Jet Streams, Western Disturbances and Their Role in Indian Weather

Formation and Nature of Jet Streams

Jet streams form at the boundary (tropopause) between the troposphere and stratosphere where strong horizontal temperature gradients exist. The temperature contrast between cold polar air and warm tropical air creates a strong pressure gradient at upper levels, which — deflected by the Coriolis force — produces a powerful west-to-east flow. Jet streams follow the polar front and the subtropical high-pressure belt. They are not smooth currents but meander in large sinusoidal waves called Rossby waves (or planetary waves), which migrate slowly westward relative to the mean flow. These meanders are responsible for persistent weather patterns on the surface.

Subtropical Westerly Jet and Indian Climate

The STJ over India undergoes a dramatic seasonal migration. In winter, it descends to about 25°N and splits into two branches around the Tibetan Plateau — one branch flows south of the Himalayas over northern India, and the other flows north of the plateau. The southern branch, passing over the Indian subcontinent, steers Western Disturbances from west to east. As summer approaches, the Tibetan Plateau heats up dramatically, reversing the temperature gradient. By late May to early June, the STJ shifts abruptly northward to about 40–45°N — north of the Himalayas — removing the barrier to moisture advection from the Indian Ocean. This shift is directly associated with the onset of the Southwest Monsoon over Kerala around June 1.

Tropical Easterly Jet and Monsoon Activity

The Tropical Easterly Jet (TEJ) is a unique upper-level wind system found at approximately 150 hPa (about 13–14 km) between 5°N and 20°N during the Indian Summer Monsoon season (June–September). It blows from east to west — opposite to the mid-latitude jets. The TEJ is maintained by the intense heating of the Tibetan Plateau, which creates a large upper-level high-pressure system (the Tibetan Anticyclone). Upper-level divergence associated with the TEJ is balanced by surface convergence and low-level moisture inflow — the very engine of monsoon rainfall. A strong TEJ correlates with an active monsoon; weakening of the TEJ signals monsoon breaks.

Western Disturbances: Mechanism and Impact

Western Disturbances are essentially extra-tropical cyclones — mid-latitude low-pressure systems — born over warm Mediterranean, Caspian, or Black Sea waters. They are embedded in the southern branch of the STJ and travel eastward at 30–40 km/h, typically reaching north-western India within 3–5 days of their formation. As they approach the Himalayas, they are further intensified — the orographic lifting of moist air over the western Himalayas produces heavy snowfall in higher altitudes and rain in the foothills and plains. A strong WD can bring 20–50 mm of rainfall over Punjab and Haryana within 24–48 hours.

WDs are the primary source of precipitation in northern India during winter. This winter rainfall is crucial for the Rabi cropping season — wheat in particular requires 1–2 wet spells between December and February for grain filling. Himalayan snowpack replenishment from WDs is equally critical: it is a major source of summer river flows for the Indus-Ganga river system, which irrigates over 250 million hectares of agricultural land.

Important Concepts and Subtopics

Rossby Waves

Large-scale meanders in the jet stream caused by the Coriolis force varying with latitude (beta effect). Troughs (southward dips) bring cold air south; ridges (northward bulges) bring warm air north. Stationary Rossby waves are maintained by large topographic features like the Himalayas and Tibetan Plateau.

Polar Front Jet vs. Subtropical Jet

The Polar Front Jet (PFJ) is found at 60°N and is associated with extra-tropical cyclones and Rossby waves. The STJ is found at 25–35° latitude and is more stable. Western Disturbances travel in the STJ (or in troughs of the PFJ that dip southward). Both jets can interact — a PFJ trough dipping toward the STJ intensifies WDs.

Cold Wave and Fog — WD Aftermath

After a WD passes, a cold wave intrusion from Central Asia follows in its wake, bringing dense cold wave conditions to the IGP. The residual moisture from the WD, combined with the cold and light winds of the post-WD period, creates ideal conditions for radiation fog and advection fog — severe fog that has been linked to thousands of deaths annually and major transportation disruptions.

Current Relevance

• Climate change impact on WDs: IITM Pune research shows WDs are becoming more intense but less frequent due to warming; erratic winter rainfall affects Rabi yields.
• Glacial melt concern: Reduced Himalayan snowpack from fewer WDs threatens long-term water security for Indus-Ganga basin — linked to India's water crisis debate.
• Monsoon onset prediction: Monitoring STJ position is a primary tool for predicting SW monsoon onset — critical for IMD's June 1 forecast.
• Delhi fog crisis: Recurring winter fog (Dec–Jan) causing aviation, rail, and road disruption — directly linked to WD moisture + cold wave regime.
• Western Disturbance in summer: Occasional off-season WDs in April–May cause pre-monsoon hailstorms and thunderstorms over NW India — affecting wheat harvest.