Tropical easterly jet stream

Also called: TEJ, Easterly jet stream, Tropical easterly jet · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 4 "Climate"

Meaning

The Tropical easterly jet stream (TEJ) is a narrow band of very fast wind that blows from east to west high in the sky, at about 13–16 km, near 15°N over peninsular India and the Arabian Sea, and it appears only in summer.

• It forms because the Tibetan Plateau heats up strongly in summer.
• Because it exists only during the monsoon season, its strength is linked to how strong the south-west monsoon is. NCERT links it to the sudden start, or "burst", of the monsoon.

Explanation

How it forms: cause → process → result

• Cause: Tibet heats up in summer
• The Tibetan Plateau is very high. In summer it heats up strongly.

• It works like a raised heat source at about 4–5 km above sea level.

• Process: a high-pressure area forms in the upper air

• Warm air over Tibet rises.

• High above Tibet, a high-pressure area (anticyclone) forms. An anticyclone is a high-pressure system that air flows out of.

• Process: the Coriolis force turns the outflow

• Upper winds flow south out of this high.
• The Coriolis force (the turning effect of the Earth's spin) turns them to the right in the Northern Hemisphere. Air moving south is therefore turned towards the west.

• South of Tibet, these winds become strong easterlies (winds blowing from the east).

• Result: the TEJ

• The easterlies gather into a fast, narrow band at about 13–16 km, near 15°N, over peninsular India and the Arabian Sea.

• This band is the Tropical easterly jet stream.

• Discovery: it was first described by P. Koteswaram (1958).

Its key features

• Height: about 13–16 km, in the upper troposphere, just below the tropopause (the boundary between the troposphere below and the stratosphere above). It is the highest of the four main jets in the study list.
• Direction: easterly. It is the only easterly jet among the four main jets. The polar front jet and subtropical jet are westerly, and the Somali jet is south-westerly.
• Season: summer only. This is unusual, because most jets are strongest in winter.
• Pressure levels: a satellite study of June 1977 ("Monsoon-77") found that the jet's cloud patterns lined up with the strongest winds at the 150 and 100 mb levels (roughly 13–16 km) [3].

Its link with the monsoon

• Conditions it needs:
• Strong summer heating of the Tibetan Plateau.

• The subtropical westerly jet (STJ) must first move away from north India. In early summer the STJ jumps north of the Tibetan Plateau. This removes the westerly "lid" over north India and makes room for the summer circulation, including the TEJ.

• Monsoon strength: the TEJ exists only in summer, so its strength is linked to how strong the monsoon is.

• "Right entrance" sector: the Monsoon-77 study found the monsoon was active in the "right entrance" sector of the jet. This is the part where the jet starts to speed up [3].
• NCERT view: the India book links the TEJ to the sudden onset, or "burst", of the monsoon. The burst means heavy rain arrives suddenly, not slowly.
• Team effort: the TEJ does not bring the monsoon by itself. It works with:
• the STJ's early-summer jump north, and
• the low-level Somali jet, which carries much of the monsoon's moisture.

Where it occurs

• Source region: the Tibetan Plateau. Its summer heating creates the upper-air high that drives the jet.
• Main belt: near 15°N, over peninsular India (south India) and the Arabian Sea.
• Height: about 13–16 km, near the 150 and 100 mb pressure levels [3].
• Time: summer monsoon season only. It is absent in winter, when the southern branch of the subtropical westerly jet blows over north India instead.
• Studied example: the June 1977 "Monsoon-77" satellite study over the Indian monsoon region [3].

Don't confuse with

• Subtropical westerly jet stream (STJ): it is westerly, flows at 12–14 km near 30°, and sits over India mainly in winter. It steers western disturbances. The TEJ is easterly, higher, further south (~15°N) and blows only in summer.
• Somali (Findlater) jet: it is a low-level jet at only 1–1.5 km. It is south-westerly, crosses the equator along the East African coast, and carries monsoon moisture (Findlater, 1969). The TEJ is an upper-level easterly jet at 13–16 km.
• Polar front jet stream: it is westerly, at 9–12 km, near 40–60°, and blows all year. It guides temperate cyclones. It has nothing to do with the Indian monsoon burst.
• Easterly trough: this is a low-pressure trough in the easterly winds, not a fast wind band. One such trough combined with a western disturbance over Delhi and north-west India in December 2024 [2].

Prelims Hooks

• Only easterly jet among the four main jets: the TEJ. It is a summer jet near 15°N over peninsular India and the Arabian Sea.
• Cause: summer heating of the Tibetan Plateau, which forms an upper-air anticyclone. First described by P. Koteswaram (1958).
• Jets from lowest to highest: Somali jet (1–1.5 km) → polar front jet (9–12 km) → subtropical westerly jet (12–14 km) → tropical easterly jet (13–16 km).
• Trap: "Jet streams are strongest in summer" is false in general. Jets are strongest in winter. The TEJ is the exception: it exists only in summer.
• Trap: western disturbances are steered by the southern branch of the STJ, not by the TEJ.
• Monsoon-77 (June 1977): the monsoon was active in the jet's "right entrance" sector, and the strongest winds were at 150 and 100 mb [3]. NCERT links the TEJ to the monsoon "burst".

Mains Points

• Upper-air control of the Indian monsoon (GS-I):
• The STJ's early-summer jump north of Tibet, the TEJ high above, and the low-level Somali jet together set up and supply the south-west monsoon.
• Summer heating of Tibet drives the TEJ, which shows how relief (the shape and height of land) controls Asian climate.

• NCERT links the TEJ to the sudden burst of the monsoon, and its strength is linked to monsoon strength.

• Timing shifts and monsoon risk (GS-I/III):

• If the STJ is late to move north, or the TEJ is weak, the monsoon can be delayed or weakened.
• This affects kharif sowing, water storage and drought risk.
• Tracking the TEJ, including its active "right entrance" sector [3], is useful for monsoon forecasting and early warning for farmers.

Related concepts

• Jet stream
• Polar front jet stream
• Subtropical westerly jet stream
• Somali jet
• Rossby waves
• Polar vortex

Read more

• Jet streams, Rossby waves and the polar vortex