- The term ‘monsoon’ is believed to originate from the Arabic word for season, ‘mawsim.’ Monsoons refer to seasonal winds that undergo a directional reversal with the changing seasons, representing periodic winds.
- These winds exhibit a dual system, moving from the sea to the land in summers and from the land to the sea during winters.
- Some scholars liken monsoon winds to a large-scale manifestation of land and sea breezes.
- Historically, monsoons played a crucial role for traders and seafarers navigating between different locations.
- While monsoons are present in various regions like the Indian subcontinent, central-western Africa, and Southeast Asia, they are most prominent in the Indian subcontinent.
ONSET OF MONSOON
- During April and May, when the sun is directly above the Tropic of Cancer, the expansive landmass north of the Indian Ocean experiences intense heat.
- This phenomenon leads to the development of a strong low-pressure system in the northwestern part of the subcontinent.
- This low-pressure system attracts southeast trade winds from across the Equator, as the Indian Ocean to the south of the landmass registers high pressure due to gradual water heating.
- These conditions contribute to the northward shift of the Intertropical Convergence Zone (ITCZ).
- The ITCZ, functioning as a low-pressure belt, influences precipitation patterns in the tropics through its oscillations along the equator.
- As the southwest monsoon advances, it can be regarded as a continuation of the southeast trade winds redirected towards the Indian subcontinent upon crossing the Equator.
- Between longitudes 40°E and 60°E, these winds traverse the Equator.
- The southwest monsoon typically reaches the Kerala coast by June 1st and progresses swiftly, reaching Mumbai and Kolkata between June 10th and 13th.
- By mid-July, the entire subcontinent is enveloped by the southwest monsoon.
SOUTH WEST MONSOON SEASON
- The monsoon season spans from June to September. The heightened temperatures over the northwestern plains in May intensify the existing low-pressure conditions.
- By early June, this intensified low-pressure system becomes potent enough to attract the trade winds from the Southern Hemisphere originating in the Indian Ocean.
- These southeast trade winds traverse the equator, entering the Bay of Bengal and the Arabian Sea. Upon entering Indian airspace, they become entangled in the atmospheric circulation over the region.
- Carrying a substantial amount of moisture from the equatorial warm currents, these winds then travel in a southwesterly direction after crossing the equator.
- It is this southwesterly trajectory that lends its name to the phenomenon, known as the southwest monsoons.
Southwest Monsoon burst
- The commencement of the southwest monsoon season is marked by a sudden and abrupt onset of rainfall.
- The initial rain has a notable impact, significantly reducing temperatures.
- The term “break” or “burst” of the monsoons denotes the sudden arrival of moisture-laden winds accompanied by intense thunder and lightning.
- In the coastal regions of Kerala, Karnataka, Goa, and Maharashtra, the monsoon may make its entrance as early as the first week of June, while in the interior areas, it may occur around the first week of July.
- Between mid-June and mid-July, daytime temperatures experience a notable drop of 5°C to 8°C.
- The relief and thermal low pressure prevailing over northwest India influence the southwesterly direction of these winds as they approach the land.
- As the monsoon approaches, it bifurcates into two branches: the Arabian Sea branch and the Bay of Bengal branch.
Northeast Monsoon
- The northeast monsoon marks its entry into India from the northeast, with the wind direction moving from the sea towards the land.
- Carrying moisture from the Indian Ocean, the northeast monsoon predominantly affects the southern regions of India.
- This monsoon system is confined to the southern parts of the country, specifically bringing rain to Tamil Nadu, Puducherry, Karaikal, Yanam, Andhra Pradesh, Kerala, Mahe, and the south interior Karnataka, spanning from October to December.
- The precipitation during this period is associated with low-pressure systems, depressions, and cyclones, earning it the designation of the winter monsoon.
- For Tamil Nadu, this season serves as the primary rainy period, accounting for 48% (447.4mm) of the state’s annual rainfall over these three months.
- The northeast monsoon plays a crucial role in sustaining water resources and agriculture in this region.
