climate of India -described as the ‘monsoon’ type

Indian Monsoons are Convection cells on a very large scale.

are periodic or secondary winds with seasonal reversal in wind direction.
2 divisions,
southwest monsoon season -seasonal in character-occurs between June and
September.-formed due to intense low pressure system over Tibetan plateau.-
bring rainfall to west coast and North Indian plains
retreating monsoon season -October and November-associated with high
pressure cells over Tibetan and Siberian plateaus.-bring rainfall to mainly
south-eastern coast

Factors responsible for south-west monsoon

Factors related to Formation-

 Intense heating of Tibetan plateau during summer months.

 Permanent high pressure cell in South Indian Ocean (east to north-east
of Madagascar in summer).

Factors related to onset-

 Subtropical Jet Stream (STJ).

 Tropical Easterly Jet (African Easterly Jet).
 Inter Tropical Convergence Zone.

Southern Oscillation (SO):reversal in pressure conditions -wherein eastern

Pacific has lower pressure in comparison to eastern Indian Ocean.

Factors related to intensity-

 Strengths of Low pressure over Tibet and high pressure over southern
Indian Ocean.
 Somali Jet (Findlater Jet).
 Somali Current (Findlater Current).
 Indian Ocean branch of Walker Cell.
 Indian Ocean Dipole.

Factors responsible for north-east monsoon formation

 Formation and strengthening of high pressure cells over Tibetan

plateau and Siberian Plateau in winter.
 Westward migration and subsequent weakening of high pressure cell
in Southern Indian Ocean.
 Migration of ITCZ to south of India

Mechanism of Indian Monsoons

Classical Theory: Sir Edmund Halley’s Theory

Summer Monsoon
  In summer -sun’s apparent path -vertically over Tropic of Cancer-high
temperature and low pressure in Central Asia.
 High pressure over Arabian Sea and Bay of Bengal- winds flow from
Oceans towards landmass in summer.
 This air flow from sea to land bring heavy rainfall to Indian
subcontinent.

Winter Monsoon

 In winter -sun’s apparent path is vertically over Tropic of Capricorn.

 north western part of India grows colder than Arabian Sea and Bay of
Bengal - - flow of monsoon reversed.
 basic idea behind Classical theory -similar to land and sea breeze
formation -except that -in case of monsoons -day and night -replaced
by summer and winter.

Drawbacks:
monsoons not develop equally everywhere on earth
fails to explain sudden burst of monsoons + delay in on set of monsoons
sometimes,

Air Mass Theory

 According to this theory, -monsoon is simply a modification

of planetary winds of tropics.

based on migration of ITCZ based on seasons.

Role of ITCZ [img-monsoons periodic winds]

 southeast trade winds in southern hemisphere and northeast trade

winds in northern hemisphere meet each other near equator.-meeting
place- known as Inter-Tropical Convergence Zone (ITCZ).
 is the region of ascending air, maximum clouds and heavy rainfall.
 location of ITCZ- shifts north and south of equator with change of
season.
 In summer season-sun shines vertically over Tropic of Cancer - ITCZ
shifts northwards.
 southeast trade winds of southern hemisphere- cross equator - blow in
southwest to northeast direction -under influence of Coriolis force.
 displaced trade winds -called south-west monsoons
 front where south-west monsoons meet north-east trade winds -known
as the Monsoon Front (ITCZ). Rainfall occurs along this front.
 In month of July -ITCZ shifts to 20°- 25° N latitude -is located in Indo-
Gangetic Plain - south-west monsoons blow from Arabian Sea and Bay
of Bengal. ITCZ in this position -often called Monsoon Trough
[maximum rainfall].
 seasonal shift of ITCZ -given the concept of Northern Inter-Tropical
Convergence Zone (NITCZ) in summer (July – rainy season) and
Southern Inter-Tropical Convergence Zone (SITCZ) in winter (Jan – dry
season).
 NITCZ is zone of clouds and heavy rainfall that effect India.
Jet Stream Theory

How Jet Streams Affect Weather?

 Jet streams have distinct peaks (ridges) and troughs.

