Jet Streams and Indian Weather

Jet Streams are narrow bands of high-velocity, meandering winds that circulate in the upper troposphere or lower stratosphere, typically at altitudes ranging from 9 to 14 kilometers. Moving from west to east at speeds that can exceed 180 km/h to 400 km/h, these upper-air currents are driven by strong global thermal gradients and the Earth’s rotation (Coriolis force). The Indian subcontinent’s weather patterns are fundamentally shaped by two major seasonal jet streams: the Subtropical Westerly Jet Stream and the Tropical Easterly Jet Stream. These upper-air currents act as the structural framework controlling the onset, stability, and withdrawal of the Indian Monsoon, as well as winter weather anomalies.

The Subtropical Westerly Jet Stream (STWJ)

The STWJ is a global upper-air current that operates across the mid-latitudes. Its seasonal migration and structural split play a decisive role in both winter precipitation and the timing of the summer monsoon onset over India.

Winter Configuration and the Bifurcation Mechanism

During the winter months (November to February), the apparent southward migration of the Sun shifts the global thermal equator down. This forces the STWJ to move south and flow directly across southern Asia. When this high-velocity wind stream encounters the physical, high-altitude obstruction of the Tibetan Plateau and the Himalayas (which average 4 to 5 km above sea level), it undergoes a physical bifurcation:

• The Northern Branch: Flows east along the northern periphery of the Tibetan Plateau across Central Asia and China.
• The Southern Branch: Establishes itself south of the Himalayas, blowing from west to east across northern and northwestern India at an altitude of nearly 12 km, centered around 25° N latitude.

Impact on Winter Weather: Western Disturbances

The southern branch of the STWJ acts as an atmospheric conveyor belt, transporting mid-latitude, non-tropical cyclonic storms originating over the Mediterranean Sea into India.

• Precipitation: These storms, known as Western Disturbances, are guided by the jet stream across Pakistan and into northwest India, bringing essential winter rain to the plains of Punjab and Haryana, and heavy snowfall to the Western Himalayas.
• Anti-Cyclonic Subsidence: The presence of the jet south of the Himalayas creates an upper-air configuration that forces air downward over the northern plains. This anti-cyclonic subsidence maintains cold, stable, and dry winter conditions, punctuated only by the passage of these disturbances.

The Summer Retreat and Monsoon Trigger

As spring transitions to summer (March to May), the solar footprint moves north toward the Tropic of Cancer. The continental landmass of India heats up rapidly, causing the southern branch of the STWJ to weaken and slide northward. By the end of May, the southern branch completely withdraws from its position south of the Himalayas, shifting to the north of the Tibetan Plateau. This sudden northward displacement is the critical upper-air structural change that triggers the “Burst” of the Southwest Monsoon. Without this retreat, the lower-tropospheric moist monsoon winds remain trapped over the Indian Ocean.

The Tropical Easterly Jet Stream (TEJ)

Unlike the global STWJ, the Tropical Easterly Jet Stream is a seasonal, regional upper-air anomaly that develops exclusively during the summer monsoon months (June to September). It runs in the opposite direction—from east to west—across Peninsular India.

The Tibetan Plateau Heat Engine

The genesis of the TEJ is directly linked to the intense heating of the Tibetan Plateau during summer. Lying at an average elevation of over 4,000 meters, this massive rocky landmass absorbs intense solar radiation and acts as a high-altitude heat source. The air above the plateau heats up rapidly and rises, establishing a powerful, warm thermal anti-cyclone in the upper troposphere (around 13 km altitude). The air escaping from the southern flank of this Tibetan anti-cyclone is deflected by the Coriolis force, forming a high-velocity easterly wind stream known as the Tropical Easterly Jet.

Spatial Track and Dynamics

The TEJ extends from the southwest South China Sea across Peninsular India (stabilizing around 14° N to 15° N latitude, near Chennai) and continues toward East Africa.

• The Mascarene High Link: After crossing India, the air mass within the TEJ flows over the equatorial Indian Ocean and descends over the western Indian Ocean near Madagascar. This downward flow reinforces the Mascarene High, a critical lower-level high-pressure system.
• The Monsoon Pull: The intensified Mascarene High drives the Southeast Trade Winds across the equator with greater velocity, strengthening the lower-level Southwest Monsoon currents pushing toward the Indian mainland.

Steering Monsoon Depressions

The TEJ governs the trajectory of low-pressure cyclonic depressions that form over the head of the Bay of Bengal. It steers these rain-bearing systems westward and northwestward across the central plains of India, ensuring uniform moisture distribution across the agricultural heartland. A strong, well-developed TEJ is consistently correlated with a stable and healthy monsoon season.

The Somali Jet (Low-Level Jet Stream)

While the STWJ and TEJ operate in the upper troposphere, the Somali Jet (also known as the Findlater Jet) is a specialized low-level jet stream that operates in the lower troposphere, around an altitude of 1.5 km.

Origin and Path

During the summer monsoon, intense low pressure over the Indian landmass draws air from the southern Indian Ocean. This air mass is funneled northward along the eastern coast of Africa as a high-velocity coastal current. It sweeps past the coast of Somalia before turning sharply eastward across the Arabian Sea toward the west coast of India.

Impact on Rainfall

The Somali Jet channels high-velocity, moisture-saturated maritime air directly into the Indian peninsula. The strength of this low-level jet determines the intensity of the southwest monsoon’s arrival:

• Strong Somali Jet: Causes heavy orographic rainfall along the windward slopes of the Western Ghats (Konkan and Malabar coasts).
• Weak Somali Jet: Leads to a delayed onset or a temporary lull in rainfall along the western coast.

Structural Comparison: Upper-Air Jet Streams affecting India

High-Yield Facts for UPSC Prelims

The Hydrodynamic Reversal Concept

The transition from winter to summer over India represents a rare vertical reversal of the upper-air wind profile. In winter, the upper atmosphere is entirely dominated by the westerly winds of the STWJ. In summer, this is replaced by the easterly flow of the TEJ. This complete seasonal turnaround is a defining feature of the tropical monsoon climate.

The Mechanism of Monsoon “Breaks”

A temporary weakening or northward shift of the Tropical Easterly Jet can trigger a dry spell known as a “Break in the Monsoon.” When the TEJ shifts or weakens, upper-air support for lower-level cyclonic activity collapses, causing rainfall to cease over Central India while heavy rain shifts to the Himalayan foothills.

Teleconnection with El Niño

During an El Niño year, the thermal anti-cyclone over Tibet often fails to develop fully due to altered global heating patterns. This results in a weak and erratic Tropical Easterly Jet, which fails to properly reinforce the Mascarene High, frequently causing deficient or drought-like monsoon rainfall across India.

Jet Streak Interaction with Local Convection

In late April and May, before the STWJ completely retreats north of the Himalayas, its high-velocity core can interact with hot, convective air masses rising from the warming northern plains. This upper-air divergence destabilizes the atmosphere, acting as a trigger for violent pre-monsoon thunderstorms like the Kalbaishakhi in West Bengal.