Monsoon is the seasonal reversal of
wind system accompanied by seasonal changes in atmospheric circulation and
precipitation.Monsoons cause excessive rainfall in
many parts of the world including Asia, Australia, North America, South
America, and Africa. The primary mechanism behind monsoon is a shift in global
wind patterns.
During most of the year, winds blow
from land to ocean making the air dry. Winds originating from land are called
continental. During certain months of the year, the winds begin to blow from
the ocean to the land making the air moist. Winds originating over a body of
water are called maritime. This moist ocean air is what causes monsoonal rains
over many countries.
Why Do Wind Patterns Shift in a
Monsoon?
Differential heating occurs when the
sun heats the land and oceans. Incoming solar radiation heats landmasses faster
than large bodies of water. As the land gets heated throughout the summer, a
large low pressure system builds over the land. The heat from the sun also
warms the surrounding ocean waters, but the effect happens much more slowly due
to the high heat capacity of water. Therefore, the oceans as well as the layer
of air above the oceans stay cooler. The cooler air above the oceans is moist
and denser creating a high pressure zone relative to the pressure above the
landmass.
Winds flow from high pressure areas
to low pressure areas due to the pressure gradient. Once the temperature
conditions on the land and oceans change, the resultant pressure changes cause
the winds to change from a land-to-ocean direction to an ocean-to-land direction.
Monsoon season does not end as abruptly as it begins. While it takes time for
the land to heat up, it also takes time for that land to cool in the fall. This
makes monsoon season a time of rainfall that diminishes rather than ends.
The southwest monsoons occur from June to September. The Thar Desert as well as adjoining
areas of the northern and central Indian subcontinent heats up considerably
during the hot summers, which causes a low pressure area over the northern and
central India. To fill this void, the moisture-laden winds from the Indian
Ocean rush into the subcontinent. These winds, rich in moisture, are drawn
towards the Himalayas. The moisture-laden winds on reaching the southernmost
point of the Indian Peninsula, due to its topology, become divided into two
parts: the Arabian Sea Branch and the Bay of Bengal
Branch.
The Arabian Sea Branch of the
Southwest Monsoon first hits the Western coasts of Kerala, thus making it the
first state in India to receive rain from the Southwest Monsoon. The monsoon winds originating over the Arabian Sea further split into
three branches:
(i) The first branch is obstructed by the Western Ghats. These winds climb
the slopes of the Western Ghats and soon, they become cool, and as a result,
the windward side of the Sahyadris and Western Coastal Plain receive 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.
(ii) Another branch of the Arabian Sea monsoon strikes the coast north of
Mumbai. Moving along the Narmada and Tapi valleys, these winds cause rainfall
in extensive areas of central India. Thereafter, they enter the Ganga plains
and mingle with the Bay of Bengal branch.
(iii)A third branch of this monsoon
wind strikes the Saurashtra Peninsula and the Kuchchh. It then passes over west
Rajasthan and along the Aravallis, causing only a scanty rainfall due to the
parallel orientation of the Aravallis. 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 Bay of Bengal Branch of
Southwest Monsoon flows over the Bay of Bengal heading towards North-East India
and Bengal, picking up more moisture from the Bay of Bengal. 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 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 of its branches 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.
Northeast Monsoon takes place from December to early March when the surface high-pressure
system is strongest. Around September, with the sun fast retreating south, the
northern land mass of the Indian subcontinent begins to cool off rapidly. With
this, air pressure begins to build over northern India; the Indian Ocean still
holds its heat. This causes the cold wind to sweep down from the Himalayas and
Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the
Deccan peninsula. This is known as the Northeast Monsoon or Retreating
Monsoon.
Jet streams are fast flowing, narrow air currents found in the upper
atmosphere. It is a river of wind that blows horizontally through the upper
layers of the troposphere, generally from west to east, at an altitude of 6 to
11 km. This rapid current is typically
thousands of kilometers long, a few hundred kilometers wide, and only a few
kilometers thick. The main jet streams are located near
the tropopause, the transition between the troposphere (where temperature
decreases with height) and the stratosphere (where temperature increases with
height).
