What is Monsoon in India?

 Why is it in the news?

1. According to India Meteorological Department (IMD), the Southwest Monsoon is likely to reach Kerala on May 27, earlier than the usual date of June 1. Thus, it is the first time since 2009 that Monsoon would arrive earlier. 

2. The Monsoon typically covers the entire country by July 8. It starts withdrawing from northwest India around September 17 and ends by October 15. IMD clarified that there is no direct relationship between the onset date and the total rainfall over the country during the season. The Monsoon arriving early and late in Kerala does not mean it will cover other parts of the country accordingly. It is characterised by large scale variability and global regional and global features. 

3. According to IMD, the forecast for 2025 Monsoon season is above normal cumulative rainfall, thereby ruling out the possibility of El Nino conditions which are associated with below normal rainfall in the Indian subcontinent. 

4. If India has 87 cm annual rainfall it is called normal. If rainfall is less than 90% of the average of 87 cm annual rainfall, it is called deficient. Between 90% to 95% rainfall is called below normal while rainfall between 105% and 110% is called above normal more than 110% is considered excess rainfall. 

5. Monsoon is crucial for India’s agricultural sector which supports the livelihood of about 42.3% of the population and contributes 18.2% of the GDP. In addition, a good monsoon is essential for replenishing reservoirs critical for drinking water and power generation across the country. 

What is Monsoon?

1. Monsoon refers to the seasonal reversal of winds accompanied by significant rainfall driven by the pressure difference between the Indian subcontinent and the Indian ocean. The word monsoon has been derived from the Arabic word Mausim which means season. The scientists are of the view that monsoon is nothing but the reversal of the wind patterns. The south east trade wind crosses the equator and becomes southwest wind because of the coriolis effect in the summer season and this moisture laden wind causes heavy rainfall in India, south Asia and Southeast Asia

2. In summer, when the land heats up faster than the ocean, it creates a low pressure zone over northern India. The moisture laden winds from the southwest moved towards mainland India and this caused heavy rainfall. 

3. There are two types of Monsoons in India- Southwest Monsoon during summer season and Northeast Monsoon in winter season. 

Southwest monsoon has two branches: the Arabian Sea branch and Bay of Bengal branch. It contributes 70-90% annual rainfall in India. It begins in Kerala usually on 1st June and then it covers the entire country by mid July. It weakens by September. It started withdrawing from Northwest India. It gradually retreats from Northern and Central India by mid October. The Monsoon completely withdraws from southern India by November or early December. The retreating Southwest Monsoon overlaps with the North East Monsoon bringing significant rainfall to Tamilnadu, coastal Andhra Pradesh, Karnataka and Kerala. 

Northeast Monsoon : It covers the months of October-December in India. Reversal of winds leads to 10-20% annual rainfall in India. While crossing the Bay of Bengal the northeast wind picks up the moisture and causes heavy rainfall in Tamil Nadu, Andhra Pradesh, Karnataka and Kerala. Tamil Nadu receives about 50-60% of its annual rainfall during this period. 

4. An important feature of the monsoon in India is its variability. It should be noted that there is less variability in annual rainfall in those areas which witness heavy rainfalls compared to those areas which get scanty annual rainfall. For example, the areas of Meghalaya state has lesser variability of annual rainfall than the area in Rajasthan or particularly in Jaisalmer. The variability percentage of rainfall is calculated by dividing the deviation of the mean rainfall by average rainfall multiplied by 100 The deviation of the mean rainfall Average rainfall100 . For example if the average (mean) annual rainfall of Punjab is 75 cm (by taking the average rainfall of the past 10 years) and if the annual rainfall in 2024 is 35 cm, the standard deviation is 40 cm. So, according to the above formula, 4075100 = 53%. Thus, the annual variability is 53%. 

Different theories of origins of Monsoons in India

1. Classical theory propounded by Sir Edmund Hailley - According to this theory, the Monsoon originates in India on account of differential heating of land and sea. In summer, the Indian subcontinent heats up faster than the Indian Ocean. This creates pressure ingredients. Thus, the moisture laden winds blow from the high pressure belt of the Indian Ocean to the low pressure belt of the Indian mainland, causing heavy rainfall. This is called the SouthWest Monsoon. In the winter season, the land cools faster than the ocean. Thus, the winds blow from the high pressure belt of Indian landmass to the low pressure belt of the Indian ocean. It is called Northeast Monsoon since it blows on the mainland, it is dry unless it crosses the Bay of Bengal. 

