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Elaborate Notes
Modern Theory of Monsoon
The modern understanding of the Indian monsoon has evolved significantly from the classical thermal concept, which viewed it simply as a large-scale land and sea breeze. The dynamic concept, developed through the works of meteorologists like Hermann Flohn (1951), M.T. Yin (1949), and P. Koteswaram (1960s), incorporates the role of upper-air circulation, jet streams, and the thermal and mechanical effects of the Tibetan Plateau.
Winter Season (November - February)
- Pressure and Temperature Conditions: During this period, the sun’s vertical rays are over the Southern Hemisphere. The Eurasian landmass, particularly the Siberian and Central Asian region, cools down rapidly, leading to the formation of a strong, persistent high-pressure cell known as the Siberian High. Over the Indian subcontinent, especially the northern plains, temperatures are low, resulting in high-pressure conditions.
- Anticyclonic Circulation: The high pressure over the landmass induces subsidence of air (sinking of air from upper levels). This subsidence leads to the formation of an anticyclonic circulation at the surface, causing winds to diverge outwards from the high-pressure center. These winds blow from the land towards the surrounding oceans (Bay of Bengal and Arabian Sea).
- Subtropical Westerly Jet Stream (STWJ):
- In the upper troposphere (~9-13 km altitude), the STWJ is a powerful, high-velocity wind belt flowing from west to east. The massive Tibetan Plateau acts as a physical barrier, bifurcating the STWJ.
- One branch flows to the north of the plateau.
- The southern branch, which is stronger during winter, flows south of the Himalayas and is positioned over the Northern Ganga Plains (around 25°N latitude).
- This southern branch of the STWJ plays a crucial role in maintaining and intensifying the surface high pressure over northern India by causing air to subside. This reinforces the land-to-sea wind flow.
- North-East Monsoon: The resultant surface winds blow from the northeast to the southwest, hence the name North-East Monsoon. As these winds originate over land, they are predominantly cold and dry, leading to clear skies and low humidity over most of India.
- Winter Rainfall: While most of the country remains dry, the North-East monsoon winds, after crossing the Bay of Bengal, pick up moisture. This moisture-laden air strikes the Eastern Ghats and the coast of Tamil Nadu, southern Andhra Pradesh, southeastern Karnataka, and southeastern Kerala, causing significant winter rainfall. This is the primary rainy season for the Coromandel Coast.
Spring Season (March - May) / Pre-Monsoon
- Transition Period: This season marks the northward apparent migration of the sun. The Indian landmass begins to heat up rapidly, causing surface temperatures to rise and atmospheric pressure to fall.
- Weakening of the STWJ: The heating of the Tibetan Plateau and the northward shift of the sun causes the southern branch of the Subtropical Westerly Jet Stream to weaken. By late May, it completely withdraws from its position over the northern plains and shifts to the north of the Tibetan Plateau. This northward shift is a critical trigger for the subsequent onset of the summer monsoon.
- Pre-Monsoon Showers: The intense surface heating leads to strong convectional air currents. This local convection, combined with moisture from nearby water bodies, results in thunderstorms and short bursts of rain in various parts of the country. These are not part of the main monsoon system but are localized phenomena.
- Mango Showers: Occur in Kerala and coastal Karnataka. The showers are beneficial for the early ripening of mangoes.
- Blossom Showers (or Cherry Blossoms): Occur primarily in Kerala and nearby areas, aiding in the flowering of coffee plants.
- Nor’westers (Kalbaishakhi): These are violent thunderstorms in West Bengal and Assam, occurring in the afternoon. The name ‘Kalbaishakhi’ means ‘calamity of the month of Baisakh’. In Assam, they are known as Bardoli Chheerha. They are crucial for the cultivation of jute, rice, and tea.
- Loo: These are strong, hot, and dry winds that blow during the day over the northern plains (from Punjab to Bihar). They are a result of intense heating and advection, often causing temperatures to rise to 45-50°C.
Summer Season (May - August) / South-West Monsoon
- Intense Low Pressure: By late May and early June, the intense heating of the northwestern part of the subcontinent (including the Thar Desert and adjoining Pakistan) leads to the formation of a very strong low-pressure trough. This is often called the Monsoon Trough.
- Northward Shift of the Inter-Tropical Convergence Zone (ITCZ): The ITCZ, a low-pressure belt near the equator, follows the apparent movement of the sun. By summer, it shifts northward and lies over the Ganga Plain, effectively becoming the monsoon trough. This trough acts as the primary target for moisture-laden winds.
