Jet Streams and Indian Monsoon: Mechanism, Types & UPSC Notes (2026)
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Mar 30, 2026
Introduction
The atmospheric circulation of the Earth is governed by complex interactions between temperature, pressure, and planetary rotation. Among these, jet streams play a decisive role in shaping global and regional weather systems. In the Indian context, the behaviour of jet streams is closely linked with the onset, intensity, and variability of the monsoon, making them a crucial element in understanding seasonal rainfall patterns.
What are Jet Streams?
Jet streams are narrow bands of high-speed winds that flow in the upper troposphere, generally between 9–16 km altitude. They follow a wavy path and are primarily driven by temperature gradients between air masses.
- They flow from west to east (westerlies) in both hemispheres
- Wind speeds can reach 150–400 km/h
- They are most prominent in mid-latitudes
- Their path is not straight but meandering (Rossby waves)
Types of Jet Streams
Jet streams are classified based on their location and origin:
| Type of Jet Stream | Location | Key Characteristics |
|---|---|---|
| Polar Front Jet (PFJ) | 50°–60° latitudes | Strong, highly variable, associated with cyclones |
| Subtropical Jet (STJ) | 25°–30° latitudes | Stable, flows over India in winter |
| Tropical Easterly Jet (TEJ) | Over tropics (India during monsoon) | Flows east to west, linked with monsoon |
| Polar Night Jet | Polar regions | Seasonal, forms during winter |
Subtropical Westerly Jet (STJ) and Indian Monsoon
During winter, the Subtropical Westerly Jet (STJ) flows south of the Himalayas and significantly influences weather conditions.
- Located at about 25°N latitude, over northern India
- Brings western disturbances, causing winter rainfall in north-west India
- Creates dry conditions over most parts of India
Role in Monsoon
- The northward shift of STJ from India marks the onset of the southwest monsoon
- Its withdrawal allows the development of low-pressure conditions over the northern plains
Tropical Easterly Jet (TEJ) and Monsoon
With the onset of summer, a new jet stream called the Tropical Easterly Jet (TEJ) develops over the Indian subcontinent.
- Flows from east to west, unlike most jet streams
- Develops due to intense heating of the Tibetan Plateau
- Located around 10°–15°N latitude
Role in Monsoon
- Strengthens the monsoon circulation
- Enhances rainfall over India, especially central and peninsular regions
- Its strength is directly related to monsoon intensity
Mechanism: Jet Streams and Monsoon Linkage
The relationship between jet streams and monsoon can be understood through seasonal transitions:
Winter Season
- Presence of STJ over India
- Dominance of high pressure over land
- Winds flow from land to sea → dry conditions
Pre-Monsoon Transition
- Increasing solar heating shifts pressure belts northward
- STJ moves north of the Himalayas
- Creates favourable conditions for monsoon onset
Summer Monsoon
- Formation of low pressure over north-west India
- Establishment of TEJ
- Strong moist south-westerly winds bring rainfall
Factors Influencing Jet Streams and Monsoon
Temperature Gradient Difference between equatorial and polar regions drives jet streams.
Tibetan Plateau Heating Acts as a heat source — crucial for TEJ formation.
Coriolis Force Deflects winds and maintains westerly/easterly flow patterns.
El Niño–Southern Oscillation (ENSO) Alters jet stream patterns and affects monsoon variability.
Indian Ocean Dipole (IOD) Influences moisture supply and interacts with jet stream dynamics.
Jet Streams and Monsoon Variability
Jet streams are not static — their fluctuations lead to variations in monsoon behaviour:
- Weak TEJ → Weak monsoon rainfall
- Delayed northward shift of STJ → Late monsoon onset
- Erratic jet stream patterns → Uneven rainfall distribution
- Blocking patterns → Prolonged dry or wet spells
Comparison: STJ vs TEJ
| Feature | Subtropical Westerly Jet (STJ) | Tropical Easterly Jet (TEJ) |
|---|---|---|
| Direction | West to East | East to West |
| Season | Winter | Summer |
| Location | North of India (winter) | Over peninsular India |
| Impact | Dry conditions, western disturbances | Strengthens monsoon rainfall |
| Role | Prevents monsoon | Supports monsoon |
Contemporary Relevance
Climate change is altering jet stream patterns, leading to increased extreme rainfall events and greater monsoon unpredictability. Recent observations show shifts in jet stream position and an increased frequency of monsoon breaks.
Understanding jet streams is essential for weather forecasting, disaster management, and agricultural planning.
##Conclusion
Jet streams act as invisible atmospheric highways that regulate seasonal weather patterns. Their seasonal migration and interaction with land-sea thermal contrasts determine the onset, strength, and variability of the monsoon. A clear understanding of this relationship provides crucial insights into rainfall behaviour, climatic anomalies, and long-term environmental changes.
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