2. Limitations - Although high temperature and the consequent low pressure takes the northwest in its grip from the middle of April onwards,no rainfall starts in Northern India till the middle of June. 

3. Dynamic Theory - Propounded by Hermann Flohn, According to this theory in summer ITCZ (InterTropical Convergence Zone) which is a low pressure belt near the Equator, shifts northward over India due to the apparent movement of the Sun. This creates a low pressure zone or monsoon trough over Northern India and the Gangetic plains. This monsoon trough attracts moisture laden winds from the Arabian Sea and Bay of Bengal leading to heavy rainfall. In winter, the ITCZ moves south and the monsoon trough weakens causing the retreat of Southwest Monsoon and the onset of the Northeast monsoon. 

4. Limitation - The above theory does not take into account the upper atmospheric circulation of jet streams and the southern oscillation. 

5. Jet stream theory - This theory was propounded by Joseph Pedlosky. According to this theory,  The monsoon in India is influenced by the seasonal movement of Jet streams. In winter, the subtropical westerly jet stream dominates over northern India, thereby blocking Monsoon. In summer, the jet streams shift northward, allowing the tropical easterly jet stream to form over south India. The easterly jet stream strengthens the Monsoon by enhancing the flow of moist southwest winds and supports the development of the Monsoon trough. 

6.Limitations -While this theory explains the role of Tibetan plateau as the heat source and focuses on upper air dynamics, it does not fully explain surface level wind pattern or oceanic influences. 

7. Tibetan Plateau Theory - According to this theory, the Tibetan plateau acts as a high altitude heat source in summer, thereby, intensifying the Monsoon. It is situated at 4000 meter altitude. Its area is 1000 km wide in the East and 600 km in the West covering the distance of 2000 km between the east and west. The huge mass of Tibetan plateau and high altitude attracts 2-3 degree C more insolation than adjoining areas. This creates pressure gradients between the land (low pressure) and the Indian ocean (high pressure) cooling in moist southwest winds. The Himalayas play a key role by blocking dry and cold winds from central Asia and forcing moist southwest winds to rise and condense causing heavy rainfall. 

 

How Monsoon in India is affected

The Indian Monsoon is influenced by a variety of factors. These factors can lead to variations such as excessive rainfall, deficient rainfall, floods or droughts, thereby impacting agriculture, water resources and overall the Indian economy. The following factors are described below 

1. El Nino - Southern Oscillation (ENSO) - Warmer sea surface temperatures in the central and eastern pacific ocean weaken the trade wind thereby weakening monsoon by altering global wind patterns,thereby, reducing pressure gradient over India. Under normal conditions the Peru is the cold water current while over the western pacific (Borneo, Indonesia and Eastern Australia), the ocean current is warm and deep. The occurrence of El Nino reverses the condition and therefore develops warm conditions over the eastern pacific ocean (Peru and Chile Coast) and cool conditions in the western pacific (Australia, New Guinea and Indonesia). Whenever this warm ocean current (El Nino) is produced near the Peru coast, the amount of precipitation along the coastal areas of South America is usually high, while the eastern coast of Australia, New Guinea and Indonesia record drought conditions. 

The shifting of atmospheric circulation over the southern pacific region is known as southern oscillation. It is a precursor to the occurrence of  El Nino events. It involves a periodic fluctuation in air pressure between the western and eastern tropical pacific ocean. It is defined as the difference in air pressure between Tahiti (in the eastern Pacific and Darwin) (Australia) in the western Pacific. So the southern oscillation index is pressure at Tahiti- pressure at Darwin. Positive SOI is high pressure over Tahiti and low pressure over Darwin indicates La Nina conditions. Negative SOI indicates low pressure over Tahiti and high pressure over Darwin. This indicates El Nino conditions. This El Nino condition suppresses convection over the Indian Ocean resulting into weak monsoon in India while positive SOI (La Nina) enhances convection over the Indian ocean leading to the stronger monsoon in India. Thus, SOI helps in Monsoon prediction and early warning of droughts and floods in India. 