 Ridges occur where warm air mass pushes against cold air mass.
 Troughs occur where cold air mass drops into warm air.
 region on earth below trough -at low pressure - region below ridge -at
high pressure.
 occurs due to weakening of jet stream -due to lesser temperature
contrast between sub-tropics and temperate region
 trough region creates cyclonic condition (low pressure) at surface of
earth whereas ridge regions creates anticyclonic condition.
 Troughs create upper level divergence -is associated with convergence
at surface (low pressure – cyclonic conditions) and ridges create upper
level convergence - associated with divergence at surface (high
pressure – cyclonic conditions).
 These ridges and troughs -give rise to jet streaks- responsible for
cyclonic and anticyclonic weather conditions at surface.
 winds leaving jet streak are rapidly diverging- creating a lower pressure
at upper level (Tropopause) - air below -rapidly replaces upper
outflowing winds-creates low pressure at surface- surface winds rush
inwards- Coriolis effect creates cyclonic rotation [low pressure cells].
 winds entering jet streak -rapidly converging because of high pressure
at upper level (Tropopause) - leads to divergence (high pressure)
at surface (anticyclonic condition).- Coriolis effect creates anticyclonic
rotation-associated with clear weather.

Role of Sub-Tropical Jet Stream (STJ)

 Sub-Tropical Jet stream -a narrow band of fast moving air -flowing

from west to east [Westerlies].
 STJ in northern hemisphere -flows between 25° to 35° N in upper
troposphere
 burst of monsoons depends upon upper air circulation which is
dominated by STJ.

Seasonal Migration of Sub-Tropical Jet Stream – STJ

 In winter -STJ flows along southern slopes of Himalayas -but in

summer -shifts northwards- flows along northern edge of Himalayas in
early summer -and northern edge of Tibetan Plateau in late summer
(July-August)
 Northward movement of subtropical jet-first indication of onset
of monsoon over India

Sub-Tropical Jet Stream – STJ in Winter

 Westerly jet stream -blows at a very high speed during winter -

over sub-tropical zone.
 This jet stream -bifurcated by Himalayan ranges and Tibetan Plateau.
  two branches reunite off east coast of China.
 northern branch-blows along northern edge of Tibetan Plateau.
 southern branch blows to south of Himalayan ranges along 25° north
latitude.
 strong latitudinal thermal gradient - responsible for development of
southerly jet.
 southern branch stronger than northern branch.
 Air subsiding beneath this upper westerly current gives dry out blowing
northerly winds from subtropical anticyclone over northwestern India
and Pakistan.

Why no south-west monsoons during winter?

 Reason 1: ITCZ left India ( winds that blow over India -mostly offshore
–– land to ocean –– so they carry no moisture).
 Reason 2: During winter-southern branch of STJ -strong -and is
to south of Himalayas. -ridge of jet lies over north-western India -is
associated with strong divergence of winds -creates a high pressure
region over north India.
 Reason 3: There is already a strong high pressure over Tibet. [High
Pressure due to STJ + High Pressure over Tibet = strong divergence =
no rainfall]

Sub-Tropical Jet Stream – STJ in Summer

 summer in March- STJ [upper westerlies] start northward march.

 southerly branch of STJ remains positioned south of Tibet, although
weakening in intensity.
 weather over northern India -hot, dry - due to incoming solar
radiation + hot winds like loo.
 Over India, -Equatorial Trough (ITCZ)- pushes northwards-
with weakening of STJ south of Tibet-but burst of monsoon not
take place -until upper-air circulation -switched to its summer pattern.
 By end of May -southern jet breaks -later is diverted to north of Tibet
Plateau -there is sudden burst of monsoons
 ( ridge moves northwards into Central Asia = high pressure over north-
west India moves northwards into Central Asia = makes way for south-
west monsoon winds).
  Easterly jet emerges over peninsular India with northward migration of
STJ.
 upper air circulations reversed with emergence of Easterly jet
[convergence in upper layers replaced by divergence == divergence in
lower layers replaced with convergence == high pressure at lower
layers replaced by low pressure system].
 easterly winds become active in upper troposphere - are associated
with westerly winds in lower troposphere (south-west monsoon winds).
 Western and eastern jets flow to north and south of Himalayas
respectively. .
 results in active south-west monsoon and heavy rainfall is caused.

Why no south-west monsoons in March – May (summer)?

 There is good sun’s insolation from March – May but still there is no s-
w monsoons.

Reason: ridge region of Southern branch of STJ creates strong divergence

(high pressure) in north-west India.
diverging air blocks incoming winds and prevents strong convergence of winds
along ITCZ.