- Onset of South-West Monsoon: The low pressure over the Indian landmass is so intense that it attracts the trade winds from the Southern Hemisphere. These are the southeast trade winds. Upon crossing the equator, they are deflected to their right due to the Coriolis force and blow into India as the South-West Monsoon winds.
- Role of Jet Streams:
- Tropical Easterly Jet Stream (TEJ): The intense heating of the Tibetan Plateau in summer leads to the formation of an upper-air high-pressure system over it, which results in an easterly flowing jet stream over peninsular India. This TEJ is responsible for creating divergence in the upper atmosphere, which in turn causes air to rise from the surface, enhancing the surface low pressure and strengthening the monsoon. The work of P. Koteswaram was seminal in establishing the role of the TEJ.
- Somali Jet: This is a low-level, cross-equatorial jet stream that forms off the coast of East Africa (Somalia). It strengthens the high-pressure cell near Madagascar (the Mascarene High), which is the source region for the southwest monsoon winds. A stronger Somali Jet pushes more moisture towards the Indian subcontinent at a faster rate.
- Bursting of the Monsoon: The sudden arrival of the monsoon winds, accompanied by a significant drop in temperature, violent thunder, and lightning, is termed the ‘burst of the monsoon’. It typically occurs first on the Malabar Coast (Kerala) around the 1st of June.
- Break in the Monsoon: During the rainy season, there can be periods of several days with little to no rainfall. This is known as a ‘break’. This can occur if:
- The monsoon trough shifts to the foothills of the Himalayas, causing heavy rain there but dry spells in the plains.
- Temperature stratification (inversion) reappears locally, preventing convection.
- Winds blow parallel to the topography (e.g., the Aravallis) instead of striking it.
Autumn Season (September - October) / Retreating Monsoon
- Southward Shift of ITCZ: As the sun begins its apparent southward journey, the low-pressure trough (ITCZ) over the northern plains weakens and gradually shifts southwards. This marks the beginning of the withdrawal of the southwest monsoon.
- Retreat of Monsoon: The withdrawal is a more gradual process than the onset. It begins from the northwestern states in early September and withdraws from the entire country, except the southern peninsula, by mid-October.
- Wind Reversal: The southwest winds are slowly replaced by the north-east winds as the high-pressure system begins to re-establish over the landmass.
- October Heat: The retreat of the monsoon is marked by clear skies and a rise in temperature. However, the land is still moist from the preceding rains. The combination of high temperature and high humidity creates an oppressive weather condition known as ‘October Heat’.
- Re-emergence of STWJ: The Subtropical Westerly Jet Stream gradually reappears over the northern plains, re-establishing the dry, stable winter conditions.
- Tropical Cyclones: This period (October-November) is known for the formation of intense tropical cyclones in the Bay of Bengal, which often strike the eastern coast of India and Bangladesh, causing widespread destruction.
Characteristics of Indian Monsoon
- Onset and Withdrawal: The onset is relatively sudden (‘burst’), while the withdrawal is a more prolonged and gradual process, taking about three months.
- Rainfall Gradient: As the monsoon winds travel inland from the sea, they progressively lose moisture. Consequently, rainfall generally decreases from the coast to the interior. For example, Kolkata receives about 120 cm, Patna 102 cm, Allahabad 91 cm, and Delhi 56 cm.
- Duration Gradient: The duration of the rainy season also decreases from south to north and from east to west. Kerala may experience the monsoon for nearly five months, whereas the extreme west of Rajasthan receives it for less than two.
- Variability:
- Temporal Variation: The timing of onset and withdrawal varies from year to year. The total amount of rainfall also fluctuates significantly annually, leading to alternating periods of floods and droughts.
- Spatial Variation: The distribution of rainfall is highly uneven. Mawsynram and Cherrapunji in Meghalaya receive over 1100 cm annually, while parts of the Thar Desert receive less than 15 cm.
- Distribution of Monsoon Rainfall (Branches): The peninsular shape of India divides the southwest monsoon winds into two main branches.
- Bay of Bengal Branch:
- It initially moves parallel to the Coromandel Coast, which is why this coast remains largely dry during the southwest monsoon season.
- A part of it strikes the Arakan Yoma in Myanmar and is deflected towards the Indian subcontinent.
- It enters West Bengal and Bangladesh from a south/southeasterly direction and gets bifurcated by the Himalayan foothills and the Meghalaya plateau.
- Eastern Branch: It moves northwards and strikes the Garo, Khasi, and Jaintia hills of the Meghalaya Plateau almost perpendicularly. The funnel-like shape of these hills forces the moist air to rise rapidly, causing extremely heavy orographic rainfall, making Mawsynram the wettest place on Earth.
