Introduction to Monsoon climate:

India is a vast subcontinent and experiences a diverse range of climatic variations influenced by the distinctive monsoon climate patterns. With changing seasons, habits and attire change, reflecting the profound impact of weather on our daily lives. In India, summer requires light attire, and winter requires woolen. These variations arise from the ever-fluctuating elements of weather, such as temperature, pressure, and wind direction. 

Central to this part is the monsoon climate, characterised by its unique seasonal reversal of wind, which bestows India with its hot and humid monsoonal climate, a characteristic shared with other regions of south and southeast Asia.

Climate: Factors and Influences Shaping Long-term Weather Conditions

• Definition: Climate refers to the sum total of weather conditions and variations over a large area for a long period of time (more than thirty years). 
• Factors Influencing Climate: The six major factors influencing climate are latitude, altitude, pressure and wind systems, distance from the sea (continentality), ocean currents, and relief features.

India’s Monsoon Climate: A Seasonal Symphony

• The Climate of India is “monsoon” type, which is found mainly in South Asia and South-East Asia. 
  • The word “monsoon” is derived from the Arabic word “mausim” which means seasons.

Geographical factors and their impact on India’s Monsoon Climate

• Latitude and India’s Climate Zones: The Tropic of Cancer divides India, placing the northern part in the sub-tropical and temperate zones, and the southern part in the tropical zone. 
  • Consequently, the south experiences constant high temperatures, while the north sees more extreme seasonal variations contributing to the unique monsoon climate.
• The Himalayas as a Climate Shield: They act as a protective barrier that shields India from cold northern winds originating near the Arctic. 
  • They also help trap the monsoon winds, which release their moisture within the subcontinent.
• Land-Water Distribution and Monsoon Winds: India’s land and sea experience differential heating, leading to varying air pressure zones. 
  • This causes the reversal of monsoon winds based on seasons.
• Proximity to the Sea: Coastal Climate Balance: The coastal areas, due to their closeness to the sea, have a balanced climate.
  • However, interiors like Delhi and Kanpur, which are distant from the sea, experience more pronounced seasonal contrasts due to the monsoon climate.
• Altitude Effects: Cooling in Elevated Regions: Elevated regions are cooler due to the decline in temperature with height. 
  • Thus, places like Darjiling are cooler than plains regions on the same latitude, like Agra.
• Relief and Its Impact on Temperature, Rainfall, and Wind: India’s topography influences temperature, rainfall, and wind patterns. 
  • For instance, the windward side of the Western Ghats receives high rainfall, while the leeward side remains relatively dry, showcasing the intricate relationship between geographical factors and the monsoon climate.

Factors Related to Air Pressure and Wind in India’s Monsoon climate:

• Surface Air Pressure and Winds: This involves understanding how air pressure and winds are distributed across the earth’s surface.
• Upper-Air Circulation and Global Weather Controllers: This factor encompasses the global weather controllers, including the movement of different air masses and the role of jet streams.
• Weather Disturbances: Western Cyclones and Monsoon Climate: Inflow of western cyclones generally known as disturbances during the winter season.
  • And tropical depressions during the south-west monsoon period into India, creating weather conditions favourable to rainfall.
• These mechanisms and their effects vary and can be best understood by studying them in the context of India’s winter and summer seasons.

Winter Weather Mechanisms and Monsoon Climate Impact in India

Winds and Pressure Dynamics: Surface Patterns and their Monsoon Climate impact

•  Influence of Central and Western Asian Pressure: In winter, weather patterns in India are influenced by pressure distributions in Central and Western Asia, shaping the distinctive monsoon climate impact.
• High-Pressure Influence North of the Himalayas: A high-pressure zone forms north of the Himalayas, causing dry continental winds to flow towards the Indian subcontinent, south of the mountain range.
• Arrival of Dry Continental Air Mass: Winter Impact on Indian Weather: The surface winds blowing out of the high pressure center over Central Asia reach India in the form of a dry continental air mass. 
• Trade Wind Interaction: Dry Continental Influence on Northwestern India: These continental winds come in contact with trade winds over northwestern India,extending to the middle Ganga valley, making the region dry.