Monsoon winds often refers to two main branches—the Arabian Sea branch and the Bay of Bengal branch—
The Arabian Sea monsoon winds exhibit three distinct streams as they impact the Indian subcontinent:
Western Ghats Stream:
- Impact: Strikes India’s west coast, delivering substantial rainfall, exceeding 250 cm.
- Topographical Effect: Interacts perpendicularly with the Western Ghats, resulting in orographic rainfall. The windward side witnesses abundant rainfall (400 to 500 cm annually), while the leeward side experiences a significant reduction to approximately 30-50 cm.
- Aridity Belt: Immediate leeward side of the Western Ghats encounters severe aridity.
- Post-Passage: As the air rises again after passing the Western Ghats, rainfall amounts increase further eastward.
Narmada—Tapi Trough Stream:
- Path: Flows through the Narmada—Tapi troughs, a narrow rift valley, before reaching central India.
- Rainfall Distribution: Due to the absence of a major orographic obstacle along the rift, it does not yield significant rainfall near the coast. However, it brings rain to certain parts of central India, such as Nagpur.
Aravali Range Parallel Stream:
- Path: Runs parallel to the Aravali Range.
- Rainfall Characteristics: Does not contribute substantially to rainfall, resulting in Rajasthan being primarily a desert state.
- Orographic Effect: Some orographic effect occurs on the southeastern edge of the Aravali Range. Mt. Abu receives about 170 cm of rain, while the surrounding plains receive a comparatively lower amount ranging from 60 to 80 cm.
BAY OF BENGAL BRANCH OF MONSOON WIND
The monsoon winds from the Bay of Bengal Branch exhibit a dual-stream pattern, impacting Meghalaya and the Ganga plain while contributing to the rain shadow effect along the Tamil Nadu coast:
First Stream to Meghalaya:
- Path: Reaches Meghalaya after traversing the Ganga-Brahmaputra delta.
- Effect: The orographic effect induces substantial rainfall in the region. Cherrapunji, receiving 1,102 cm of annual rainfall, primarily experiences this between June and August.
- Topography: Cherrapunji and Mawsynram, the latter being the current rainfall champion at 1,221 cm, are situated at the northern end of a deep valley running from south to north on the southern slopes of the Khasi hills.
Himalayan Foothills Stream:
- Bay of Bengal Branch: The second stream flows along the Himalayan foothills.
- Deflection by Himalaya: Deflected westward by the Himalayas, this stream brings widespread rainfall to the Ganga plain.
- Rainfall Distribution: Moving from east to west along the plain, the rainfall from this stream decreases steadily.
Additionally:
Rain Shadow Effect in Tamil Nadu:
- Effect of Currents: The rain shadow effect results from the parallel flow of the Arabian Sea current and the Bay of Bengal current along the coast.
- Outcome: This effect keeps the Tamil Nadu coast relatively dry during the southwest monsoon season as the currents create a rain shadow, reducing rainfall in this region.
FEATURES OF MONSOONAL RAINFALL
The characteristics of monsoonal rainfall in India are diverse and crucial to the country’s agrarian economy:
- Seasonal Pattern:
- Southwest monsoons bring rainfall between June, August, and September.
- Topographical Influence:
- Relief or topography plays a significant role in determining monsoon rainfall.
- Regional Variances:
- Windward side of the Western Ghats and the Eastern Himalayas receive heavy rainfall, exceeding 250 cm.
- Distance from the Sea:
- Monsoon rainfall tends to decrease with increasing distance from the sea.
- City-specific Rainfall:
- During the southwest monsoon, cities experience varying rainfall: Kolkata (119 cm), Patna (105 cm), Allahabad (76 cm), and Delhi (56 cm).
- Rainfall Characteristics:
- Monsoon rains occur in short bursts lasting a few days, separated by ‘breaks’ with no rain.
- Cyclonic depressions from the Bay of Bengal cause these breaks.
- Depression Impact:
- Frequency, intensity, and the path of cyclonic depressions determine rainfall distribution.
- Summer Rainfall Impact:
- Torrential downpours during summer cause significant runoff and soil erosion.