Jet stream develops where air masses of
differing temperatures meet.The position of this
upper-level jet stream denotes the location of the strongest surface
temperature contrast. Thus, surface temperatures
determine where the jet stream will form. The greater the temperature
difference, the faster is the wind velocity inside the jet stream. During the winter months, Arctic and tropical air masses create a
stronger surface temperature contrast resulting in a strong jet stream.
However, during the summer months, when the surface temperature variation is
less dramatic, the winds of the jet are weaker.
Inter Tropical Convergence Zone
(ITCZ)
The Inter Tropical Convergence Zone
(ITCZ) is a low pressure zone located at the equator, where trade winds
converge, and so, it is a zone where air tends to ascend. In July, the ITCZ is
located around 20°N-25°N latitudes over the Gangetic plain. This trough
encourages the development of thermal low over north and northwest India. Due
to the shift of ITCZ, the trade winds of the southern hemisphere cross the
equator between 40° and 70°E longitudes and start blowing from southwest to
northeast due to the Coriolis force. In winter, the ITCZ moves southward, and
so the reversal of winds from northeast to south and southwest, takes place.
Onset/Burst of the Monsoon
The shift in the position of the ITCZ
is related to the phenomenon of the withdrawal of the westerly jet stream from
its position over the north Indian plain, south of the Himalayas. The easterly
jet stream sets in along 15°N latitude only after the western jet stream has
withdrawn itself from the region. This easterly jet stream is held responsible
for the burst of the monsoon in India.
The southwest monsoon sets in over
the Kerala coast by 1st June and moves swiftly to reach Mumbai
and Kolkata between 10th and 13th June. By mid- July, southwest monsoon engulfs
the entire Subcontinent.
EI-Nino
EI-Nino is a complex weather system
that 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 replaces temporarily the cold Peruvian current or
Humboldt Current. This current increases the temperature of water on the
Peruvian coast by 10°C which results in:
(i) The distortion of equatorial atmospheric circulation;
(ii) irregularities in the evaporation of sea water;
(iii) Reduction in the amount of planktons thus reducing the number of fishes
in the sea.
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.
Break in the Monsoon
During the south-west monsoon period
after having rains for a few days, if rain fails to occur for one or more
weeks, it is known as 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:
(i) 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.
(ii) Over the west coast the dry spells are associated with days when winds
blow parallel to the coast.
Characteristics of Monsoonal Rainfall
(i) Monsoon rainfall is seasonal in character, which occurs between June and
September.
(ii) It is largely governed by relief or topography. For instance the
windward side of the Western Ghats registers a rainfall of over 250 cm. Again,
the conical hills around Mawsynram make it the wettest place on earth.
(iii)The monsoon rainfall decreases
with increasing distance from the sea. Kolkata receives 119 cm during the
southwest monsoon period, Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.
(iv) The wet spells in the
monsoon rains are interspersed with rainless interval known as ‘breaks’. These
breaks in rainfall are related to the cyclonic depressions mainly formed at the
head of the Bay of Bengal.
(v) The summer rainfall comes in a heavy downpour leading to considerable
run off and soil erosion.
(vi) Over three-fourths of the
total rain in the country is received during the southwest monsoon. Thus it has
a strong bearing on our agrarian economy.
(vii) Its spatial distribution is also uneven which ranges from 12 cm to even
400 cm.
(viii)There can be delays in the
beginning of the rains over the whole or a part of the country. Sometimes the
rains end considerably earlier than usual, causing great damage to standing
crops and making the sowing of winter crops difficult.
Distribution of Rainfall
The average annual rainfall in India
is about 125 cm, but it has great spatial variations
Areas of High Rainfall: The Western Ghats, the sub-Himalayan areas is the northeast and
the hills of Meghalaya are the regions where the rainfall exceeds 200 cm. In
some parts of Khasi and Jaintia hills, the rainfall exceeds 1,000 cm. In the
Brahmaputra valley and the adjoining hills, the rainfall is more than 200 cm.
