Jet Stream UPSC: Causes, Types, Effects, and Significance

Jet Stream is a geostrophic wind that typically blows west to east over the upper troposphere at heights between 20,000 and 50,000 feet. When air masses with different temperatures come into contact, Jet Streams are produced. The Jet Stream’s internal wind speed increases as the temperature difference widens. Jet Streams extend from 20 degrees latitude to the poles in both hemispheres. Therefore, the surface temperatures frequently influence the location at which Jet Streams would develop. As per the World Meteorological Organization (WMO), the term "jet stream" refers to a robust and narrow current concentrated along a nearly horizontal axis in the upper troposphere or stratosphere. It is distinguished by pronounced vertical and lateral wind shear and includes one or more velocity maxima.

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In this article on the Jet Stream, we shall discuss what it is, its causes, types, effects on various aspects, seasonal migration and uses. This will be very useful for the aspirants of the UPSC Prelims Exam.

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What is a Jet Stream?

The Jet Streams are strong winds that appear like a narrow river of winds with one or more velocity maxima, centred roughly along a horizontal axis in the upper troposphere or stratosphere. They develop along the edges of neighbouring air masses with notable temperature changes due to differential heating of the earth’s surface.

Causes of Jet Stream

Some of the causes of jet stream are listed below.

Coriolis Effect

The coriolis forces eflects the north-south transport of heat from the equator to the poles into the predominantly east-west motion of the Jet Stream is called coriolis effect. This leads to the formation of Coriolis Force.

Landmasses

• Land masses impede the movement of the jet due to friction and temperature variations, and the earth’s rotation highlights these changes. The movement veers off track as a result of this.
• The interactions between the meandering sections of the stream and the neighbouring landmasses, which produce a condition of flux, are what generate temperature changes.
• In winter, the stratosphere’s temperature can affect the stream’s strength and placement. The polar stratosphere’s temperature drops as the Jet Stream’s force grows stronger.
• The warmth of the land and ocean masses may have an impact on the Jet Stream’s power and amplitude (such as the El Nino Southern Oscillation).

Types of Jet Stream

Mainly, there are two types of Jet Streams Subtropical Westerly Jet Stream and Polar Front Jet Stream as discussed below:

Subtropical Westerly Jet Stream

• Above 30 to 35 degrees latitudes in both hemispheres, they travel north of the subtropical high-pressure band in the upper troposphere. i
• They shift from west to east more frequently than the Polar Jet Stream.
• The earth’s rotation produces it and flows most of the year.

Polar Front Jet Stream

• They develop above the zone of convergence between the tropical warm air mass and the polar cold air mass in latitudes between 40 and 60 degrees.
• The intersection of two separate air masses causes the temperature gradient to be so severe.
• These have a more changeable position than the subtropical jet and migrate in an easterly direction.

Other Types

Polar Night Jet Stream

• Polar Night Jet Stream meanders through the upper stratosphere over the poles.
• They are in the convergence zone above the sub-polar low pressure belt.

Coastal Low-Level Jet Stream

• Strong coastal parallel winds are produced by coastal low-level jets, which are closely tied to a sharp contrast between high temperatures over land and lower temperatures over the sea. These are associated with thermal lows over land and high-pressure systems in the ocean.
• The valley exit jet, a strong, high down-valley air circulation, passes above the junction of the valley and the surrounding plain. They are capable of tremendous heights and speeds.

Mid Level African Easterly Jet Stream

• It reaches West Africa between 10°N and 20°N during the Northern Hemisphere’s summer. The Great Plains of East and South Africa are passed by the poleward low-level jet at night.

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Effect of Jet Streams on Air Travel

• When airplanes are flying, Jet Streams can be pretty beneficial. This is evident from the fact that aircrafts traveling eastward may see a speed boost from the Jet Stream’s strong winds, cutting the length of their entire voyage.

Effect of Jet Streams on Weather

• In general, the stronger the Jet Stream, the lower the pressure in the adjoining areas, which enhances the chances of rain and stormy weather.
• On the other hand, a weaker Jet Stream may cause areas of higher pressure to take precedence, often accompanied by clear skies, calm winds, and less turbulent weather.

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Seasonal Migration of STJ (SubTropical Jet Streams)

In the winter, STJ flows along the southern Himalayan slopes. In the summer, it dramatically changes northward, running along the Himalayan edge in early June and along the northern edge of the Tibetan Plateau in late summer (July-August).

STJ in Summer

• As summer approaches, the STJ [upper westerlies] begin their northward movement in March.
• Despite its waning vitality, the southern branch of STJ is still south of Tibet.
• Because of more exposure to the sun and sweltering winds like the loo, northern India frequently suffers hot, dry, and stormy weather.
• The weak STJ [upper westerlies] south of Tibet with the Equatorial Trough (ITCZ) sweeps northward across India. Still, the monsoon does not begin to blow until the upper-air circulation changes to its summer pattern.
• The southern jet breaks at the end of May and later on gets diverted to the north of the Tibet Plateau, causing a quick onset of monsoons (the ridge moves northward into Central Asia, which causes high pressure over north-western India to move northward into Central Asia, which creates space for south-west monsoon winds).
• An easterly jet appears over peninsular India due to the STJ’s northward migration.
• With the arrival of the Easterly jet, the upper air currents are reversed. (Convergence in upper layers is replaced by divergence, and the divergence in lower layers is replaced with convergence. The high pressure at lower layers is replaced by standard pressure system).
• Westerly winds are associated with Easterly winds in the lower troposphere when Easterly winds become most active (southwest monsoon winds).
• The western and eastern jets flow in opposite directions, north and south of the Himalayas.
• When the eastern jet strengthens and sets close to latitude 15°N, the southwest monsoon becomes more active and produces significant rainfall.

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STJ in Winter

• Over the subtropical region, the westerly Jet Stream blows very high all winter.
• The Tibetan Plateau and Himalayan Mountains split this Jet Stream in two. Off China’s east coast, the two branches come together.
• Along the Tibetan Plateau’s northern edge blows the north branch of the Jet Stream.
• Along 25° north latitude, the southern branch blows south of the Himalayan mountain ranges.
• A significant latitudinal heat differential is one of many elements that affect how the southerly jet forms (differences in temperature).

Significance of Jet Stream

• They significantly affect regional and local weather patterns.
• Intensity-wise, Jet Streams and temperate cyclones are closely associated.
• Severe storms are created when Jet Streams and surface wind systems collide.
• They also provide a reasonably accurate picture of the frequency of El Nino and La Nina periods.
• Pilots also use them when they need to fly in the direction of the Jet Stream.
• Although Jet Streams are unpredictable and capable of causing unexpected movement even when the weather appears calm and clear, aviators avoid them when flying in the opposite direction from them.

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Conclusion

• The role of jet stream in Indian monsoon is largely significant.
• The monsoons depend on the upper air circulation, which SubTropical Jet Streams control.
• The tropical easterly jet stream, located between 8 and 35 degrees north latitude, is connected to the southwest monsoon in India.
• The northeast monsoon is connected to the subtropical westerly Jet Stream, which blows between 20 and 35 degrees latitude in both hemispheres during the winter.

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