- Economic Significance:
- Southwest monsoons contribute over three-quarters of the country’s total rainfall, playing a critical role in India’s agrarian economy.
- Spatial Distribution:
- Uneven distribution of rainfall ranging from 12 cm to more than 250 cm.
- Variable Onset and End:
- The onset of rains can be delayed, impacting the entire country or specific regions.
- Rains can end earlier than expected, causing damage to crops and difficulties in winter crop sowing.
RETREATING MONSOON
The retreating monsoon season in India, occurring in October and November, is characterised by the withdrawal of the southwest monsoon. Here are the key features of the retreating monsoon in both the southern and northern halves of India:
Retreating Monsoon in the Southern Half of India:
Weakening Process:
- By the end of September, the southwest monsoon weakens as the low-pressure trough over the Ganga plain moves southward due to the sun’s southward march.
- The monsoon departs western Rajasthan by the first week of September.
- By the end of September, it retreats from Rajasthan, Gujarat, the Western Ganga plain, and the Central Highlands.
Southern Progression:
- The low-pressure system covers the northern parts of the Bay of Bengal by early October.
- It moves over Karnataka and Tamil Nadu by early November.
- The center of low pressure is completely removed from the Peninsula by the middle of December.
Weather Conditions:
- Clear skies and rising temperatures indicate the retreat of the southwest monsoon.
- Despite the ground being wet, oppressive weather sets in due to high temperatures and humidity, known as the ‘October heat.’
Retreating Monsoon in the Northern Half of India:
Temperature Changes:
- In the second half of October, temperatures begin to drop rapidly, especially in northern India.
Dry Weather with Exceptions:
- Weather in north India during the retreating monsoon is generally dry.
- However, rain is associated with it in the eastern part of the Peninsula.
Rainiest Months:
- October and November are the rainiest months in this area during the retreating monsoon.
- Rain is linked to the passage of cyclonic depressions originating over the Andaman Sea and making their way to the southern Peninsula’s eastern coast.
Role of Tibet in Indian Monsoons:
Barrier Effect:
The Tibetan Plateau acts as a massive highland barrier, influencing the Indian monsoons significantly.
Insolation Difference:
Due to its elevated height, the plateau receives 2-3°C more insolation compared to neighbouring areas.
Mechanical Barrier and Heat Source:
- Mechanical Barrier: The plateau affects the atmosphere both as a mechanical barrier and as a high-level heat source.
- Heat Source: The protruded height of the plateau contributes to its role as a high-level heat source.
Jet Stream Dynamics:
- Subtropical Jet Stream (STJ): In June, the subtropical jet stream withdraws from India and positions itself along 40°N, north of the Tibetan Plateau.
- Accentuated Northward Displacement: The plateau accentuates the northward displacement of the jet stream, leading to the burst of monsoon in June. The Himalayas play a crucial role in this process.
Winter Influence:
- High Pressure Cell Formation: In October, the winter cooling of the Tibetan Plateau establishes a high-pressure cell (anticyclonic condition).
- Strengthening of N-E Monsoons: This high-pressure cell over Tibet strengthens the North-East monsoons.
Summer Heating and Low Pressure:
- Summer Heating: The Tibetan Plateau heats up in summer, being 2-3°C warmer than adjoining regions.
- Low Pressure Cell: The plateau generates an intense low-pressure cell due to its heating, contributing to the monsoon dynamics.
Air Circulation and Moisture Uptake:
- Air Ascent and Convergence: As heated air ascends, it spreads outwards in the upper troposphere (divergence) and gradually descends over the equatorial Indian Ocean (subsidence).
- Return Current: The descending air approaches the west coast of India as a return current from a south-westerly direction, known as equatorial westerlies.
- Moisture Uptake: This wind picks up moisture from the Indian Ocean, leading to rainfall in India and neighbouring regions.
In summary, the Tibetan Plateau’s dual role as a barrier and a heat source significantly influences the dynamics of the Indian monsoons, playing a crucial part in the seasonal patterns and precipitation over the Indian subcontinent.
