Jet streams, Rossby waves and the polar vortex

Detail

1. What a jet stream is

• Jet stream: a narrow band of very fast wind high in the sky. It winds and bends as it moves around the Earth.
• Height: the upper troposphere, about 9–16 km up, just below the tropopause (the boundary between the troposphere below and the stratosphere above).
• Speed: the fastest part is called the core. Core speeds are often 150–300 km/h.

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

• Why jets form:

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

• The result is a fast, narrow band of wind along that boundary.

• Geostrophic wind: a wind that blows parallel to the isobars (lines of equal pressure). It forms when 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). High in the air there is almost no friction, so this balance holds. That is why jets are near-geostrophic and flow roughly west to east around the globe. (NCERT, Class 11 Ch 9: geostrophic flow is the basis of upper-air winds.)

• Discovery: Wasaburo Ooishi measured jets over Japan in the 1920s using weather balloons. Second World War bomber pilots later made them famous: flying west over the Pacific, they found headwinds strong enough to slow them almost to a stop.
• Use in flying: eastbound flights are faster because they ride the westerly jets. Westbound flights try to stay out of them.

2. The main jets: structure and location

Jet	Height	Latitude / place	Season	Direction	Key role
Polar front jet stream	9–12 km	~40–60°, above the polar front	All year; strongest in winter	Westerly	Guides temperate cyclones; brings cold outbreaks when it dips south
Subtropical westerly jet stream (STJ)	12–14 km	~30°	Over India mainly in winter	Westerly	Splits around Tibet; southern branch steers western disturbances; its jump north helps the monsoon set in
Tropical easterly jet stream (TEJ)	13–16 km	~15°N, over peninsular India and the Arabian Sea	Summer only	Easterly	Driven by heating of Tibet (Koteswaram, 1958); NCERT links it to the monsoon "burst"
Somali (Findlater) jet	Low level, 1–1.5 km	East African coast → Arabian Sea	Summer monsoon	South-westerly; crosses the equator	Carries much of the monsoon's moisture (Findlater, 1969)

a) Polar front jet stream

• Polar front: the boundary where cold polar air meets warmer mid-latitude air. It lies at roughly 40–60° latitude.
• The jet flows above the polar front. It has the sharpest temperature contrast, so it is the most changeable jet.
• Its wavy path guides temperate cyclones (low-pressure storms of the mid-latitudes). Storms form and move along the jet's track.
• When the jet dips south, cold polar air flows south behind it. This brings cold outbreaks to North America, Europe and Central Asia.

b) Subtropical westerly jet stream (STJ)

• It flows at about 12–14 km, near 30° latitude, above the poleward edge of the Hadley cell (the tropical circulation loop: air rises at the equator and sinks near 30°).
• Winter over Asia:
• 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.

• Western disturbances (WDs): low-pressure systems from the Mediterranean region. They bring winter rain and snow to north India.

• The southern branch of the STJ steers them eastward into north-west India.
• A WD is a trough (a dip of low pressure) in the middle- and upper-level westerlies. "Active subtropical jet stream dynamics" help it strengthen and move east [2].

• 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. The moisture came from both the Arabian Sea and the Bay of Bengal [2].

• Early summer:

• The jet leaves its position south of the Himalaya.
• It jumps north of the Tibetan Plateau.
• This removes the westerly "lid" over north India, which helps the monsoon set in. Full monsoon detail is in indian-climate-monsoon.

c) Tropical easterly jet stream (TEJ)

• Cause, step by step:
• In summer the Tibetan Plateau heats up strongly. It acts as a raised heat source at about 4–5 km.
• The warm air rises, and a high-pressure area (anticyclone) forms in the upper air over Tibet.
• Upper winds flow out of this high and are turned by the Coriolis force. They become strong easterlies to the south.

• These easterlies form the TEJ at about 13–16 km, near 15°N, over peninsular India and the Arabian Sea.

• It was first described by P. Koteswaram (1958).

• It exists only in summer, so its strength is linked to how strong the monsoon is.
• A satellite study of June 1977 ("Monsoon-77") found that:
• jet cloud patterns lined up with the strongest winds at the 150 and 100 mb levels (roughly 13–16 km);

• the monsoon was active in the "right entrance" sector of the jet, where it starts to speed up [3].

• NCERT: the India book links the TEJ to the sudden onset, or "burst", of the monsoon.

d) Somali jet (Findlater jet)

• Low-level jet: a fast wind band near the ground, about 1–1.5 km up. It is not an upper-air jet.
• Path:
• It starts as south-east trades in the southern Indian Ocean.
• It crosses the equator near the East African coast (Kenya and Somalia), where the East African highlands channel it.
• The Coriolis force turns it to the right, so it becomes a south-westerly.

• It blows across the Arabian Sea to the west coast of India.

• Role: it carries much of the monsoon's moisture to India.

• It was first described by J. Findlater (1969).

3. Rossby waves

• Rossby waves (planetary waves): giant loops in the upper westerlies and the jets.
• Cause, step by step:
• The Coriolis effect is weak near the equator and strong near the poles.
• When air moving east is pushed north or south, this change with latitude pulls it back.
• The air overshoots and swings back again, so the flow snakes in large loops.

• Mountain ranges (the Rockies, Tibet) and land–sea heating contrasts help start the loops.

