UPSC Prelims 2020 Important topics: Jet Streams and its effect on weather system

A ribbon of fast-moving winds in the upper atmosphere which influences the global atmospheric phenomenon.

Jet Streams

Jet streams are the strong air streams present in the upper troposphere, lower stratosphere, and possibly even mesosphere which help to complete the global circulation and characterised by strong vertical and lateral wind shear. They are relatively narrow belts of swift-moving winds having velocity in the range of 100km/hr to 900km/hr.

Discovery of Jet Streams

  • Meteorologists came to know about the existence of jet stream during the second world war when American Fighter jets were obstructed by the flow of the Jetstream during their Bombing mission over Japan. Although previously also, the study of high-speed cirrus cloud (high altitude cloud, found at a height greater than 6000 M) pointed out the existence of high-velocity wind in the upper troposphere in as early as in 1904.
  • They are called Jet streams based upon the analogy of water jet.

Characteristics of the Jet Streams

  • Jet streams follow curved and meandering path and such path is because of Coriolis effect and due to the tendency of large scale system to conserve the angular momentum.
  • The height of Jetstream varies and in general, their height decreases from the equator towards poles since the height of troposphere decreases from the equator towards poles.
  • They are embedded in the upper air westerlies or in the core of westerlies where the maximum velocity is found since away from core there is increased wind shearing.
  • Their length profile varies from 1600-3200KM.
  • The high velocity is due to intense pressure gradient and thermal contrast.
  • Jet streams are characterised by great seasonal variations as during colder part of the year they migrate towards the equator and their velocity also gets increased while during summer the wind speed of the Jetstreams is reduced to approximately half of what it is in the winter season and they reach far into tropics.

Development of the Jet Streams

  • The temperature gradient from the equator towards the poles, the surface high pressure at the poles and genesis of the circumpolar whirl above the poles caused by tropospheric low pressure are the major causes of the origin of the Jet Streams.
  • Due to the subsidence of the cool air over the Arctic region, the surface high pressure is intensified while upper air low pressure develops in the upper troposphere.
  • Because of this phenomenon a cyclonic system of air circulation, whose direction is from west to east in the form of whirl develops around upper tropospheric low pressure and the equatorward meandering part of this upper air circulation is called Jet Stream.

Types of Jet Streams

On the basis of locational aspect jet streams are divided into 5 types

1.Polar Front Jet Stream

2.Subtropical westerly Jet Stream

3.Tropical Easterly Jet Stream

4.Polar Night Jet Stream

5.Findlater/ Cross equatorial Jet Stream

Polar Front Jet Stream

  • This jet stream was discovered by Rossby and also known as the primary jet stream.
  • It has an average speed of 215km/hour and in some cases, the speed may become double of the average speed.
  • Length profile of these Jets is in the range of 1600-3200 km and depth is around 1-2 km.
  • Sometimes, Polar front Jetstream encircles the entire globe and there is diurnal variation in the position and speed of this Jet Stream.
  • Polar front jet streams are associated with Polar Fronts. As Polar cold airmass and warm tropical airmass begin to converge upon the subpolar lows, the warm air is lifted above the cold air and it tires to enter the polar region, while the cold air gradually pushes towards the warm air. The rising warm air due to geostrophic effect becomes westerly and known as Rossby wave. And Polar Front Jet are embedded in the Rossby waves (jet stream lies at the core of westerlies which has the highest speed).

Influence on weather

  • They have an important role in the management of latitudinal heat budget of the earth and it accomplishes it through the creation of various cyclones and anticyclones in the Middle-Latitude.
  • It plays a very important role in the distribution of precipitation in the middle latitude.
  • Cyclogenesis in the Temperate region is associated with the crest of Polar Front jet and maximum cyclonic rainfall occurs ahead of the trough.
  • At the same time, these jets also determine the path and movement of Temperate Cyclone.

Subtropical westerly Jet Stream

  • They originate at the meeting point of Hadley and Ferrel Cell. The immense temperature contrast between two air masses produce a sudden change in the pressure gradient, thus sudden change in the velocity.
  • On account of permanency of the Hadley cell, this Jetstream remains intense throughout the year although, with the shifting of the pressure belt, its path also shifts North-South.

Influence on the weather

  • It plays an important role in the distribution of precipitation over the subtropical latitude.
  • It has a major role in bringing western disturbances to India during the winter season and thus provides crucial rainfall which is vital for Rabi Crop.
  • The trough of the subtropical jet stream experiences high pressure and thus associated with the Tropical Deserts; and below the crest, the low-pressure system (cyclogenesis) may develop.
  • The contrast between Hadley and Ferrel cell become greater over the continents hence as the jet enters above the western coast of the continents, they suddenly accelerate, and it causes the dynamic sinking of air and due to this reason tropical desert has high pressure.

Tropical Easterly Jet Stream

  • It is located at an altitude of 10-12 km above the Gangetic Plain and roughly parallel to the 250 North Latitude.
  • It originates due to excessive heating of Tibetan Plateau during the summer season due to which low pressure(converging) sets up at the surface and relatively higher pressure(diverging) in the upper troposphere. The part of this southward diverging air comes under the impact of Coriolis effect and it is forced to deflect from east to west leading to the formation of a tropical Jetstream.
  • One of the divergent limbs of Tropical Easterly Jetstream falls over the North -East India and which due to Coriolis deflection becomes Easterly.

Influence on Weather

  • TEJ steers the low-pressure depression found over the Bay of Bengal into the Indian Subcontinent. Due to this, much of the rainfall is received in the Indo-Gangetic Plain and Central India.
  • It has a relationship with oscillation and shifting of ITCZ (Inter-Tropical Convergence Zone) and Monsoon Trough.
  • During the onset of winter, it suddenly disappears while its arrival heralds the onset of the South West Monsoon.

Polar Night Jet Stream/ Stratospheric Jet Stream

  • This Jetstream is found during winter above poles at an altitude ranging from 40-50km.
  • They are called polar nights as jet streams are formed in winter nights of 6 months at poles. It is found due to the thermal contrast between sub-polar layers and polar layers, and become westerly due to Coriolis force.
  • This jet has been found in the stratosphere little below the maximum concentration of ozone thus also known as Stratospheric Jet Stream.

Influence on Weather

  • Polar Night Jet Stream help in increasing the intensity of Polar Anticyclone.

Findlater/ Cross equatorial Jet Stream/Somali Jet Stream

  • Formed due to the unequal heating of continent of Africa and located at a very low height of 3-4 km, since the warm air above the Mozambique Channel produces a horizontal thermal contrast with the cold air from the African Highlands.
  • It also crosses the equator and becomes south-westerly in direction and due to this reason, it is known as Cross Equatorial Jet Stream.

Influence on Weather

  • Helps in the intensification of S-W Monsoon over Indian Subcontinent.
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