JET STREAMS
The collision of warm and cold air masses in the atmosphere gives rise to Jet Streams. To the north and south of the Himalayas, the western and eastern jet streams flow, respectively. The eastern jet stream intensifies and reaches 15 degrees north latitude. This event triggers increased activity in the southwest monsoon, leading to substantial rainfall. A profound understanding of the Jet Stream is essential for grasping the dynamics of temperate cyclones and the patterns of the Indian monsoons.
- In the higher layers of the atmosphere, jet streams manifest as relatively slender belts of potent winds.
- These high-altitude air currents typically travel from west to east, although their trajectory often meanders northward and southward.
- Ranging from 160 to 480 kilometers in width and 900 to 2150 kilometers in thickness, with a central velocity exceeding 300 kilometers per hour, jet streams constitute formidable atmospheric phenomena.
- The strength of these air currents imposes restrictions on aircraft routes that oppose their movement.
Jet streams are closely linked with significant tropopause breaks, delineating key transitions in the Earth’s atmosphere.
Factors Influencing Jet Streams:
- Landmasses and Coriolis Effect:
- The presence of landmasses and the Coriolis effect are two primary factors influencing jet stream dynamics.
- Landmasses disrupt the flow of the jet stream due to friction and temperature differences, with the Earth’s rotation accentuating these effects, leading to meandering movements.
- Interaction with Landmasses:
- Interaction with landmasses generates flux and temperature variations in the meandering sections of the jet stream.
- Stratospheric Temperature:
- The temperature of the stratosphere in winter impacts the strength and position of the jet stream. A cooler polar stratosphere is associated with a stronger jet stream.
- Landmasses and Oceans:
- The warmth of landmasses and oceans, as observed in phenomena like the El Niño Southern Oscillation, can influence the strength and amplitude of the jet stream.
Characteristics of Jet Streams:
- Direction of Circulation:
- Jet streams circulate from west to east due to the Earth’s rotation.
- Latitudinal Range:
- Generally observed between the poles and 20 degrees latitude in both hemispheres, making them circumpolar.
- Seasonal Changes:
- During the summer, the jet stream narrows due to a northward shift, expanding during the winter and reaching up to 20 degrees latitude.
- Path Characteristics:
- The trajectory of the jet stream is wavy and curving, exhibiting a sinusoidal pattern.
- Width and Depth:
- Jet streams can be up to 40 kilometers wide and 2-3 kilometers deep.
- Wind Speed:
- Possessing extremely high average wind speeds, jet streams register a winter low of around 120 km/h and a summer high of 50 km/h.
- Velocity Distribution:
- Maximum wind velocity is measured in the crest and trough of the jet stream, emphasizing its dynamic and fluctuating nature.
TYPES OF JET STREAMS
Polar Front Jet Streams:
- Formed above the convergence zone of polar cold air mass and tropical warm air mass (40 to 60 degrees latitude).
- Occurs where contrasting air masses collide, creating a steep thermal gradient.
- More variable in position than the subtropical jet, moving in an easterly direction.
Subtropical Westerly Jet Streams:
- Located north of the subtropical high-pressure belt in the upper troposphere (above 30 to 35 degrees latitude).
- More regular in circulation compared to the polar front jet stream, moving from west to east.
- Produced by the Earth’s rotation and persists for most of the year.
Tropical Easterly Jet Streams:
- Upper-level easterly wind during late June to early September.
- Develops during the Asian monsoon, centered on 15°N, 50-80°E, extending from Southeast Asia to Africa.
- Strongest development occurs about 15 kilometers above the Earth’s surface, with wind speeds up to 40 meters per second.
- Emerges after the SubTropical Jet (STJ) shifts north of the Himalayas (early June).
Polar Night Jet Streams:
- Forms during winter months when nights are longer.
- Travels at a higher altitude during summer (approximately 24,000 meters).
- Created by extreme air pressure differences in the stratosphere during dark polar nights.
- Races eastward at an altitude of about 48 kilometers.
