Jet Stream UPSC: Geography IAS (Climatology)

• Jet streams are narrow, fast, meandering ribbons of wind in the upper troposphere and lower stratosphere, thousands of kilometres long, a few hundred kilometres wide and a few kilometres deep, bounded by slower air.
• They are the cores of the upper westerlies: most blow from west to east around the globe at about 9–14 km, just below breaks in the tropopause. The tropical easterly jet of the Asian summer is the main exception.
• In the WMO definition a jet carries speeds of at least about 30 m/s (≈108 km/h); cores commonly run at 150–250 km/h and have exceeded 400 km/h in winter.
• Discovery: Wasaburo Ooishi tracked fast upper winds with balloons over Japan in the 1920s; Heinrich Seilkopf (1939) coined the term Strahlströmung (“jet current”); B-29 crews bombing Japan in 1944–45 met headwinds that nearly stalled them.

Origin and Characteristics

Geostrophic Flow Aloft

• Above about 1–2 km the wind is free of surface friction and is controlled by two forces: the pressure gradient force (PGF) and the Coriolis force.
  • Air starting from rest moves from high to low pressure; the Coriolis force turns it (to the right in the northern hemisphere) more and more as it gains speed.
  • When the two forces become equal and opposite, the air blows parallel to the isobars: the geostrophic wind. Upper-air westerlies and jets are almost geostrophic.

• On a rotating Earth this deflection breaks a single pole-to-equator overturning into three cells per hemisphere (Hadley, Ferrel and polar), and the upper poleward flow turns into westerlies.

Why Jets Form

• Thermal wind: where a strong north–south temperature contrast exists, the pressure surfaces slope more steeply with height, so the pressure gradient, and with it the westerly wind, increases upward up to the tropopause. The jet is where this contrast is sharpest.
  • Its strength is proportional to the temperature contrast through the whole layer beneath, which is why jets lie above frontal zones and are strongest in winter.
• Angular momentum: air moving poleward in the upper branch of the Hadley cell is drawn closer to the Earth’s axis and speeds up eastward, building the subtropical jet at the cell’s poleward edge.
• Jets therefore sit at the tropopause breaks between cells: the subtropical jet where the tropical and middle tropopauses overlap (about 30°), the polar-front jet above the polar front (about 40–60°).

Characteristics

• Speed: stronger in winter, when the temperature contrast is steepest; summer speeds fall to about half. Cores are fastest off the east coasts of Asia (Japan) and North America in winter.
• Path: circumpolar but wavy; jets appear as north-westerly at some longitudes and south-westerly at others, and the polar-front jet often splits or breaks.
• Seasonal shift: jets move equatorward and strengthen in winter, poleward and weaken in summer, following the sun.
• Shear: speed falls off sharply around the core, giving clear-air turbulence; strong jets can break through the tropopause, letting moisture and cirrus into the lower stratosphere.
• Southern hemisphere jets, with no large high-latitude continents, are more symmetrical and steadier.

Types of Jet Streams

Polar-Front Jet

• Forms above the polar front, where cold polar and warm tropical air meet, between about 40° and 60°, at about 9–10 km (near 250–300 hPa).
• Westerly, the most variable jet: it meanders widely, can encircle the globe, and moves north and south with the front. It steers mid-latitude cyclones.

Subtropical Westerly Jet (STJ)

• Lies at the poleward edge of the Hadley cell, about 25–35°, at about 12–14 km (near 200 hPa), above the subtropical high-pressure belt.
• More continuous and steady than the polar-front jet, and not tied to any surface front. In the northern hemisphere it is strongest in winter; over Asia it splits around the Tibetan Plateau in winter and jumps north of it in summer.

Tropical Easterly Jet (TEJ)

• An easterly jet of the northern summer (June to September) over South Asia and Africa, with its core near 15°N at about 100–150 hPa (roughly 14–16 km), described by P. Koteswaram (1958).
• It flows out of the Tibetan (South Asian) anticyclone that develops in the upper troposphere over the heated plateau, and can be traced westward across the Arabian Sea to West Africa. Its strength goes with the vigour of the Indian monsoon.

Polar Night Jet

• A winter, stratospheric westerly jet at about 60° latitude, roughly 20–50 km up, circling the cold stratospheric polar vortex in the long polar night.
• A sudden stratospheric warming can weaken or reverse it, splitting the vortex and sending cold air south weeks later; the warming of February 2018 preceded Europe’s “Beast from the East” cold wave.

