Subtropical westerly jet stream

Geography glossary

Also called: STWJ, Subtropical westerly jet, Westerly jet stream · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 4 "Climate"

Meaning

The subtropical westerly jet stream (STJ) is a narrow band of very fast west-to-east wind. It blows high in the sky, about 12–14 km up, near 30° latitude, and sits above the poleward edge of the Hadley cell. Over India it matters mainly in winter. It splits around the Tibetan Plateau, and its southern branch steers western disturbances into north-west India. In early summer it jumps north of Tibet, and this helps the south-west monsoon set in.

Explanation

How it forms

  • Jet stream: a narrow band of very fast wind in the upper troposphere, just below the tropopause (the boundary between the troposphere below and the stratosphere above).

  • The fastest part is called the core. Core speeds are often 150–300 km/h.

  • Cause → process → result:

  • Warm air and cold air meet along a sharp boundary.
  • This creates a steep pressure difference high in the air.

  • A fast, narrow band of wind forms along that boundary.

  • Why it flows west to east:

  • High in the air there is almost no friction.
  • The pressure gradient force (the push from high pressure to low pressure) is balanced by the Coriolis force (the turning effect of the Earth's spin).

  • The wind therefore blows parallel to the isobars (lines of equal pressure). This is a geostrophic wind. Jets are near-geostrophic, so they flow roughly west to east around the globe.

  • Link to the Hadley cell:

  • The Hadley cell is the tropical circulation loop. Air rises at the equator and sinks near 30°.

  • The STJ flows above the poleward edge of this cell, near 30° latitude.

  • Season: jets are strongest in winter, when the temperature difference between the equator and the poles is greatest.

Winter: the split around Tibet

  • The Tibetan Plateau stands in the jet's path.
  • The jet splits into two branches, one north of Tibet and one south of it.

  • The southern branch blows over north India, along the southern side of the Himalaya.

  • Steering western disturbances (WDs):

  • WDs are low-pressure systems from the Mediterranean region. They bring winter rain and snow to north India.
  • A WD is a trough (a dip of low pressure) in the middle- and upper-level westerlies.
  • The southern branch of the STJ steers WDs eastward into north-west India.
  • "Active subtropical jet stream dynamics" help a WD strengthen and move east [2].

Early summer: the jump north

  • The jet leaves its position south of the Himalaya.
  • It jumps north of the Tibetan Plateau.
  • This removes the westerly "lid" over north India.

  • With the lid gone, the monsoon can set in.

  • The STJ works together with the tropical easterly jet and the Somali jet to set up and supply the south-west monsoon. A change in the timing of the STJ's shift can delay or weaken the monsoon.

Where it occurs

  • World: near 30° latitude, at about 12–14 km height, above the poleward edge of the Hadley cell. It is one of the main upper-air jets.
  • Tibetan Plateau: the barrier that splits the jet into northern and southern branches in winter.
  • North India and the southern side of the Himalaya: the path of the southern branch in winter.
  • North-west India: where the southern branch steers western disturbances that come from the Mediterranean region.
  • Case, 26–29 December 2024: a WD met an easterly trough (a low-pressure trough in the easterly winds to the south). Together they caused deep thunderclouds and severe weather over Delhi and north-west and central India. Moisture came from both the Arabian Sea and the Bay of Bengal [2].
  • North of the Tibetan Plateau: where the jet moves in early summer, before the monsoon sets in.

Don't confuse with

  • Tropical easterly jet stream (TEJ): it blows from the east, at 13–16 km near 15°N over peninsular India and the Arabian Sea. It appears only in summer and is driven by the heating of Tibet (Koteswaram, 1958). The STJ is westerly and works over India mainly in winter.
  • Polar front jet stream: it is lower (9–12 km) and lies farther poleward (~40–60°), above the polar front. It guides temperate cyclones, not western disturbances.
  • Somali (Findlater) jet: a low-level jet at only 1–1.5 km. It crosses the equator along the East African coast and carries monsoon moisture. It is not an upper-air westerly jet.
  • Polar vortex: a large low-pressure area of cold air circling each pole, in the stratosphere and upper troposphere. The STJ blows in the upper troposphere, just below the tropopause, not in the stratosphere.

Prelims Hooks

  • Height order of jets: Somali jet (1–1.5 km) → polar front jet (9–12 km) → subtropical westerly jet (12–14 km) → tropical easterly jet (13–16 km).
  • Western disturbances are steered by the southern branch of the STJ in winter. They are not steered by the TEJ or the polar front jet. This is a common trap.
  • In winter the STJ splits around the Tibetan Plateau. The southern branch runs along the southern side of the Himalaya over north India.
  • Its early-summer jump north of Tibet removes the westerly "lid" and helps the monsoon set in.
  • Jets are strongest in winter, not summer. They blow just below the tropopause, not in the stratosphere.
  • Eastbound flights are faster because they ride the westerly jets. Jets were first measured by Wasaburo Ooishi (Japan, 1920s).

Mains Points

  • Winter rain and the rabi crop (GS-I):
  • The STJ's southern branch steers western disturbances into north-west India.

  • These bring winter rain and snow. They help the rabi crop and feed the Himalayan snowpack.

  • Monsoon onset (GS-I):

  • The STJ's early-summer jump north of Tibet, the TEJ and the Somali jet together set up and supply the south-west monsoon.

  • If the jet's shift comes late or changes, the monsoon can be delayed or weakened.

  • Extreme weather and early warning (GS-III):

  • A WD can join an easterly trough and draw in moisture from both the Arabian Sea and the Bay of Bengal. This happened over Delhi and north-west and central India on 26–29 December 2024 [2].
  • Tracking the jet and WDs is therefore key to impact-based forecasting and early-warning systems for winter storms.

Related concepts

Read more

Sources

  1. 1Class 11, Ch 4 "Climate" (primary)
  2. 2Decoding the dynamics: Interaction of Western disturbances and Easterly troughs driving extreme weather over India (MAUSAM, IMD, 2025)mausamjournal.imd.gov.in · tier 1