Jet streams: Characteristics, Types and Significance

What is a jet stream? What are the characteristics of jet streams? Why is it important for various climate phenomena? Read to know more.

What is a jet stream?

The understanding of Jet streams is very important for understanding temperate cyclones and Indian monsoons.

According to the World Meteorological Organisation (WMO), “a strong narrow current concentrated along a quasi-horizontal axis in the upper troposphere or the stratosphere characterized by strong vertical and lateral wind shear and featuring one or more velocity maxima is called the jet stream.”

Jet streams are strong, upper tropospheric flows that are quasi-horizontal. They originate due to differential heating of the earth’s surface and form at the boundaries of adjacent air masses with significant temperature differences.

The factors that influence the flow of the jet stream are the landmasses and the Coriolis effect.

• Landmasses interrupt the flow of the jet through friction and temperature differences, whilst the spinning nature of the earth emphasizes these changes. This causes the meandering movement.
• The meandering sections of the stream continue to change as they interact with landmasses creating a state of flux and subsequent temperature differences.
• In winter, the temperature of the stratosphere can also affect the strength and position of the stream. The cooler the polar stratosphere, the stronger the jet stream.
• The warmth of the landmasses and oceans (such as the El Nino Southern Oscillation) can also have an impact on the strength and amplitude of the stream.

Characteristics of Jet streams

• The circulation of Jet streams is from west to east due to the effect of Earth’s rotation. Generally, the circulation is observed between poles and 20 degrees latitude in both hemispheres.
• These are also called circumpolar because they move around the poles in both hemispheres.
• Their circulation path on the trajectory is wavy and meandering.
• The extent narrows down during the summer season because of the northward shifting while these extend up to 20 degrees latitude during winters.
• These may range from 40 km in width and 2-3 km in depth.
• The average wind speed is very high with a lower limit of about 120 km per hour in winter and 50 km per hour in summer.
• The maximum velocity of wind is recorded in the crest and trough of the jet stream.
• In winter, it flows along the southern slopes of the Himalayas and shifts northwards in summer, flowing along the edge of the Himalayas (early June). And in late summers between July and August along the northern edge of the Tibetan Plateau.
• The periodic movement of the Jet Stream often indicates the onset and subsequent withdrawal of the monsoon.
• The northward movement of the subtropical stream is the first indication of the onset of the monsoon over India.

Types of Jet streams

The origin of the Jets is supported by three kinds of gradients:

• The thermal gradient between the pole and equator
• The pressure gradient between the pole and equator
• The pressure gradient between surface and subsurface air over the poles.

The two major types are:

1. The subtropical jet stream
2. The mid-latitude or polar front jet stream

Subtropical Westerly Jet stream

• They move in the upper troposphere to the north of the subtropical high-pressure belt in both hemispheres that is above 30 degrees to 35 degrees latitude.
• Their circulation from west to east and more regular than the polar front jet stream.
• It flows through most of the year and is produced by the rotation of the earth.

Polar front Jet stream

• They are formed above the convergence zone (40 degrees and 60 degrees latitude) of the surface polar cold air mass and tropical warm air mass.
• The thermal gradient is steep because of the convergence of two contrasting air masses.
• These move in an easterly direction and have a more variable position than the subtropical jet.

Other types

• There is another type of upper-level jet called polar night jet which forms mainly during the winter months when the nights are much longer. It meanders through the upper stratosphere over the poles and is present in the convergence zone above the sub-polar low-pressure belt.
• A barrier jet in the low levels forms just upstream of mountain chains, with the mountains causing the jet to be oriented parallel to the mountains.
• Coastal low-level jets are related to a sharp contrast between high temperatures over land and lower temperatures over the sea and play an important role in coastal weather, giving rise to strong coast parallel winds. These are associated with oceanic high-pressure systems and thermal lows over land.
• A valley exit jet is a strong, down-valley, elevated air current that flows above the intersection of the valley and its adjacent plain. They can achieve high speeds and heights.
• The mid-level African easterly jet occurs during the summer in the Northern Hemisphere between 10°N and 20°N above West Africa. While the nocturnal poleward low-level jet occurs in the Great Plains of East and South Africa.

Climatic Significance

They have an immense influence on local and regional weather conditions.

There is a close relationship between the intensity of temperate cyclones and jet streams. Severe storms occur when Jet streams interfere with surface wind systems.

They also help in giving a relatively clear picture of the occurrence of the events of El Nino and La Nina

They are also used by aviators if they have to fly in the direction of the flow of the jet streams. But aviators avoid them when flying in the opposite direction of the jet streams because Jet streams are unpredictable and can cause sudden movement even when the weather looks calm and clear.

The monsoon of South Asia is largely affected and controlled by Jet streams.

The monsoons also depend on the upper air circulation which Sub Tropical Jet Streams dominate. The southwest monsoon in India is related to the tropical easterly stream which is between 8-35 degrees North latitudes. The northeast monsoon in the winters is related to the subtropical westerly Jet Stream which blows between 20-35 degree latitudes in both hemispheres.

They are also known to bring ozone-depleting substances to the stratosphere, aiding the ozone layer depletion by acting as a vehicle.

Also read: Atmospheric River; Heat Index