Mains Practice Questions

Approach:

• Introduce the answer by briefing about Indian monsoon and role of subtropical westerly and tropical easterly jet streams
• Give role of Subtropical Westerly Jet Stream (STWJ) and Tropical Easterly Jet Stream (TEJ) in e onset and withdrawal of the Indian monsoon.
• Conclude suitably.

Introduction

The Indian monsoon is a seasonally reversing wind system primarily driven by differential heating between land and ocean, with critical modulation by upper-tropospheric jet streams.

• Among them, the Subtropical Westerly Jet Stream (STWJ) and the Tropical Easterly Jet Stream (TEJ) play a pivotal role in determining the timing, strength, and spatial distribution of the onset and withdrawal of the Indian summer monsoon (ISM).

Body:

Role of Subtropical Westerly Jet Stream (STWJ):

• A high-velocity westerly wind belt, active in winter and spring months. Found at latitudes of 25°–35° N, approximately 200 hPa (12–14 km) altitude.
  • Associated with upper-level divergence, cold air advection, and western disturbances.
  • Role in Monsoon Onset
    • Winter Dominance Suppresses Monsoon Formation
      • During winter and early summer, STWJ lies over the Indian subcontinent, contributing to cool and dry weather.
      • Its presence aloft suppresses vertical convection, thereby inhibiting pre-monsoon rainfall.
    • Northward Shift is a Trigger for Onset
      • By late May or early June, due to intense heating of the subcontinent and Tibetan Plateau, the STWJ shifts northward into the mid-latitudes.
      • This vacates upper-air space, enabling ascent of moist air and low-pressure formation at the surface.
    • Enhances Tropical Convective Activity
      • Its withdrawal allows ITCZ (Inter-Tropical Convergence Zone) to move northward, aligning over the Indian landmass.
      • This leads to the burst of the monsoon, beginning typically over the Kerala coast.
  • Role in Monsoon Withdrawal
    • Southward Re-establishment
      • In September–October, with cooling of the landmass, STWJ shifts southward, reinstating stable, dry upper-level air over the subcontinent.
    • Promotes Drying and Stability
      • The re-entry of STWJ suppresses convection and marks the transition from southwest monsoon to northeast monsoon (especially over Tamil Nadu).

Role of Tropical Easterly Jet Stream (TEJ):

• Strong easterly winds peaking during June to August. Located between 5°–20° N, at altitudes of 100–150 hPa (6-9km).
  • Originates due to strong thermal gradients between the equatorial Indian Ocean and the heated Tibetan Plateau (forming the Tibetan High).
  • Role in Monsoon Onset
    • Formation of Upper-Level Easterlies
      • The TEJ sets in after STWJ’s northward shift, usually by late May.
      • It reflects the establishment of a warm-core upper-level high over Tibet, a prerequisite for monsoon onset.
    • Supports Deep Convection
      • TEJ induces upper-level divergence, encouraging ascent of moist air from the Arabian Sea and Bay of Bengal.
      • Promotes vigorous convective rainfall across the Indian subcontinent.
    • Strengthens Monsoonal Circulation
      • TEJ enhances vertical wind shear, which is favorable for organized monsoon activity and widespread rainfall.
    • Onset Confirmation Parameter
      • The India Meteorological Department (IMD) uses TEJ strength and positioning as a parameter to confirm monsoon onset.
  • Role in Monsoon Withdrawal
    • Gradual Weakening in September
      • As surface heating declines, the thermal contrast over the Tibetan Plateau weakens, leading to the disintegration of TEJ.
    • Collapse of Monsoonal Vertical Support
      • The disappearance of upper-level divergence results in weakened convection and a gradual drying of the atmosphere.
    • Withdrawal from Northwest to Southeast
      • TEJ’s weakening facilitates the organized retreat of monsoon from northwest India first, progressing toward the southern peninsula.

Conclusion:

The Subtropical Westerly Jet and the Tropical Easterly Jet form a dynamic upper-atmospheric framework that governs the onset, intensity, and withdrawal of the Indian monsoon. Their inverse movement (STWJ retreats as TEJ strengthens) is a classic feature of monsoon dynamics.