 During summer season in Northern Hemisphere-low pressure areas

develop at ground surface -in north-west India - due to intense heating
of ground surface during April, May, and June.
 As long as -position of upper air jet stream - maintained (to south of
Himalayas)- dynamic anti-cyclonic conditions persist over north-west
India.
 winds descending from upper air high pressure [because of ridge of
STJ] -obstructs ascent of winds from surface low pressure areas, -result
-weather remains warm and dry.
 This is why -months of April and May -generally dry and rainless -in
spite of high temperatures (low pressure on land) and high evaporation.

Role of Tropical Easterly Jet (TEJ) [African Easterly Jet]

TEJ-caused by high temperatures over Tibetan Plateau during summer.

important role in kick starting southwest monsoon.

Tropical Easterly Jet (TEJ)

high velocity winds in lower troposphere called low-level jets (LLJs).

most prominent are -Somali Jet and African Easterly Jet [Tropical Easterly
Jet].
TEJ-in summer -over southern Asia and northern Africa.-between 5° and 20°N.
intensity from June through October

 TEJ comes into existence -quickly after -STJ shifted to north

of Himalayas (Early June).
  TEJ flows from east to west over peninsular India -at 6 – 9 km- and
over Northern African region.
 formation of TEJ results in reversal of upper air circulation patterns
[High pressure switches to low pressure] -leads to quick onset of
monsoons.

Indian Monsoons – Role of Tibet

 Tibetan Plateau -an enormous block of highland -act as a formidable

barrier.
 Due to its height -receives 2-3°C more insolation than neighboring
areas.
 plateau affects atmosphere in two ways: (a) as a mechanical barrier, (b)
as a high- level heat sources.
 At beginning of June -subtropical jet stream -completely withdrawn
from India -occupies position along 40° N (to north of Tibetan
Plateau).
 plateau accentuates northward displacement of jet stream-Hence burst
of monsoon in June -prompted by Himalayas -not by thermally
induced low pressure cell over Tibet. (Tibetan plateau is responsible for
south-west monsoons. But it is STJ that facilitates sudden outburst of
monsoons with its sudden northward migration)
 In middle of October -plateau- important factor in advance of jet -south
of Himalayas or bifurcating it into two parts.
 winter Tibetan Plateau -cools rapidly -produces high pressure cell. (
anticyclonic condition established)-strengthens N-E monsoons.
 in summer-Tibet gets heated-2°C to 3°C warmer than air
over adjoining regions-generates area of rising air
(convergence)(intense low pressure cell).
 During its ascent -air spreads outwards in upper troposphere
(divergence) -gradually sinks (subsidence) over equatorial part
of Indian Ocean.
 finally approaches west coast of India -as a return current -from a
south-westerly direction -is termed as equatorial westerlies.
 picks up moisture from Indian Ocean -causes rainfall in India and
adjoining countries.
 When summer temperature of air over Tibet -high for a sufficiently
long time-helps in strengthening easterly jet -results in heavy rainfall in
India.
 easterly jet -not come into existence- if snow over Tibet Plateau does
not melt- This hampers occurrence of rainfall in India.
 Therefore-any year of thick snow over Tibet-weak monsoon and less
rainfall.

Indian Monsoons – Role of Somali Jet

 Polar and subtropical jet streams are permanent jet streams-

influence weather of temperate regions.
 Temporary jet streams -are narrow winds -with speeds more than 94 kph
in upper, middle and lower troposphere-include Somali Jet
and African Easterly jet or Tropical Easterly Jet.
 These two jet streams play an important role in formation and
progression of Indian Monsoons.
  progress of the southwest monsoon towards India -greatly aided
by onset of Somali jet- that transits Kenya, Somalia and Sahel.
 flow from Mauritius and northern Madagascar before reaching coast
of Kenya
 strengthens permanent high near Madagascar - also helps to drive S-W
monsoons towards India- at a greater pace and intensity.
 its path around 9º N -coincides with zone of coastal upwelling.
 As strong winds drive away surface coastal waters towards east- cold
water from depths of sea rise upwards -preserve continuity of mass.
 peculiar feature of Somali Current is reversal in direction with onset
of summer monsoon.
 In winter,-current - north to south - from coast of Arabia to east
African coastline;
 In summer-reverses its direction -flows from south to north.

