Week 7... continued: Indian Monsoons

 

Jet Streams

Definition:

Jet streams are narrow, fast-moving bands of geostrophic wind in the upper troposphere, typically found at altitudes between 9–16 km. These are circumpolar, meandering air currents primarily flowing from west to east, heavily influenced by Earth’s rotation (Coriolis effect) and horizontal temperature contrasts.

Fundamental Concepts

A. Geostrophic Wind

• Forms when pressure gradient force (PGF) balances the Coriolis force in the absence of friction.
• Air moves parallel to isobars above ~2 km altitude.
• Basis for jet stream motion.

Why Jet Streams Form

• Caused by sharp temperature and pressure gradients, particularly at the boundaries of air masses.
• These gradients are strongest near:
• Polar Front (between cold polar and warm tropical air)
• Subtropical Front (due to Hadley cell dynamics)

Structural Characteristics

Feature	Details
Location	Upper troposphere (9–16 km altitude)
Speed	Averages 110–120 mph (177–193 km/h); Peaks > 250 mph (402 km/h)
Width	50–150 km
Depth	~1–5 km thick
Direction	Generally west to east
Type of Wind	Geostrophic, meandering
Boundaries	Sharp velocity contrast with surrounding slow winds

Classification of Jet Streams

A. Permanent Jet Streams

Type	Latitude	Formation Mechanism	Features
Subtropical Jet (STJ)	~30° N/S	Rising Hadley Cell air diverted poleward; strong Coriolis	Strong in winter, weak/intermittent in summer (NH)
Polar Front Jet (PFJ)	~60° N/S	Temperature gradient at polar front	Shifts equatorward in winter, poleward in summer

B. Seasonal/Temporary Jet Streams

Type	Region	Features
Tropical Easterly Jet (TEJ)	Over S. Asia (5°–20°N), summer only	Upper-tropospheric wind from east; linked to Indian monsoon
Somali Jet	Somalia-Arabian Sea (SW winds)	Low-level jet (LLJ), June–September; feeds moisture into monsoon system

Theoretical Frameworks & Mechanisms

A. Rossby Waves

• Large-scale planetary waves in jet streams caused by latitudinal variation in Coriolis force.
• Cause meandering of jet streams.
• Influence formation of cyclones and anticyclones.

B. Jet Streaks

• Regions within the jet stream where wind speeds are locally maximum.
• Causes upper-level divergence/convergence, affecting surface pressure and cyclogenesis.

Influence on Weather & Climate

A. General Impacts

• Influence storm tracks, temperature distribution, and precipitation patterns.
• Separate cold polar air from warmer tropical air.
• Guide and intensify cyclonic systems.

B. Effects in Different Regions

Region	Impact of Jet Streams
Mid-latitudes	Drive temperate cyclones; affect storm paths and intensity
South Asia	TEJ and Somali Jet crucial for monsoon onset and strength
North America	PFJ influences winter storms and heatwaves
Europe	Zonal vs. meridional flow governs stable vs. stormy weather

Jet Streams and Aviation

Benefits:

• Fuel-efficient travel when flying with the jet stream (e.g., NY to London).
• Helps in flight planning and navigation.

Hazards:

• Clear Air Turbulence (CAT) near jet streams causes bumpy rides.
• Volcanic ash can be dispersed rapidly by jet streams, posing engine risks.

Jet Streams on Other Planets

A. Jupiter

• Multiple jet streams, both eastward and westward.
• Drive large atmospheric bands and the Great Red Spot.

B. Saturn

• Strong equatorial jet streams; influence planetary banding.

Insight: Jet streams are not unique to Earth, but a universal feature of rotating planetary atmospheres with differential heating.

1. Introduction to Indian Monsoons

The monsoon is a seasonal reversal of winds, influenced by land-sea heating differences, atmospheric circulation, and oceanic currents. The Indian monsoon is vital for agriculture, water security, biodiversity, and the economy.

Types of Monsoons in India

1. Southwest Monsoon (June – September)
• Brings 75-80% of India's annual rainfall.
• Onset: Kerala by June 1st, progressing northward.
• Divided into Arabian Sea and Bay of Bengal branches.
• Covers the entire country by July.
2. Northeast Monsoon (October – December)
• Affects Tamil Nadu, Andhra Pradesh, Odisha, and parts of Kerala.
• Brings winter rainfall, crucial for crops in Tamil Nadu.

Fig No. 1 - Monsoon wind pattern during onset

Fig No. 2 - Monsoon winds withdrawal pattern

2. Mechanism of Indian Monsoons

Classical Theories

1. Thermal Concept (Halley’s Theory)

• Proposed by Edmond Halley (1686).
• Explains monsoon based on land-sea heating differences:
• Summer: Indian landmass heats up → Low-pressure formation → Moist winds from the ocean rush in → Heavy rainfall.
• Winter: Land cools faster → High-pressure system → Winds blow outward → Dry conditions.

2. Dynamic Concept (Flohn’s Theory)

• Suggests monsoons are a part of the planetary wind system.
• Monsoon winds shift due to seasonal movement of the Inter-Tropical Convergence Zone (ITCZ).
• ITCZ shifts north in summer (causing monsoons) and south in winter (dry season).

Fig No. 3 - Sea breeze (Heating of land)

Fig No. 4 - Land Breeze (heating of sea)

Fig No. 5 - ITCZ Shift over Seasons

Modern Theories

1. Jet Stream Theory

• Upper atmospheric westerly jet streams influence the Indian monsoon.
• Bursting of the monsoon occurs when:
• Subtropical jet weakens, allowing the easterly jet to strengthen.
• This brings moist air inland, triggering rainfall.

