Climate

Winds and Monsoons

Monsoon Origins

• Traditional Theory: Winds blow from land to sea in winter (continental dry).

• Flohn's Criticism: Claims land-sea heating not sufficient for global wind reversal.

• New Theory: Pressure and wind belts shift seasonally with sun's vertical rays.

  • Summer: Sun aligns over Tropic of Cancer ➔ ITCZ shifts north ➔ Low pressure over northwestern India. Intensification: High temperatures create a strong low pressure, drawing southwest monsoons from Indian Ocean.

  • Winter: ITCZ shifts south ➔ High pressure develops over northern India. Winds shift to northeast monsoons due to high pressure.

Influential Factors

• Sub-tropical westerly & tropical easterly jet streams play a key role in the seasonal wind patterns affecting India.

Mechanism of Indian Weather

• Forms of Weather Mechanism: Composed of surface pressure and winds distribution alongside upper air circulation (global weather control factors).

• Weather Conditions Leading to Rainfall: Involves western disturbances and tropical depressions, impacting precipitation levels across the region.

Winter Season Weather Mechanism Influence

• High pressure over Central/West Asia leads to wind flows from north, creating dry continental air masses. This air can reach the Middle Ganga Valley, impacting weather through trade winds, particularly influencing northwestern India.

• Upper Atmospheric Conditions: Different patterns are observed at altitude (about 3 km) with westerly jet streams influenced by the Tibet Plateau. These patterns create gusty winds and play a crucial role in attracting western disturbances, ultimately affecting Indian weather.

Summer Season Weather Mechanism

• After summer onset, the ITCZ shifts north ➔ A low-pressure zone attracts maritime tropical air (southwest monsoon).

• Upper Air Condition: The easterly jet stream attracts tropical depressions that impact rainfall distribution significantly throughout the season.

Indian Monsoonal Regime

Characteristics of Monsoon

• Onset and Advance: The region heats up, resulting in low pressure in northwestern India. ITCZ shifts north; southeast trade winds cross the equator, creating southwest monsoon winds. The advance can start as early as June in Kerala and reach pan-India by mid-July.

Rain-bearing Systems and Distribution

• Monsoon rains are typically intermittent, featuring wet and dry spells. Tropical depressions cause rainfall across the northern plains, while the western coast benefits from Arabian Sea currents. The variability in rainfall distribution is prominently driven by meteorological conditions, with northern regions experiencing diverse rainfall patterns.

Break in Monsoon

• A break occurs when monsoon conditions fail for consecutive weeks, leading to dry spells. This can be caused by various factors like tropical depression failures or dislocations of the ITCZ.

Retreat of Monsoon

• The retreat begins around September from western Rajasthan, continuing to mid-October for most parts of India. The southwest monsoon picks up moisture again from the Bay of Bengal, affecting the Tamil Nadu coast in December.

Variability of Annual Rainfall

• High Variability Observed: Rainfall variability demonstrates significant fluctuations with a co-efficient range of 15% to 30%. Wetter areas exhibit less variability, while regions like Gujarat and deserts may experience extreme variability (>40%).

• Impact on Agriculture: This variability presents challenges for agricultural practices, leading to concerns over food security in drought-prone regions. Droughts and floods represent the extreme conditions these areas face, affecting crop yield and livelihoods.

• Droughts in India: Caused by insufficient moisture leading to extended dry spells, particularly in areas receiving marginal rainfall (<75 cm). Prolonged dry periods can affect millions living in drought-prone zones.

  • Regional Impact: Vulnerability of certain regions to droughts highlights the importance of understanding rainfall patterns.

  • Mitigation Programs: Government strategies focus on early warning systems and integrated resource management to help combat the effects of droughts.

Seasonal Rhythm of Climates in India

• Seasons Recognized: Winter Season, Summer Season, Rainy Season, Retreating Monsoon.

• Winter Season: Mid-Nov to March; January is the coldest month with northern India experiencing temperatures < 21°C at night. High pressure dominates the northwest, leading to low rainfall primarily from the Bay of Bengal.

• Summer Season: March to June sees rising temperatures (41°C-47°C in NW India), characterized by oppressive hot winds and localized rains due to dust storms. Rainfall is minimal, with northern averages ≤ 2.5 cm.

• Rainy Season: Spanning June to October, the southwest monsoon accounts for ~80% of annual rainfall, crucial for agriculture.

• Retreating Monsoon: Begins in September with clearing skies and pleasant evening temperatures. Cyclonic storms from the Bay of Bengal contribute to rainfall during this period, marking a transition to post-monsoon weather conditions.