Indian Monsoons – Role of Indian Ocean Dipole[img-indian ocean dipole]

 Indian ocean Dipole- is a SST anomaly (Sea Surface Temperature

Anomaly – different from normal) -occurs in Northern or Equatorial
Indian Ocean Region (IOR).
 Indian Ocean Dipole (IOD) -defined by difference in sea surface
temperature between two areas – a western pole in Arabian Sea
(western Indian Ocean) and an eastern pole in eastern Indian Ocean
south of Indonesia.
 IOD develops in equatorial region of Indian Ocean- from April to May
peaking in October.
 With positive IOD -winds over mIndian Ocean- blow from east to west
(from Bay of Bengal towards Arabian Sea). -results in -Arabian
Sea being warmer -and eastern Indian Ocean around Indonesia
becoming colder and dry.
 In negative dipole year- reverse happens -Indonesia much warmer and
rainier.
 Positive IOD -good for Indian Monsoons -more evaporation occurs in
warm water.
 Similar to ENSO-atmospheric component of mIOD -named as
Equatorial Indian Ocean Oscillation (EQUINOO)(Oscillation of
pressure cells between Bay of Bengal and Arabian Sea).
 During positive (EQUINOO),- enhanced cloud formation and rainfall
in western equatorial ocean near African coast -while such activity -
suppressed near Sumatra.
 While EQUINOO and IOD go in step during strong positive IOD
events, they do not always do so.

How Jet Streams affect Indian Monsoons?[all jets streams together]

summer-increased solar heating of Indian subcontinent and Tibetan Plateau.

1. apparent northward movement of sun, -southern branch of SJT- which

flows to south of Himalayas-shifts to north of Himalayas.
2. When sun’s position- reach Tropic of Cancer (June), -SJT shifts
to north of Tibetan Plateau - ITCZ close to its peak position
over Tibetan Plateau.
 3. altitude of mountains -initially disrupts the jet- but once it has cleared
the summits- able to reform over central Asia.
4. Its movement towards north- associated with onset of the monsoon
over India.
5. With northward shift of SJT- Easterly Jet formed over Indian plains-
in first week of June -lasts till late October.
6. can be traced in upper troposphere -up to west coast of Africa.
7. northward shift of SJT and ICTZ -moves subtropical high pressure belt
to north of Tibetan Plateau -and Easterly Jet -creates a low pressure
region in Indian plains (Easterly Jet creates anticyclonic conditions in
upper troposphere).
8. This low pressure in northern plains + intense low of Tibetan Plateau-
sudden onset of south-west monsoons (1st of Jun – 20th of June).
9. monsoon cell situated between Indian Ocean (North of
Madagascar)(High Pressure Cell) and Tibetan plateau (Low Pressure
Cell).
10. In summer -sub-tropical easterly jet fluctuates between plains of India
and peninsular India -varying intensity of rainfall from location to
location.
11. During March to May-building up of this cell -blocked by STJ -
which blow to south of Himalayas -high pressure belt
undermines influence of low pressure cell over Tibet)
12. As long as STJ is in this position -development of summer
monsoons inhibited ( high pressure belt stays over north India).
13. With STJ migrates to north of Tibet- sub continental monsoon cell
develops (Somali Jet)
14. Warmth and moisture -fed into cell by lower level tropical jet stream -
brings with it- air masses laden with moisture from Indian Ocean.
15. end of the monsoon season -brought when atmosphere over Tibetan
Plateau begins to cool (August – October), -enables STJ to transition
back across Himalayas.
16. With southward shift of ITCZ-subtropical high pressure belt returns
back to Indian plains -rainfall ceases.
17. leads to formation of a anticyclonic winter monsoon cell -typified by
sinking air masses over India -relatively moisture free winds blow
seaward.
18. gives rise to settled and dry weather- winter months.

Projects to understand monsoons

ISMEX

Two experiments jointly, by India and former USSR in 1973 and 1977,
known as Indo-Soviet Monsoon Experiment (ISMEX) and Monsoon-77
respectively

MONEX

Data collection effort

by many researching organizations and WMO under their World Weather
Watch (WWW) programme.

Monsoon Prediction In India

IMD-designed a statistical weather model –predicting weather – based
on relationship between two weather phenomena.
2014-IMD started to use numerical models -supplement statistical models for
long-range forecasting
IMD collects weather data like temperature, humidity, wind and precipitation
679 automatic weather stations, 550 surface observatories, 43 radiosonde 24
radars and three satellites.

