precipitation all over the Indian subcontinent.
– The Western Ghats are the first highlands of India that the SW Monsoon winds
encounter. The Western Ghats rise very abruptly from the Western Coastal Plains of
the subcontinent making effective orographic barriers for the Monsoon winds.
– The Himalayas and the Tibetan plateau play more than the role of just the orographic
barriers for Monsoon. They help in its confinement onto the subcontinent. Without it,
the SW Monsoon winds would blow right over the Indian subcontinent into China,
Afghanistan and Russia without causing any rain.
– For NE Monsoon, the highlands of Eastern Ghats play the role of orographic barrier.
• 7. Near equatorial trough: Before the onset of the summer
monsoon over India, a low-pressure zone forms on either side of the
equator, roughly along 5º N and 5º S. Meteorologists refer to this as
an equatorial double trough. The double trough is frequently
observed in satellite observations of clouds. Prior to the onset of the
monsoon, the near equatorial trough north of 5º N weakens, but the
trough near 5º S remains fairly active.

8. Mascarenes High: In the southern hemisphere, off the coast of

Madagascar there arises a zone of high pressure which is referred
as the 'Mascarenes High. Around this Mascarenes High an
anticyclonic air circulation becomes predominant.
• 9.The Tibetan High
 •
10. EI-Nino and the Indian Monsoon
• EI-Nino is a complex weather system that appears once every three to seven years,
bringing drought, floods and other weather extremes to different parts of the world.

• The system involves oceanic and atmospheric phenomena with the appearance of
warm currents off the coast of Peru in the Eastern Pacific and affects weather in
many places including India. EI-Nino is merely an extension of the warm equatorial
current which gets replaced temporarily by cold Peruvian current or Humboldt
current.
This current increases the temperature of water on the Peruvian coast by 10°C.
The word EI-Nino means ‘Child Christ’ because this current appears
around Christmas in December. December is a summer month in Peru
(Southern Hemisphere). EI-Nino is used in India for forecasting long range
monsoon rainfall.

• 11. Madden Julian Oscillation (MJO):The MJO is an eastward moving

disturbance of clouds, rainfall, winds, and pressure that traverses the planet
in the tropics and returns to its initial starting point in 30 to 60 days, on
average. The MJO consists of two parts, or phases: one is the enhanced
rainfall (or convective) phase and the other is the suppressed rainfall
phase.
12. Air Sea Interactions and Temperature Inversion
13. Hadley and Walker Cells
14. Southern Oscillation and Walker Cell
• 15. La Nina: La Niña, which means “The Little Girl” or
“El Viejo” or “anti-El Niño” or simply “a cold event”
or “a cold episode is the cooling of water in the
Eastern Pacific Ocean. Here is what happens in La
Niña.
• The water in Eastern Pacific, which is otherwise cool;
gets colder than normal. There is no reversal of the
trade winds but it causes strong high pressure over
the eastern equatorial Pacific which proves beneficial
for the Indian monsoon.
• Currently, climate change can also be
observed as a major factor impacting the
monsoon mechanism. It is clearly observed to
bring variations in rainfall pattern leading to
drought and floods. The severe drought of
2002 is attributed to the failure of monsoonal
rainfall.
• Onset and advancement in File2.
 North East Monsoon
• Mechanism and Factors- in File 2.