Jet Streams

Notes

Jet streams are narrow, fast-moving bands of westerly winds in the upper troposphere (about 9-14 km, near the tropopause), like rivers of air. Their speed is generally above 150 km/h and can reach 300 km/h or more; they are stronger in winter, when the temperature contrast is greater. They arise from sharp temperature and pressure gradients and are deflected by the Coriolis force; they meander in Rossby waves.

Major jet streams

Polar front jet: at about 60° latitude, 9-12 km; the strongest, and it steers mid-latitude cyclones.
Subtropical westerly jet (STJ): at about 30° latitude, 10-16 km.
Tropical Easterly Jet (TEJ): an easterly jet at about 8-15°N and 14-16 km in the Indian summer.
Polar night jet: in the stratosphere.
The Somali (Findlater) jet is a low-level jet (about 1.5 km) that carries moisture across the equator to India in summer.

Role in the Indian climate

Winter: the STJ is split by the Tibetan Plateau into two branches. The southern branch blows along the southern edge of the Himalaya and steers Western Disturbances to north-west India, bringing winter rain and snow.
Summer: the STJ shifts north of the Tibetan Plateau (around June), which is linked to the burst of the monsoon. The TEJ then sets in over peninsular India and supports the monsoon.
Changes in the jets help explain the onset, break and withdrawal of the monsoon.

Other effects: steering of weather systems and cyclones; shaping heat waves and cold waves; aviation (tailwinds and clear-air turbulence); and possibly more wavy, slow-moving patterns with Arctic warming.

Key Points

Jet streams: upper-troposphere westerlies (9-14 km), above 150 km/h.
Types: polar front jet, subtropical jet, tropical easterly jet, polar night jet.
Winter: STJ's southern branch steers Western Disturbances.
Summer: STJ shifts north of Tibet; TEJ sets in; monsoon bursts.
Findlater (Somali) jet carries moisture to India.

MCQs

Q1. The Tropical Easterly Jet is associated with:

(a) The winter monsoon
(b) The summer monsoon of India
(c) Western Disturbances
(d) The polar vortex
Ans: (b).

Q2. Western Disturbances reach north-west India in winter with the help of the:

(a) Tropical Easterly Jet
(b) Subtropical Westerly Jet
(c) Somali Jet
(d) Polar Night Jet
Ans: (b).

Q3. Jet streams flow at a height of about:

(a) 1-2 km
(b) 9-14 km
(c) 30-40 km
(d) 80-100 km
Ans: (b).