Factors Responsible for Inaccurate Monsoon Forecast

lack of data due to insufficient monitoring stations.
Automatic weather stations of substandard quality
major data gaps-g dust, aerosols, soil moisture and maritime conditions not
monitored.
models developed by western scientists-no fine tuning done to suit Indian
conditions
Lack of competent software professionals and scientists

Recent Indian Initiatives

Monsoon Mission of India-Ministry of Earth Sciences,-new approaches (high

resolution, super parameterizations, data assimilation etc.)
IMD in collaboration with (ICAR) provides district-level agro-meteorological
advisories to farmers
(IMD) provides meteorological support to (CWC) for issuing flood warnings.
Indo-US expedition-Indian Ocean Research Vessel, 'Sagar Nidhi',-seeking to
find answers to vagaries of Bay of Bengal-fed southwest monsoon
National Supercomputing Mission-predict timely and accurate monsoon
forecasts

Global Warming and Monsoon

Excess Rainfall-(IITM) Pune,-For every 1oC rise in temperature, -atmosphere

can hold 7% more moisture.
Germany’s Potsdam Institute for Climate Impact Research (PIK)-every
additional degree-increase monsoon rainfall by 5%.
rapid heating of global ocean-weakening land-sea temperature gradient-thus
drying monsoon circulatio
increase in moisture in air because of rise in SST.
Inconsistent rainfall-In 2021,- 7 states received deficient rainfall.-Central and
South Peninsula regions witnessed excess rainfall.
Ministry of Earth Sciences-frequency of localised extreme single-day rain
events - increased by 75% between 1950 and 2015.
September a ‘new August’, October an anomaly-Monsoon rainfall data for last
10 years-increasing rain in September-widely varying patterns in October.
monsoon uncertainty-IPCC 6th Assesment-monsoon deficits occurring with
greater frequency in South Asia.
Rare Events-Cyclone Gulab-only third cyclone in 21st century to form in
September,-crossed over peninsular India to Arabian Sea,
ongoing La-Niña event in Pacific Ocean -entered its third year- one of the
longest ever La-Niña episodes in recorded history.
Shift of monsoon weather systems:West Bengal, Jharkhand, Bihar and Uttar
Pradesh -experiencing worst monsoon season of the century.
Madhya Pradesh, Gujarat, Rajasthan and parts of Maharashtra recorded excess
rainfall

impacts of Variability in Monsoon?

Floods, Landslides, Lightning: Loss of Lives and Livelihoods

reduces yields of short-duration kharif crops such as groundnut, urad, soybean,
and maize
Agriculture Output-both kharif and rabi crop pattern seasonal in nature
give rise to pest attacks and diseases.
Quality of Grains:heat stress affect plant physiological processes-leading to
spikelet sterility, non-viable pollen and reduced grain quality.
concerns over food security.
forecasting complications for the IMD

Way Forward

invest more resources in better prediction of Monsoon forecast

Western Disturbances

 In winter season,-(STJ) bifurcated into two branches -obstruction

of Himalayas and Tibetan Plateau.
 One branch -flows to south of the Himalayas- second branch is - north
of Tibetan Plateau
 ridge of jet stream creates anticyclonic (with clockwise air circulation)
conditions over North-West India.
 Consequently-winds descend over north-western parts of India-
development of atmospheric stability and dry conditions (anticyclonic
condition = no rainfall).
 But (STJ) causes periodic changes in general weather conditions.
 STJ drives temperate low pressures over Mediterranean Sea towards
east across Afghanistan, Pakistan and reach north-west India.
 These storms are residual frontal cyclones -move at height of 2000
metres from mean sea level.
 On an average, 4 to 6 cyclonic waves reach north-western India between
October and April each year.

Weather associated with Western Disturbances

precipitation
abrupt decrease in air temperature over North-West India.
heavy snowfall in the Himalayan Region
cold wave to north Indian plains.

Importance of Western Disturbances

beneficial for standing rabi crops,

southern branch of jet stream-responsible for steering of-[Western

Disturbances] from Mediterranean Sea.
Some of the Western disturbances -continue eastwards -redeveloping in zone
of jet stream confluence -near east coast of China).
Winter rain and heat storms in north-western plains + occasional heavy
snowfall in hilly regions
cold waves in whole of northern plains.