• Number: usually four to six waves circle the globe.

• Ridges and troughs:
• A ridge is a loop bending toward the pole. Under it, warm high-pressure weather sits.

• A trough is a loop bending toward the equator. Under it, cold, unsettled weather sits.

• What they do:

• They "transfer heat from the tropics toward the poles and cold air toward the tropics" to bring the atmosphere back toward balance [4].

• They help locate the jet stream and mark the track of surface low-pressure systems [4].

• Blocking: when Rossby waves slow down or stall, weather gets stuck in one place.

• The slow motion of these waves "often results in fairly long, persistent weather patterns" [4].
• Possible results: long heat waves, cold spells or floods.

Event	Where	What the stuck pattern did
2010 Russian heat wave + Pakistan floods	Western Russia; Indus basin, Pakistan	A blocking ridge over Russia gave long heat and wildfires. The trough downstream fed extreme monsoon rain over Pakistan.
2021 heat dome	Pacific north-west of the USA and western Canada (June–July 2021)	A stalled ridge trapped hot air, like a lid on a pot, and gave record heat.

4. The polar vortex

• Polar vortex: a large area of low pressure and cold air circling each pole, in the stratosphere and upper troposphere.
• It is a large "circumpolar westerly circulation" that exists between autumn and spring [5].

• It has swirling westerly winds around its edge. Normally these winds are strong enough to keep the coldest air in the Arctic in winter [6].

• Sudden stratospheric warming (SSW): a rapid rise in temperature in the polar stratosphere [5].

• Cause: Rossby waves from the troposphere are pushed upward into the stratosphere.
• Process: the waves break there and slow the vortex winds, much as a breaking sea wave loses its energy [4].

• Result: the vortex weakens, shifts or splits, and Arctic air spills south into the mid-latitudes.

• February 2021 Texas cold wave:

• An SSW about 30 km above the North Pole weakened the polar vortex. Cold air spilled into the mid-latitudes and warmer air entered the Arctic [6].
• Dallas fell to −16 °C on 15 February 2021, its coldest since 1989. Normal for that time of year is about 15.6 °C [6].
• About 4 million people in Texas lost power [6].
• By midnight on 16 February 2021, 73% of the continental USA was under snow. This was the largest extent in the record, which began in 2003 [6].

• The Arctic Oscillation had been strongly negative since early December 2020 [6]. (The Arctic Oscillation is a see-saw of air pressure between the Arctic and the mid-latitudes. In its negative phase, cold air escapes south more easily.)

• Another SSW disturbed the vortex in January 2024 [5].

5. Arctic warming and a "wavier" jet: an open debate

• The claim, step by step:
• The Arctic is warming faster than the rest of the world (Arctic amplification).
• The temperature contrast between the pole and the mid-latitudes gets smaller.

• The jet may then become weaker and wavier, with more blocking and more cold outbreaks.

• Status: this is still debated. Observations and models do not yet agree. (Link: climate-change.)

Prelims Hooks

• Lowest to highest jet: Somali jet (1–1.5 km) → polar front jet (9–12 km) → subtropical westerly jet (12–14 km) → tropical easterly jet (13–16 km).
• Only easterly jet among the main four: the Tropical easterly jet. It is a summer jet near 15°N over peninsular India, linked to Tibetan heating (Koteswaram, 1958).
• Somali jet is a low-level jet: it crosses the equator along the East African coast (Findlater, 1969). It is not an upper-tropospheric jet. This is a classic trap.
• Western disturbances are steered by the southern branch of the subtropical westerly jet in winter. They are not steered by the TEJ or the polar front jet.
• Jet streams 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 discovered by Wasaburo Ooishi (Japan, 1920s).
• Rossby waves: usually 4–6 around the globe. They are caused by the Earth's rotation and its change with latitude. When they stall, they cause blocking.
• Polar vortex sits in the stratosphere and upper troposphere. A sudden stratospheric warming weakens or splits it. Example: the Texas cold wave of February 2021, after an SSW about 30 km above the North Pole [6].
• Event–cause pairs: 2010 Russian heat wave and Pakistan floods → Rossby wave blocking; February 2021 Texas freeze → polar vortex weakening.

Mains Points

• Jets and Indian weather (GS-I):
• The STJ's winter position south of the Himalaya steers western disturbances. These bring rain and snow that help the rabi crop and feed Himalayan snowpack.
• The STJ's early-summer jump north, the TEJ and the Somali jet then set up and supply the south-west monsoon.

• A shift in the timing of any of these can delay or weaken the monsoon.

• Blocking as a disaster driver (GS-III):

• Stalled Rossby waves turn ordinary weather into long-lasting extremes, such as the 2010 Russia–Pakistan pair and the 2021 heat dome.

• One blocking pattern can hit regions thousands of kilometres apart. This calls for impact-based forecasting and early-warning systems.

• Polar vortex disruption and infrastructure (GS-III):

• Texas in February 2021 had record cold, about 4 million people without power and record snow cover [6].

• Grids and water systems built for a "normal" climate fail in rare extremes. Cold-proofing and better seasonal forecasting are part of climate adaptation.

• Climate change link (GS-I/III):

• Arctic amplification may weaken the pole-to-equator temperature contrast and make the jet wavier, but this is still debated.
• An answer should present it as a hypothesis, not a settled fact, and stress the need for long-term observation.