Low-Level and Local Jets

• Somali (Findlater) jet: a low-level cross-equatorial jet at about 1–1.5 km, documented by Joseph Findlater (1969). It flows north along the coast of Kenya and Somalia from May–June and turns south-west over the Arabian Sea, carrying much of the moisture of the south-west monsoon to India’s west coast.
• The African Easterly Jet, at about 3–4 km over West Africa, spawns the easterly waves that seed Atlantic hurricanes.

Rossby Waves and the Index Cycle

Rossby Waves

• The upper westerlies and their jets form long, slow waves around the globe, first explained by Carl-Gustaf Rossby (1939). Each hemisphere usually holds three to six of them, each several thousand kilometres long.
• Cause: the Coriolis effect grows with latitude. Air pushed poleward gains cyclonic spin and swings back equatorward; air pushed equatorward swings back poleward. The flow therefore oscillates in ridges (poleward bulges of warm air) and troughs (equatorward dips of cold air).
• Behaviour: long waves travel slowly eastward or stand still; the Rockies and the Tibetan Plateau anchor troughs downstream of them.
• Weather: surface lows form ahead (east) of upper troughs, where air diverges aloft; surface highs form ahead of ridges. Waves that stall create blocking and long spells of the same weather, as in the 2010 Russian heatwave and the Pakistan floods of the same summer.

Index Cycle

• The zonal index measures how strong and straight the westerlies are; it swings between high and low over a few weeks, a sequence analysed by Jerome Namias (1950).
  1. High zonal index: the jet lies far poleward, blows almost due west to east at high speed, and keeps cold polar air north of warm air; little exchange between latitudes.
  2. Developing waves: the jet slows and begins to undulate; cold air pushes south in troughs and warm air north in ridges.
  3. Large meanders: the waves deepen and sharpen; the jet lies farther equatorward, and the north–south exchange of air peaks.
  4. Low zonal index: the meanders cut off as cold lows in lower latitudes and warm blocking highs in higher latitudes; the zonal flow breaks into cells.
• The cut-off cells dissolve, the jet straightens, and the cycle restarts. Each cycle moves heat poleward and helps keep the latitudinal heat balance.

Significance of Jet Streams

Weather and Climate

• Mid-latitude cyclones: the polar-front jet fixes the position of the polar front, steers temperate cyclones, and deepens them when it lies above, by removing air aloft (upper divergence). The heaviest precipitation lies beneath and just poleward of the jet.
• Air masses: jets drive cold and warm air deep into other latitudes: cold waves, heatwaves, blizzards, tornadoes.
• Anticyclones: air piling up on the equatorward side of the jet sinks and strengthens the subtropical highs (anticyclogenesis).
• Aviation: eastbound flights ride the jet and save fuel; westbound routes avoid it; clear-air turbulence near the core is a hazard.

Jet Streams and the Indian Monsoon

• Winter (December–February): the subtropical westerly jet lies south of the Himalaya, near 25–30°N over northern India. It holds subsiding, dry air over the plains and suppresses the monsoon flow.
  • Western disturbances travel along it from the Mediterranean, bringing winter rain for rabi crops to Punjab, Haryana and western Uttar Pradesh and snow to the western Himalaya.
• Onset (late May–June): heating of the Tibetan Plateau builds an upper-tropospheric anticyclone over it. The STJ weakens and jumps north of the plateau (to about 40°N), the tropical easterly jet sets in over southern India, and the monsoon bursts within days.
  • Maung Tun Yin (1949) stressed the plateau’s mechanical role in this jump; Koteswaram (1958) stressed its role as an elevated heat source.
• Active season: a strong TEJ descends over the southern Indian Ocean and strengthens the Mascarene High, steepening the pressure gradient towards the monsoon trough; the Somali jet feeds moisture to the west coast. A weak TEJ, as in many El Niño years, goes with a weak monsoon.
• Withdrawal (September–October): the plateau cools, the Tibetan anticyclone collapses, the TEJ disappears, and the STJ returns south of the Himalaya; dry north-westerly air replaces the monsoon over north India from about 17 September (the normal start of withdrawal from north-west India).
• Interaction: a westerly trough meeting a monsoon system over the Himalaya can produce extreme rain, as in the Kedarnath disaster of June 2013 and the Himachal Pradesh floods of July 2023.

Jet Streams in a Warming World

• The Arctic has warmed nearly four times as fast as the globe since 1979, narrowing the pole–equator contrast that powers the polar-front jet.
• Jennifer Francis and Stephen Vavrus (2012) argued that this makes the jet slower and wavier, favouring stalled heatwaves, cold spells and floods.
• Later analyses, such as Russell Blackport and James Screen (2020), found no lasting increase in waviness, and the IPCC (2021) gives low confidence to the Arctic–mid-latitude link.
• Better supported is a slow poleward shift of jets and storm tracks, alongside the widening of the tropics.