- Western Branch: It is deflected westward and moves up the Ganga plain. As it moves west, its moisture content decreases, leading to a progressive reduction in rainfall from east to west.
- Arabian Sea Branch:
- Western Ghats Stream: It strikes the Western Ghats at a right angle, forcing the moist air to ascend. This causes very heavy orographic rainfall (over 250 cm) on the windward side. On the leeward side, the descending air becomes warm and dry, creating a pronounced rain-shadow region (e.g., parts of interior Maharashtra and Karnataka).
- Narmada-Tapti Stream: A second stream enters through the Narmada-Tapti river valleys and travels further inland, bringing rainfall to parts of central India.
- Saurashtra-Aravalli Stream: A third stream moves over Saurashtra and Kachchh and then towards Rajasthan. Since the Aravalli Range is oriented parallel to the direction of these winds, it does not act as an effective barrier, resulting in scanty rainfall for Rajasthan. This branch eventually meets the Bay of Bengal branch over the northern plains.
- Bay of Bengal Branch:
Western Disturbances
- Origin and Mechanism: Western Disturbances are extratropical or temperate cyclones that originate over the Mediterranean Sea and the adjoining Atlantic Ocean. They are low-pressure systems that are steered eastwards across Iran, Afghanistan, and Pakistan by the Subtropical Westerly Jet Stream.
- Timing and Impact: They are most active during the winter months (November to April) when the STWJ is positioned south of the Himalayas. Upon reaching the Indian subcontinent, their path is obstructed by the Himalayas. This obstruction forces the moist air to rise, causing precipitation.
- Precipitation Pattern: They cause light to moderate rain in the northern plains (Punjab, Haryana, Delhi, Western UP) and heavy snowfall in the higher reaches of the Himalayas (Jammu & Kashmir, Himachal Pradesh, Uttarakhand). The amount of precipitation decreases from west to east.
- Significance:
- Agriculture: This winter precipitation is of immense importance for the cultivation of rabi crops, particularly wheat and mustard, which are staples of the region.
- Water Resources: The snowfall replenishes the glaciers, which are the source of major perennial rivers like the Ganga and Yamuna, ensuring water supply during the summer months.
- Weather: They cause an abrupt drop in the night temperature and induce cold waves and fog in North India.
- Hazards: Intense Western Disturbances can sometimes lead to cloudbursts, flash floods, landslides, and avalanches in the Himalayan region. The 2010 Leh Cloudburst and the 2013 Uttarakhand Floods were triggered by interactions involving Western Disturbances.
Climatic Regions of India
India’s vast size and diverse topography result in a wide range of climatic conditions. The most widely accepted classification of Indian climate is based on the Koppen scheme, which uses temperature and precipitation data.
- Amw (Monsoon with short dry season): West coast of India, south of Goa.
- As (Monsoon with dry summer): Coromandel coast (Tamil Nadu).
- Aw (Tropical savanna): Most of the peninsular plateau, excluding the coastal strips.
- BShw (Semi-arid steppe climate): Rain-shadow areas of Western Ghats, large parts of Rajasthan, Haryana, and Gujarat.
- BWhw (Hot desert climate): Extreme western Rajasthan.
- Cwg (Monsoon with dry winter / Humid Subtropical): Most of the northern plains (Ganga plain), eastern Rajasthan, Assam.
- Dfc (Cold humid winter with short summer): Higher reaches of Arunachal Pradesh and Sikkim.
- E (Polar type): Mountain peaks of the Himalayas.
Prelims Pointers
- Modern Monsoon Theory: Based on the dynamic concept involving upper-air circulation, not just surface temperature differences.
- Winter Season: High pressure over northern plains; anticyclonic circulation; land-to-sea winds.
- Subtropical Westerly Jet Stream (STWJ): Bifurcated by the Tibetan Plateau in winter; the southern branch is over the Ganga plains.
- North-East Monsoon: Provides rainfall primarily to the Coromandel Coast (Tamil Nadu, South AP).
- Pre-Monsoon Showers:
- Mango Showers: Kerala, Karnataka (for mango ripening).
- Blossom Showers: Kerala, Karnataka (for coffee flowering).
- Nor’westers/Kalbaishakhi: West Bengal, Assam.
- Bardoli Chheerha: Name for Nor’westers in Assam.
- Loo: Hot, dry winds in the Northern Plains.
- Summer Season: Intense low pressure over NW India; ITCZ shifts over Ganga plains.
- Wind Direction Change: Southeast trade winds cross the equator, deflect right due to Coriolis force, and become South-West Monsoon winds.