2. La Nina - When the pacific sea surface temperature becomes cool , the monsoon is strengthened because of the stretching of the trade wind. This increases heavy rainfall in India. ENSO is a major driver of annual monsoon variability, sometimes reducing southwest monsoon by 10-20%.  

3. Indian ocean dipole (IOD) - Positive IOD is caused when the sea surface temperature is enhanced in the Arabian sea thereby strengthening southwest monsoon. On the other hand, the sea surface temperature of the Arabian sea becomes cooler and the eastern Indian ocean around Indonesia and Australia becomes warmer, the monsoon gets weakened leading to reduced rainfall. It should be noted that a positive IOD can offset El Nino’s negative effect while a negative IOD can exacerbate drought conditions and may lead to scanty rainfall in India. 

4. Madden-Julian Oscillation (MJO) - It causes intra seasonal variability by modulating rainfall over 30-60 days cycles. Active MJO enhances rainfall while suppressed phases of MJO cause dry spells or breaks in the Monsoon. The longer period of Monsoon breaks impact crop growth and water availability.

5. Climate change - Global warming increases land and ocean temperature, thereby altering land sea temperature and pressure gradient that drives the monsoon. Warmer temperatures and intensify low pressure systems over India, thereby increasing extreme rainfall leading to floods. Studies suggest that reduced snow cover on the Tibetan plateau and Himalayan glaciers has further weakened the heating effect of the plateau, which is the driving force of the Monsoon. Similarly, the aerosol pollution and Greenhouse gas emission are further weakening the monsoon. This may contribute to the deficient rainfall in coming years.  

6. The delay in the shifting of subtropical westerly jet streams northward may have postponed the onset of monsoon in Northern India. Similarly, a weakened easterly jet stream in summer can reduce the strength of South west monsoon winds.

7. In addition, high pressure systems or unusual atmospheric patterns in northern India can block monsoonal winds causing prolonged dry spells.

8. Regional and local factors - It has been observed that deforestation in the Western Ghat has caused reduced rainfall in Peninsular India.  Similarly, rapid urbanisation has caused urban heat islands in metropolitan cities causing the alteration in the local wind patterns, thereby impacting the rainfall. 

9.Changes in sea surface temperature increase cyclone frequency and its intensity and thus, brings heavy rainfall in Tamilnadu, Andhra Pradesh and Odisha because of the frequent rise of tropical cyclones and depression in the Bay of Bengal. Studies have shown that North East Monsoon cyclones in the winter season have become more intense in the Bay of Bengal thereby causing heavy rainfall in the coastal region of India. It has also been observed that since 2000 there have been prolonged dry spells or frequent extreme rainfall events. 

Impact of Monsoon on the Indian Economy 

1. A normal monsoon ensures the increase in farm incomes, rural demand for goods and overall growth of economic activity. It stabilises food prices by ensuring sufficient supply and keeps inflation in check. 

2. Since agriculture contributes 18.2% India’s GDP and 42.3% people are engaged in agriculture, a normal southwest monsoon enhances economic growth of India. 

3. A deficient Monsoon causes food inflation and overall weakening of the Indian economy. 

4. A normal monsoon is necessary for the replenishment of water reservoirs and recharging of ground water, vital for irritation, drinking water and hydro power. 

5. Since, India is still 45% of its land rain-fed, A normal monsoon is necessary for the overall growth of Indian Economy. The deficient monsoon leads to droughts, crop failure and food insecurity. 

6. A normal monsoon is necessary for the growth of agro based industries. A deficient Monsoon may affect rural demand, thereby impacting the Indian Economy. 

7. Poor monsoons lead to increased government spending on relief activities subsidies and crop insurance. 

8. Excessive Monsoonal rainfall causes erosion of topsoil, thereby reducing the fertility of the soil. 

Conclusion

Monsoon is the lifeline of India. Our agriculture activities are completely dependent upon annual monsoonal rainfall. The monsoon in India originates by a complex of factors. No single factor fully explains the origin of the monsoon. It impacts the overall economy of our country. However, the Southwest Monsoon in India is erratic and limited to four months from June to September. It accounts for 70-90% annual rainfall in India while the Northwest monsoon accounts for only 10-15% annual rainfall and is mostly restricted to the coastal regions of TamilNadu and Andhra Pradesh. Any deficient or excessive monsoon leads to drought or floods in major portions of India.