Indian Monsoons: Introduction and Salient Features

Introduction

Monsoon is a familiar though a little known climatic phenomenon. It is one of the oldest and most anticipated weather phenomena and yet it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving. It has to be studied at the global rather than at regional level.

The southwest summer monsoon, a four-month period when massive convective thunderstorms dominate India's weather, is Earth's most productive wet season. A product of southeast trade winds originating from a high-pressure mass centred over the southern Indian Ocean, the monsoonal torrents supply over 80% of India's annual rainfall. Attracted by a low-pressure region centred over South Asia, the mass spawns surface winds that ferry humid air into India from the southwest. 

The word monsoon (derived from the Arabic "mausam", meaning "season") is generally defined as a system of winds characterized by a seasonal reversal of direction. The Indian Meteorological Department defines it as the seasonal reversal of the direction of winds along the shores of the Indian Ocean, especially in the Arabian Sea, which blow from the southwest for half of the year and from the northeast for the other half.

Climatic Regions of India

The whole of India has a monsoon type of climate. But the combination of elements of the weather reveal many regional variations. These variations represent the sub-types of the monsoon climate. A climatic region has a homogeneous climatic condition which is the result of a combination of factors. Temperature and rainfall are two important elements which are considered to be decisive in all the schemes of climatic classification. 

Major climatic types of India are classified on the basis of Koeppen’s scheme. Koeppen based his scheme of Climatic classification on monthly values of temperature and precipitation. He identified five major climatic types, namely:

Climate Type Characteristics
Tropical climates Monthly temperature throughout the year is over 18°C
Dry climates Dry because precipitation is very low in comparison to temperature
  • If dryness is less, it is semiarid (S)
  • If it is more, the climate is arid (W)
Warm temperate climates Mean temperature of the coldest month is between 18°C and minus 3°C
Cool temperate climates
  • Mean temperature of the warmest month is over 10°C
  • Mean temperature of the coldest month is under minus 3°C
Ice climates Mean temperature of the warmest month is under 10°C

Koeppen used letter symbols to denote climatic types as given above. Each type is further sub-divided into sub-types on the basis of seasonal variations in the distributional pattern of rainfall and temperature. He used capital letters:

  • S for semi-arid and 
  • W for arid 
He used following small letters to define sub-types:
  • f (sufficient precipitation)
  • m (rain forest despite a dry monsoon season)
  • w (dry season in winter)
  • h (dry and hot)
  • c (less than four months with mean temperature over 10°C)
  • g (Gangetic plain). 
Accordingly, India can be divided into eight climatic regions: 

Code  Climate TypeAreas
Amw Monsoon with short dry seasonWest coast of India south of Goa
As Monsoon with dry summerCoromandel coast of Tamil Nadu
Aw Tripical savannahMost of the Peninsular plateaus, south of the Tropic of Cancer
Bwhw Semi-arid steppe climateNorth-western Gujarat, some parts of western Rajasthan and Punjab
Bwhw Hot desertExtreme western Rajasthan
Cwg Monsoon with dry winterGanga plain, eastern Rajasthan, northern Madhya Pradesh, most of North-east India
Dfc  Cold humid winter with short summerArunachal Pradesh
E  Polar typeJammu and Kashmir, Himachal Pradesh and Uttarakhand

Understanding the Monsson

The intensity of southwest monsoon winds of southern oscillation can be measured, among others, by measuring the difference in pressure between Tahiti (roughly 20°S and 140°W) in French Polynesia in East Pacific and port Darwin (12°30'S and 131°E) in northern Australia.

Indian Meteorological Department (IMD) can forecast the possible behaviour of monsoons on the basis of 16 indicators.

Salient features of the monsoon

Some of the salient features of the monsoon are:

  1. The onset of the monsoon
  2. Rain-bearing systems (e.g. tropical cyclones) and the relationship between their frequency and distribution of monsoon rainfall
  3. Break in the monsoon

Onset of the Monsoon

Sequence of events and the responsible factors

  • Differential heating of land and sea
    Towards the end of the nineteenth century, it was believed that the differential heating of land and sea during the summer months sets the stage for the monsoon winds. 
  • Low pressure in northwestern part
    During April and May when the sun shines vertically over the Tropic of Cancer, the large landmass in the north of Indian ocean gets intensely heated. This causes the formation of an intense low pressure in the northwestern part of the subcontinent.
  • Northward shift of ITCZ
    Since the pressure in the Indian Ocean is high as water gets heated slowly, the low pressure cell in northwestern part of subcontinent attracts the southeast trades across the Equator. These conditions help in the northward shift in the position of the ITCZ. 
  • Continuation of southeast trades as southwest monsoon
    The southwest monsoon may thus, be seen as a continuation of the southeast trades deflected towards the Indian subcontinent after crossing the Equator. These winds cross the Equator between 40°E and 60°E longitudes.
  • Entry of Monsoon into India 
    • Kerala coast: reached by 1st June
    • Mumbai and Kolkata: reached between 10th and 13th June
    • Entire subcontinent: engulfed by mid-July

Relation between southwest monsoon and jet streams

  • Westerly jet stream: Its withdrawal from the north Indian plains is related to the shift in position of ITCZ.
  • Easterly jet stream: It sets in along 15°N latitude only after the western jet stream has withdrawn itself from the region. This is held responsible for the burst of the monsoon in India.

