Jet Streams

Q.Write short note on Rossby waves and Jet stream. (12/11)

Jet Streams are fast flowing narrow stream of Geostrophic westerly winds found in upper troposphere near tropopause which encircles globe at respective latitudes.

         Peterson has defined jet streams as thermal winds produced by great temperature contrast found between meridional cells which produces intense pressure gradient.

CHARACTERISTICS

• Genesis:They are product of high pressure gradient, strong coriolis and negligible friction which is ideal for geostrophy.
• Speed: varies from 120 Km/hr to 900 Km/hr
• Seasonal Migration: During winter, they are poleward while in summer they shift equatorward.This is associated directly to the shifting of meridional cells with seasons.
• Rossby Waves: Associated with polar front jet stream.They follow meandering path which appears like waves with crest and trough.Crest and trough have north-south alignment perpendicular to the path of jet stream.This meandering jet streams are also referred as Rossby waves. This meandering is due to cold air mass from north and warm air from south invade in each others territory distorting the path of polar front jet stream.
• Crest and trough: Associated with subtropical jet stream.Jet streams near equator  tends to form crest and trough which is acceleration toward ground or moving away from it.These are formed due to contrast in pressure gradients existing between ocean and continents and angular momentum conservation principle.As it move towards,continent, gradient intensifies and it tends to make trough while moving from land to ocean, it tends to form crest.

TYPES

1.Subtropical Jet Stream

• It is formed between hadley cell and Ferrel cell due to temperature contrast resulting intense pressure gradient.
• Associated with crest and trough formation.Crest is associated with cyclogenesis and low pressure condition while trough is associated with subsidence of air and anti-cyclogenesis conditions.Formation of thar desert is also due to sub-tropical jet stream forming trough there.
• Steers Western disturbances toward india
• Associated with retreat of monsoon: retreat of monsoon occurs as it appears over norther part of india around gangetic plains.

2.Polar Front Jet Stream

• This was discovered by Rossby.
• It is formed between polar cell and ferrel cell where temperature.
• Speed varies between 90 -120 Km/hr
• It is characterized by rossby wave formation and follows index cycle.
• Index cycle describes various stages where no-meandering stream transform into rossby wave and again dissipates back into non-meandering jet stream.
• Index cycle goes through 4 stages

1. Incipient stage: non-meandering stage
2. Juvenile stage: development of small wave due to warm  air from south and cold air from polar region stats to invade each other
3. Mature stage: full fledged rossby wave due to further of invasion of cold and warm air in each other’s territory
4. Dissipation stage: cold air and warm air enter each other's territory and mix up which results in vanishing of rossby wave.

• Telepathic influence on Temperate Cyclone formation and movement.

 3.Tropical Easterlies Jet

• Originates due to excessive heating of tibetan plateau.Rising air flows outward after reaching higher troposphere.One stream flow towards gangetic plain and form Tropical easterlies jet
• It lies over gangetic plain and oscillates between shivalik and vindhyas.
• It is related to the onset of monsoon in indian subcontinent.
• It also affect monsoonal rainfall over gangetic plain by determining the monsoonal trough over gangetic plain.
• It also steers tropical cyclones into India.

     

4.Findlater Jet

• Formed between Warm Mozambique air mass  & African Highland cold air mass due to thermal contrast.
• It bifurcates due to african highland and right channel directs southwest monsoon into india

5.Polar Night Jet