Influence of Jet Stream and Altitude on Winter Weather in India:

• Altitude-Independent Air Circulation at 3 km Above Surface: At higher altitudes, roughly 3 km above the surface , a distinct air circulation pattern emerges, unaffected by surface atmospheric pressure variations.
• Westerly Winds and Jet Streams Over Western and Central Asia: Westerly winds over Western and central Asia flow across the Asian continent at latitudes north of the Himalayas, near the Tibetan highlands, known as jet streams.
• Bifurcation Due to Tibetan Highlands: Influence on India’s Winter Weather and Monsoon Climate: These streams bifurcate due to the Tibetan highlands, with the southern branch influencing India’s winter weather.

Direction of Winds in India in Winter at the Height of 9-13 km

Monsoon: India’s Seasonal Wind Phenomenon and Monsoon Climate

It refers to a climate characterised by seasonal reversals of the wind system. 

The sailors who came to India in historic times were among the first to notice the phenomenon of the monsoon.

What forces shape the dynamics of India’s Monsoon Climate?

• Differential Heating and Cooling: The differential heating and cooling of land and water creates low pressure on the landmass of India, while the seas around experience comparatively high pressure.
• Summer Shift of the ITCZ: The shift of the position of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this is the equatorial trough normally positioned about 5°N of the equator. 
• Oceanic High-Pressure Zone: The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean. 
• Tibetan Plateau Dynamics: The Tibetan plateau gets intensely heated during the summer, which results in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level. 
• Role of Westerly Jet Stream: The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.

Tracing the Onset and Climate Impact in India:

• The onset of the monsoon was attributed to the differential heating of land and sea during the summer. 

India: Normal Dates of Onset of the Southwest Monsoon

• Seasonal Pressure Variations  in Northern India: 
  • By April and May, the northern landmass heats up, creating a significant low-pressure area. 
  • Meanwhile, the slower-heating Indian Ocean retains high pressure. 
• ITCZ Migration: Northward Shift and Trade Wind Dynamics: 
  • This contrast draws the southeast trade winds towards the subcontinent.
  • It led to the northward movement of the Intertropical Convergence Zone (ITCZ). 
• Monsoon’s Transition: Journey of the Southeast Trade Winds:: 
  • After crossing the Equator between 40°E and 60°E longitudes, these winds become the southwest monsoon.  
• Jet Stream Dynamics: Pivotal Role in Monsoon Onset and Climate Impact: 
  • The withdrawal of the westerly jet stream from northern India and the subsequent setting of the easterly jet stream are crucial for the monsoon’s onset.
• Entry of the Monsoon into India: Southwest Arrival and Subcontinental Enveloping
  • The southwest monsoon sets in over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June. 
  • By mid-July, the southwest monsoon covers the entire subcontinent. (Refer Figure)

What are the varied aspects of Unity and Diversity within India’s Monsoon Climate?

• Regional Variations: Monsoonal Symphony Across States 
  • India’s monsoon climate emphasises its unity with the larger southeast Asian region, characterised by rich regional variations.
  • Example: States like Kerala and Tamil Nadu in the south differ climatically from northern states like Uttar Pradesh and Bihar, even though all experience the monsoon pattern. 
  • These variations manifest in winds, temperature, rainfall, and seasonal rhythms.
• Temperature Extremes: From Sizzling Rajasthan to Frigid Leh
  • These are evident  while Rajasthan’s western parts might sizzle at 55°C in summer, Leh can plummet to minus 45°C in winter.
• Rainfall Contrasts: Cherrapunji’s Bounty to Jaisalmer’s Scarcity
  • Places like Cherrapunji in Meghalaya might receive a staggering 1,080 cm of annual rainfall, whereas Jaisalmer in Rajasthan might get a meager 9 cm.
• Diverse Precipitation Patterns: From Himalayan Snowfalls to Northeastern Rains
  • Precipitation forms also vary, from Himalayan snowfalls to widespread rains in the north-east. 
  • Yet, despite these stark differences, India’s climate is undeniably monsoonal in its essence and rhythm.

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