9. JET STREAM
9.1. INTRODUCTION – JETSTREAM
- The strong and rapidly moving circumpolar westerly air circulation in a narrow belt of a few hundred kilometers width in the upper limit of troposphere is called jet stream.
- The circulation of westerly jet stream is confined between poles and 20° latitudes in both the hemispheres at the height of 7.5 – 14 km.
- According to World Health Organization (WHO), ‘a strong narrow current concentrated along a quasihorizontal axis in the upper troposphere or in the stratosphere characterized by strong vertical and lateral wind shear and featuring one or more velocity maxima is called jet stream’.
9.2. PROPERTIES
The jet streams are characterized by the following properties.
i) The circulation of jet stream is from west to east in a narrow belt of a few hundred kilomerters width at the height of 7.5 – 14 km in the upper troposphere.
II )On an average, jet steams measure thousands of kilometers in length, a few hundred kilometers in width and few kilometers (2 – 4 km) depth.
iii) Generally, their circulation is observed between poles and 20° latitude in both the hemispheres. These are also called circum – polar whirl because these move around the poles in both the hemispheres.
iv} The vertical wind shear of jet streams is 5 – 10m/second (18 -36km/hour), meaning thereby the wind velocity above or below jet stream decreases by 18 -36 km/hour. Lateral wind shear is 5m/second (18km/hour). The minimum velocity of jet stream is 30m/second (108 km/hour).
v) Their circulation path (trajectory) is wavy and meandering.
vi) There is seasonal change in the wind velocity in jet streams wherein these become strong during winter season and the wind velocity becomes twice the velocity during summer season. (Maximum wind velocity is 480 km per hour).
vii. The extent of jet streams narrows down during summer season because of their northward sifting while these extend up to 20° latitudes during winter season.
DISCOVERY OF JET STREAM
Jet steam was discovered during second world war when American jet bomber fighter planes while flying towards Japan (from east to west) found obstructions of an air circulation which was moving in opposite direction (West to east) resulting into marked reduction in the velocity of jet fighter planes, these planes registered marked increase in their velocity while they used to return to their bases (west to east). After careful study of this phenomenon, it was found that there was a strong upper air circulation from west to east in the upper portion of troposphere which presented obstruction in the free movement of jet fighter planes. Based on this fact, westerly strong meandering upper air circulation was called as jet stream.
9.3. TYPES OF JET STREAMS
On the basis of locational aspect, jet streams are divided into 5 types:
(1) Polar front jet streams are formed above the convergence zone (40 -60 latitudes) of the surface polar cold air mass and tropical warm air mass. The thermal gradient is steepened because of convergence of two contrasting air masses. These move in easterly direction but are irregular.
(2) Subtropical westerly jet streams move in the upper troposphere to the north of subtropical surface high pressure belt (at the poleward limit of the Hadley cell in both the hemispheres) i.e. above 30 °−35° latitudes. Their circulation is from west to east in more regular manner than the polar front jet streams.
(3) Tropical easterly jet stream develop in the upper troposphere above surface easterly trade winds over India and Africa during summer season due to intense heating of Tibetan plateau and play important role in the mechanism of Indian monsoon.
(4) Polar night jet streams, also known as stratospheric subpolar jet streams, develop in winter season due to steep temperature gradient in the strato-sphere around the poles at the height of 30 km. These jet streams become very strong westerly circulation with high wind velocity during winters but their velocity decreases during summers and the direction becomes easterly.
(5) Local jet streams are formed locally due to local thermal and dynamic conditions and have limited local importance.
9.4. INDEX CYCLES OF JET STREAMS
- The genesis of jet streams is related to temperature gradient fromequator towards the poles, surface high pressure at the poles and genesis of circumpolar whirl above the poles caused by tropospheric low pressure.
- Surface high pressure is intensified over the surface of arctic region due to subsidence of cooled heavy air during winter season in the northern hemisphere.
- On the other hand, upper air low pressure develops in the upper troposphere above the high pressure of ground surface of the arctic region.
- Due to this phenomenon a cyclonic system (west to east) of air circulation in the form of a whirl develops around upper tropospheric low pressure.
- The general direction of this circulation is form of a whirl develops around upper tropospheric low pressure.
- The general direction of this circulation is from west to east.
- The equatorward meandering part of this upper air circulation is called jet stream.
- The upper air arctic whirl becomes very strong during winter season in the northern hemisphere resulting into maximum southward extension of jet stream upto 20°N latitude.
- There are changes in the position of extent of jet stream from poles toward equator.
- The weavy (meandering) jet stream is called rossby waves.
- The period of transformation of straight path of jet stream to wavy or meandering path is called index cycle which is completed in four successive stages.
FIRST STAGE
- The position of stream is near the poles and is separated by polar cold air mass in the north and warm westerlies in the south (in Northern hemisphere).
- The circulation of jet stream is almost in straight path from west to east because Rossby wave is not developed by this time. There is step pressure gradient across this strong upper air westerly circulation which generates high zonal index.
SECOND STAGE
- Gradually, the straight path of jet stream is transformed into wavy path with March of time.
- This process initiates the beginning of the development of Rossby waves.
- With the march of time, the amplitude of jet streams increases and gradually they extend towards the equator.
- The pressure gradient is north-south.
THIRD STAGE
- It is characterized by fully developed meandering course of jet stream, with the result they are positioned near the equator (20° latitude).
- The pressure gradient in the first two stages is from north to south, but here east-west.
- There is displacement of polar cold air masses towards equator and tropical warm air masses towards the poles.
FOURTH STAGE
- Fourth stage is characterized by cutting off meanders of jet stream from main path due to their more and more meridional circulation (north to south) resulting into their circulation in independent circular pattern in the form of cyclonic and Anticyclonic circulation.
- Thus, there develop several cellular circulation patterns which follow cyclonic pattern in the south and Anticyclonic pattern in the north.
- Such cut off low (cyclonic) or cut off high (anticyclonic) air circulation patterns obstruct west to east flow of jet streams.
9.5. SIGNIFICANCE OF JET STREAMS
Though jet streams have not been properly studied as yet but they are supposed to have immense influence on local and regional weather conditions as follows:
(1) There is close relationship between the intensity of mid-latitude (temperate) cyclones and jet
streams. These cyclones become very strong and stormy when the upper air tropospheric jet streams are positioned above temperate cyclones of ground surface and yield more precipitation than normal.
(2) There are fluctuations in the local weather conditions due to changes in the form and nature of ground surface cyclones and anticyclones caused by upper air jet streams.
(3) Jet streams cause horizontal convergence and divergence in the upper troposphere. The upper air convergence forms upper air anticyclones while upper air cyclones are developed due to upper air divergence.
(4) The vertical circulation of air in jet streams occurs in 2 ways. E.g. cyclonic pattern characterized by upward vertical air movement while there is downward vertical air movement in Anticyclonic pattern of air circulation. This vertical air circulation causes rapid rate of mixing of air between troposphere and stratosphere, which helps in transport of anthropogenic pollutants from troposphere to stratosphere. For e.g. the transport of CFC into stratosphere.
(5) The monsoon of South Asia is largely affected and controlled by jet streams
The pressure gradient in the first two stages is from north to south, but here east-west.There is displacement of polar cold air masses towards equator and tropical warm air masses towards the poles.