2. El Niño & Southern Oscillation (ENSO)

• El Niño: Warming of the Pacific Ocean weakens monsoon rainfall.
• La Niña: Cooling of the Pacific Ocean strengthens monsoons.

3. Indian Ocean Dipole (IOD)

• Positive IOD: Warmer western Indian Ocean → Strong monsoons.
• Negative IOD: Warmer eastern Indian Ocean → Weaker monsoons.

4. Madden-Julian Oscillation (MJO)

• Short-term movement of convection affecting rainfall on a 30-60 day cycle.

3. Changing Monsoon Patterns in India (Case Study)

Observations of Change

1. Delayed Onset & Erratic Rainfall: Monsoons are arriving later and more unpredictably.
2. Increasing Frequency of Droughts & Floods: Uneven rainfall distribution.
3. Declining Total Rainfall: Some regions receive less than their historical average.

Causes of Change

1. Climate Change & Global Warming
• Rising temperatures affect land-sea heat contrast.
• Changes in wind patterns impact moisture transport.
2. Deforestation & Urbanization
• Heat island effects alter local weather.
• Loss of vegetation reduces moisture recycling.
3. Anthropogenic Aerosols
• Pollution affects cloud formation, reducing rainfall.
4. Variability in Oceanic Phenomena
• Stronger El Niño & IOD effects shift monsoon behaviour.

4. Comparative Analysis of Classical & Modern Theories

Feature	Classical Theories (Thermal & Dynamic)	Modern Theories (Jet Streams, ENSO, IOD)
Basis	Land-sea heating contrast	Atmospheric & oceanic interactions
Primary Factors	Temperature differences	Jet streams, ocean currents, pressure systems
Accuracy	Simplistic explanation	More accurate & scientifically backed
Relevance Today	Partially valid	More comprehensive approach

6. Advanced Insights & Further Exploration

• Recent Research: Studies on the impact of Arctic ice melt on Indian monsoons.
• Cutting-Edge Developments: Use of AI and Machine Learning in monsoon forecasting.
• Suggested Readings:
• "Monsoon: The Indian Ocean and the Future of American Power" – Robert D. Kaplan.
• Research papers from IMD (Indian Meteorological Department) & IPCC Reports.

 

Changing Monsoon Patterns in India (Case Study & Additional Examples)

Primary Case Study: Shifting Rainfall Patterns in Central India

Background:

• Central India (Madhya Pradesh, Chhattisgarh, eastern Maharashtra) has witnessed increased intensity of extreme rainfall events despite a decline in total monsoon rainfall.

Findings:

• A study by the Indian Institute of Tropical Meteorology (IITM) found that short-duration extreme rainfall events increased threefold between 1950–2015.
• Anthropogenic aerosols and urban heat islands are contributing factors.

Implications:

• Frequent flash floods in cities like Bhopal and Nagpur.
• Stress on infrastructure not designed to handle high-intensity rainfall.

Case Study 2: Declining Monsoons in the Western Ghats

Background:

• The Western Ghats, once a high rainfall zone, has seen a decline in average monsoon rainfall since the late 1990s.

Key Drivers:

• Deforestation and land-use changes.
• Weakened Arabian Sea branch of the southwest monsoon.
• Influence of positive IOD events being outpaced by global warming trends.

Impacts:

• Water shortages in Kerala and coastal Karnataka during early monsoon months.
• Disruption of hydropower generation and agriculture, especially cash crops like coffee and spices.

Case Study 3: Delayed Monsoon Onset in Northeast India

Background:

• Northeastern states like Assam, Meghalaya, and Manipur have experienced delayed monsoon onset and reduced early season rainfall.

Contributing Factors:

• Weak Bay of Bengal branch.
• Shifting ITCZ patterns.
• Reduced recycling of moisture due to deforestation in the region.

Consequences:

• Rice transplanting delays.
• Increased flood risk in later stages due to compressed rainfall windows.
• Ecological stress on wetland ecosystems.

Case Study 4: Urban Flooding in Mumbai (2005 & 2021)

Event Description:

• Mumbai 2005: Recorded 944 mm rainfall in a single day (July 26).
• Mumbai 2021: Recurring intense rainfall events flooded key transport systems.

Underlying Causes:

• High-intensity short-duration rainfall due to convective storms enhanced by climate change.
• Poor drainage infrastructure and urban planning lapses.

Takeaways:

• Need for climate-resilient infrastructure.
• Integration of early warning systems and smart city drainage modeling.

Case Study 5: Positive Indian Ocean Dipole and the 2019 Monsoon Recovery

Context:

• The 2019 monsoon season began with a 25% deficit in June, raising drought fears.

Turnaround:

• Positive IOD event developed mid-season, pushing warm waters towards Africa and enhancing Arabian Sea monsoon winds.

Outcome:

• Above-normal rainfall in September helped replenish reservoirs.
• Agricultural revival in states like Maharashtra and Gujarat.

Key Lessons from Case Studies

Theme	Insights
Climate Influence	ENSO, IOD, and climate change cause spatial and temporal monsoon variability.
Regional Diversity	Different regions face unique challenges – floods in Central India, droughts in the West, delayed rains in the Northeast.
Urban Risks	Cities are increasingly vulnerable to monsoon-induced disasters due to poor adaptation.
Need for Adaptation	Sustainable agriculture, reforestation, and resilient infrastructure are key responses.