Local Jet Streams:
- Formed locally due to thermal and dynamic conditions.
- Only locally significant, influenced by specific geographical features.
Other Types:
- Barrier Jets: Formed upstream of mountain chains, oriented parallel to the mountains in low levels.
- Coastal Low-Level Jets: Associated with temperature contrasts between land and sea, influencing coastal weather.
- Valley Exit Jets: Strong, down-valley, elevated air currents flowing above valleys and plains.
- African Easterly Jets: Mid-level jet occurring over West Africa during the summer (10°N to 20°N), and a nocturnal poleward low-level jet in East and South Africa’s Great Plains.
Impact of Jet Streams on the Indian Climate:
Winter Months – Westerly Jet Stream:
- Effect: The westerly jet stream during winter brings western cyclonic disturbances from the Mediterranean Sea, entering the Indian subcontinent from the west and northwest.
- Signs: Arrival of cyclonic disturbances is preceded by an increase in night temperatures.
Summer Months – Sub-tropical Easterly Jet Stream:
- Effect: An easterly jet stream, known as the sub-tropical easterly jet stream, flows over peninsular India during the summer, passing over 14°N.
- Occurrence: Tropical cyclones are part of this easterly flow, affecting the monsoon season and occurring in October and November.
- Impact: These disturbances influence coastal regions of the country.
Jet Streams Movement in India:
- Winter Flow: Jet streams flow along the southern slopes of the Himalayas in winter and shift northwards in the summer, along the Himalayan edge (early June).
- Late Summer Flow: In late summer, between July and August, jet streams flow along the northern edge of the Tibetan Plateau.
- Monsoon Prediction: The periodic movement of jet streams is often used to predict the onset and withdrawal of the monsoon season in India.
Significance of Jet Streams:
- Weather Pattern Influence:
- Jet streams significantly impact local and regional weather patterns, playing a crucial role in the Indian climate.
- Relationship with Temperate Cyclones:
- Jet streams and temperate cyclones have a close relationship in terms of intensity and are interconnected in influencing weather phenomena.
- Collisions with Surface Wind Systems:
- Jet streams colliding with surface wind systems can result in the development of severe storms, affecting weather conditions.
- Role in El Nino and La Nina Events:
- Jet streams assist in providing a clear picture of the occurrence of El Nino and La Nina events, contributing to the understanding of climate phenomena.
- Aviation Considerations:
- Aviators use jet streams for efficient navigation when flying in the direction of the jet stream’s flow.
However, when flying against the jet streams, aviators avoid them due to their unpredictable nature, which can cause sudden movements even in seemingly calm and clear weather.
FAQs – Monsoons, Jet Streams, and Their Impact on Indian Climate
1. What is the origin of the term ‘monsoon,’ and what does it refer to?
The term ‘monsoon’ is believed to originate from the Arabic word for season, ‘mawsim.’ Monsoons refer to seasonal winds characterized by a directional reversal with changing seasons, representing periodic winds.
2. How do monsoons impact historical trade and navigation?
Monsoons historically played a crucial role for traders and seafarers navigating between different locations. They influenced trade routes and maritime activities, especially in regions like the Indian subcontinent, central-western Africa, and Southeast Asia.
3. What triggers the onset of the southwest monsoon in India?
The onset of the southwest monsoon in India is triggered by the intense heat experienced by the expansive landmass north of the Indian Ocean during April and May. This leads to the development of a strong low-pressure system in the northwestern part of the Indian subcontinent.
4. How does the southwest monsoon progress over the Indian subcontinent?
The southwest monsoon progresses swiftly from the Kerala coast by June 1st, reaching Mumbai and Kolkata between June 10th and 13th. By mid-July, the entire subcontinent is enveloped by the southwest monsoon, marking the onset of the monsoon season.
5. What characterizes the burst or onset of the southwest monsoon season?
The southwest monsoon season’s onset is marked by a sudden and abrupt arrival of rainfall, known as the “burst” or “break” of the monsoons. This is accompanied by moisture-laden winds, thunder, and lightning, significantly reducing temperatures.
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