Rain-bearing Systems and Rainfall Distribution

There are two rain-bearing systems in India.

Bay of Bengal system
It causes rainfall over the plains of north India. Much of the rainfall along these plains is due to tropical depressions originating from the Bay of Bengal. 
  • Frequency of these tropical depressions varies from year to year. 
  • Paths over India are mainly determined by the position of ITCZ which is generally termed as the monsoon trough. As the axis of the monsoon trough oscillates, there are fluctuations in the track and direction of these depressions.
Arabian Sea system
It brings rain to the west coast of India. Much of the rainfall along the Western Ghats is orographic as the moist air is obstructed and forced to rise along the Ghats. 

The intensity of rainfall over the west coast is related to two factors:

  • The offshore meteorological conditions.
  • The position of the equatorial jet stream along the eastern coast of Africa
Trends of Monsoon over India

Intensity and amount of rainfall vary from year to year. The rain which comes in spells, displays following trends: 

  1. West coast: rain declines from west to east.
  2. North Indian Plain and northern part of the Peninsula: rain declines from southeast towards northwest.

Break in the Monsoon

It is the failure of rain for one or more weeks, midway during the south-west monsoon. These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons. Following are some of the reasons: 

  • Northern India: rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region. 
  • West coast: winds blow parallel to the coast.
EI-Nino and 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 Humbolt current. This current increases the temperature of water on the Peruvian coast by  10°C. This results in:

  1. the distortion of equatorial atmospheric circulation
  2. irregularities in the evaporation of sea water; 
  3. reduction in the amount of planktons which further reduces the number of fish 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. In 1990-91, there was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of the country ranging from five to twelve days.

Distribution of Rainfall

The average annual rainfall in India is about 125 cm, but it has great spatial variations

Areas of High Rainfall

  • The highest rainfall occurs along the west coast, on the Western Ghats, as well as in the sub-Himalayan areas is the northeast and the hills of Meghalaya. Here 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 less then 200 cm. 

Areas of Medium Rainfall

  • Rainfall between 100-200 cm is received in the southern parts of Gujarat, east Tamil Nadu, northeastern Peninsula covering Odisha, Jharkhand, Bihar, eastern Madhya Pradesh.
  • Other areas include northern Ganga plain along the sub-Himalayas and the Cachar Valley and Manipur. 

Areas of Low Rainfall

  • Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Deccan Plateau receive rainfall between 50-100 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

The degree to which rainfall amounts vary across an area or through time. There are two types (or components) of rainfall variability, areal and temporal.

  • Areal VariabilityThe variation of rainfall amounts at various locations across a region for a specific time interval. (time does not vary) 
  • Temporal Variability: The variation of rainfall amounts at a given location across a time interval. (area does not vary)

Both temporal and areal variability of precipitation may be measured in various ways.  The resulting numerical value can be used to characterize the climate of a region in various ways.  The variability of rainfall is computed with the help of the following formula:

\( C.V. = (Standard Deviation \div Mean) \times 100 \), where C.V. is the coefficient of variation

The values of coefficient of variation show change from the mean values of rainfall and variability of rainfall in India. The actual rainfall in some places deviates from 20-50 per cent.

  • Variability of less than 25 per cent: It exists on the western coasts, Western Ghats, northeastern Peninsula, eastern plains of the Ganga, northeastern India, Uttarakhand and Himachal Pradesh and south-western part of Jammu and Kashmir. These areas have an annual rainfall of over 100 cm. 
  • Variability of over 50 per cent: It 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. 
  • Variability of 25-50 per cent: and these areas receive an annual rainfall between 50 -100 cm.

Monsoons and the Economic Life in India

  • Entire agricultural cycle of India revolves around Monsoon. It is because about 64 per cent people of India depend on agriculture for their livelihood and agriculture itself is based on southwest monsoon. 
  • Except Himalayas all the parts of the country have temperature above the threshold level to grow the crops or plants throughout the year
  • Regional variations in monsoon climate help in growing various types of crops. 
  • Variability of rainfall brings droughts or floods every year in some parts of the country. 
  • Agricultural prosperity of India depends very much on timely and adequately distributed rainfall. If it fails, agriculture is adversely affected particularly in those regions where means of irrigation are not developed. 
  • Sudden monsoon burst creates problem of soil erosion over large areas in India. 
  • Winter rainfall by temperate cyclones in north India is highly beneficial for rabi crops. 
  • Regional climatic variation in India is reflected in the vast variety of food, clothes and house types.