- Tropical Easterly Jet Stream (TEJ): Develops over peninsular India in summer due to the heating of the Tibetan Plateau; strengthens the monsoon.
- Somali Jet: A low-level jet off the coast of East Africa that intensifies the cross-equatorial flow towards India.
- Monsoon Burst: The sudden onset of monsoon rains, often with thunder and lightning.
- Monsoon Break: A period of dry spell during the rainy season.
- Retreating Monsoon: Marked by the southward shift of the ITCZ and ‘October Heat’.
- Monsoon Branches:
- Arabian Sea Branch: Hits Western Ghats; causes rain-shadow on the leeward side; moves parallel to Aravallis.
- Bay of Bengal Branch: Moves parallel to the Coromandel coast; strikes Meghalaya hills (Mawsynram); moves west up the Ganga plains causing rainfall to decrease from east to west.
- Western Disturbances:
- Origin: Temperate cyclones from the Mediterranean Sea region.
- Path: Steered to India by the Subtropical Westerly Jet Stream.
- Season: Active in winter (November - April).
- Impact: Rain in northern plains, snow in the Himalayas.
- Significance: Crucial for Rabi crops like wheat and mustard.
- Koppen Climate Codes for India:
- Amw: West Coast
- As: Coromandel Coast
- Cwg: Northern Plains
- BWhw: Thar Desert
Mains Insights
Analytical Perspectives on the Indian Monsoon
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Historiographical Debate: From Thermal to Dynamic Concept
- Classical Theory (Halley, 1686): Viewed the monsoon as a large-scale land and sea breeze driven by the differential heating of the Indian subcontinent and the Indian Ocean. It was a simple, surface-level explanation.
- Modern Dynamic Theory (Post-WWII): Emphasizes the role of upper-air circulation. The discovery of jet streams and a better understanding of global atmospheric circulation led to this paradigm shift. The works of Flohn, Yin, and Koteswaram were pivotal. The modern theory incorporates the mechanical effect of the Himalayas and Tibetan Plateau (bifurcating the STWJ) and its thermal effect (acting as an elevated heat source in summer, creating the TEJ). This shift highlights the complexity and global teleconnections of the monsoon system.
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The Monsoon as the “Real Finance Minister”: Socio-Economic Significance
- Economic Backbone: The Indian economy is heavily reliant on agriculture, which is predominantly rain-fed. A good monsoon boosts agricultural output, increases rural income, controls food inflation, and stimulates demand for industrial goods and services. A failed monsoon can have a cascading negative effect, potentially reducing the nation’s GDP.
- Water Security: The monsoon replenishes reservoirs, recharges groundwater, and feeds perennial rivers, ensuring water availability for drinking, irrigation, and hydroelectric power generation.
- Dual Nature - A Unifying as well as a Disruptive Force: While the monsoon unifies the country in a single climatic and agricultural rhythm, its variability causes widespread disruption. Excess rainfall leads to floods (e.g., in Bihar, Assam, Kerala), while deficits lead to droughts (e.g., in Maharashtra’s Vidarbha, Rajasthan), creating a cycle of disaster and relief management.
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Impact of Climate Change on the Monsoon
- Increased Variability and Extremes: Global warming is intensifying the monsoon’s erratic nature. Studies indicate a trend towards more frequent and intense extreme rainfall events over short periods, interspersed with longer dry spells. This increases the risk of both floods and droughts within the same season.
- Shifting Patterns: There is evidence of a weakening of the overall monsoon circulation but an increase in localized, heavy downpours. The timing of onset and withdrawal is also becoming more unpredictable, which poses a significant challenge for agricultural planning and sowing schedules.
- Policy Imperative: This necessitates a shift in policy from mere prediction to building resilience through better water management (e.g., interlinking of rivers, rainwater harvesting), promoting climate-resilient agriculture (e.g., drought-resistant crops), strengthening disaster management infrastructure, and improving the accuracy of long-range forecasting.
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Western Disturbances: A Double-Edged Sword
- Boon for Winter Agriculture: They are vital for North India’s food security, providing the necessary moisture for the critical Rabi crop season. Without them, the region would be significantly more arid.
- Bane in the form of Disasters: Their increasing intensity, possibly linked to climate change, is a growing concern. They can trigger avalanches, landslides, and devastating flash floods in the ecologically fragile Himalayas. The interaction of Western Disturbances with monsoon systems can also lead to catastrophic events, as seen in the 2013 Uttarakhand tragedy. This highlights the need for robust early warning systems and sustainable development practices in the Himalayan region.