Areas of Medium Rainfall : Southern parts of Gujarat, east Tamil Nadu, northeastern Peninsula
covering Orissa, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain
along the sub-Himalayas, the Cachar Valley and Manipur experience rainfall
between 100 and 200 cm.
Areas of Low Rainfall: Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir,
eastern Rajasthan, Gujarat and Deccan Plateau receive rainfall between 50 and
100 cm.
Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Karnataka
and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50
cm. Snowfall is restricted to the Himalayan region.
Variability of Rainfall
A characteristic feature of rainfall
in India is its variability. The variability of rainfall is computed with the
help of the following formula:
The values of coefficient of
variation show the change from the mean values of rainfall. The values of
coefficient of variation show variability of rainfall in India. A variability
of less than 25 per cent exists on the western coasts, Western Ghats,
northeastern Peninsula, eastern plains of the Ganga, northeastern India, Uttaranchal
and Himachal Pradesh and south-western part of Jammu and Kashmir. These areas
have an annual rainfall of over 100 cm. A variability of over 50 per cent
exists in the western part of Rajasthan, northern part of Jammu and Kashmir and
interior parts of the Deccan plateau. These areas have an annual rainfall of
less than 50 cm. Rest of India have a variability of 25-50 per cent and these
areas receive an annual rainfall between 50 -100 cm. Thus, in general, higher
the rainfall, lower is the variability.
The monsoon trough is that portion of the Intertropical Convergence Zone (ITCZ) which
is depicted by a line in the weather map showing the locations of relatively
minimum sea level pressure in a monsoon region. Thus it is a convergence zone
between the wind patterns of the southern and northern hemispheres. Westerly
monsoon winds lie in its equatorward portion while easterly trade winds lie
poleward of the trough. Right along its axis, heavy rains can be found.
Depressions and tropical cyclones often form in the vicinity of the monsoon
trough, with each capable of producing heavy rainfall in a relatively short
time frame.
Monsoon Depression
It is a depression that forms within
the monsoon trough. It denotes weak cyclonic disturbances that form over the
Bay of Bengal and generally track northwestward towards the Indian
subcontinent. These may intensify into tropical cyclones if they remain over
warm ocean water long enough. Most monsoon depressions that develop in the
western North Pacific eventually acquire persistent central convection and
accelerated core winds, marking their transitions into conventional tropical
cyclones.
Mango showers are the pre-monsoon showers in the Indian states of Karnataka and
Kerala that help in the ripening of mangoes. Also known as ‘April rains’ or
‘Summer showers’, they are a result of thunderstorms over the Bay of Bengal.
Kalbaishakhi
During summer the eastern and north
eastern states of the country like West Bengal, Assam, Bangladesh and parts of
Orissa and Bihar experience dramatic appearance of a special type of violent
thunderstorm known as Norwesters. It is locally known as ‘Kalbaishakhi’ or
calamity of the month of Baisakh.Originating from the Chhotanagpur plateau and
influenced by the westerlies, this warm wind moves eastwards and is responsible
for heavy rain and hailstones in West Bengal, Assam and Orissa. These winds
many a time cause destruction of life and property. Apart from its
destructive effects it is somewhat helpful for the pre-kharif crops like jute,
Aus paddy, summer til and a large number of vegetables and fruits and a sudden
drop in temperature gives relief after the mid-day heat.
Cherry Blossom is a local wind that blows over the interior Karnataka during the summer
season and is extremely good for coffee cultivation.
While travelling towards the Indian
Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and
pours it over peninsular India and parts of Sri Lanka. Cities like Chennai,
which get less rain from the Southwest Monsoon, receive rain from this Monsoon.
About 50% to 60% of the rain received by the state of Tamil Nadu is from the
Northeast Monsoon.
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