JET STREAMS

JET STREAMS

A jet stream is a fast-moving wind current flowing horizontally at high altitudes (20,000–50,000 feet) in the upper layers of the troposphere. Jet streams typically move from west to east and form where air masses with contrasting temperatures meet.

The greater the temperature difference between these air masses, the faster the wind speed within the jet stream. Jet streams can be found in both hemispheres, extending from 20 degrees latitude to the poles.

Genesis of Jet Streams:

1. Thermal Gradient: The temperature difference between the poles and the equator.
2. Pressure Gradient: Differences in pressure between the poles and the equator.
3. Surface and Subsurface Gradient: Pressure differences between the air at the surface and the subsurface air over the poles.

Mechanism of Jet Streams:

• Air Warming: In tropical areas, air warms and rises, fueling the formation of jet streams.
• Tropopause Influence: As air reaches the tropopause, it is drawn towards colder polar air at the poles.

Types of Jet Streams:

1. Subtropical Jet Streams:
   • When?: These are best developed in winter and early spring. Their maximum speed can reach approximately 300 knots, especially when they merge with polar-front jets.
   • Effect: A subsiding motion accompanies subtropical jets, resulting in predominantly fair weather in regions where they pass.
   • Anomalies: Sometimes, subtropical jet streams drift northward and merge with polar-front jets.
2. Tropical Easterly Jet Stream:
   • When and Where?: This type of jet stream is found near the tropopause over Southeast Asia, India, and Africa during the summer.
   • Effect: It signifies a deep layer of warm air to the north of the jet and cooler air to the south over the Indian Ocean.
   • Mechanism: The heating and cooling differences and resulting pressure gradients drive this jet stream.
3. Polar-Night Jet Stream:
   • Mechanism: These jet streams meander through the upper stratosphere over the poles.
   • Where?: They exist in the convergence zone above the subpolar low-pressure belt.

Significance of Jet Streams:

1. Climatic Significance:

• Monsoon: Jet streams play a significant role in the onset and withdrawal of monsoon winds, influencing rainfall patterns in regions like India.
• Ozone Layer Depletion: Jet streams are known to have transported ozone-depleting substances to the stratosphere, which contribute to ozone layer depletion.
• Cloud Formation: Jet streams aid in the formation of noctilucent clouds—cloudlike formations made of ice crystals visible in the deep twilight at high altitudes in the stratosphere.
• Cyclones: Jet streams intensify cyclones and anticyclones by affecting the formation of pressure troughs and ridges.
• Alternative Pressure Gradients: As the air mass moves, it undergoes alternating expansion and compression, creating regions of high and low pressure.

2. Climate Change:

• Atmospheric blocking: Anthropogenic climate change weakens the north-south temperature gradient (Arctic amplification). This weakening causes the jet stream to “waver,” increasing the likelihood of atmospheric blocking.
• Intensification of jet streams: Climate change also cools the polar lower stratosphere while warming the tropical upper troposphere, strengthening the jet stream.
• Turbulence at cruising altitudes: The imbalance caused by these effects can increase turbulence, especially in mid-latitudes, leading to unpredictable flight conditions.
• Climatic extremes: Jet streams generate extreme weather when polar cold air meets tropical warm air. This can lead to storms, droughts, and floods.
  • Examples: Devastating floods in Germany and Belgium, heat waves in the U.S. and Canada.
• Prolonged rains and heat waves: Omega-shaped jet streams result in alternating hot and cold weather patterns, often causing dangerous weather to persist for extended periods.

3. Jet Streams and Aviation:

• Application in aviation: Aviators exploit jet streams for faster travel in the same direction but avoid flying against them.
• Bumpy flights: Unpredictable jet streams can cause sudden turbulence, making flights bumpier even during seemingly calm conditions.
• Affects aviation routes: Volcanic ash from eruptions may be sucked into jet streams, impacting aviation safety.

4. Effects of Jet Streams on Weather:

• Indian monsoon: The onset and strength of monsoons depend on the upper air circulation, dominated by the Subtropical Jet Stream (STJ).
  • Southwest monsoon: Flows between 8°N and 35°N, linked to the tropical easterly stream.
  • Northeast monsoon (winter monsoon): Linked to the subtropical westerly jet stream, flowing between 20°N and 35°N in both hemispheres.

Additional Effects of Jet Streams:

• Mid-latitude cyclones: Jet streams play a crucial role in determining the intensity of mid-latitude cyclones. When positioned above temperate cyclones, the upper air tropospheric jet streams intensify storms.
• Latitudinal heat balance: Jet streams maintain heat balance across different latitudes by enabling the exchange of air masses.
• Weather disturbances: Polar-front jet streams affect mid-latitude weather, often triggering severe storms when they interfere with surface winds.
• Temperate cyclones: The path and distribution of precipitation in temperate cyclones are influenced by jet streams.
• Air masses: Jet streams affect air masses, which can result in prolonged droughts or floods.

Characteristics of Jet Streams:

• Thermal contrast: Jet streams are generated due to variations in temperature between air cells, such as the Hadley and Ferrel cells.
  • Example: Hadley and Ferrel cell dynamics.
• Rossby waves: The meandering or whirl of jet streams is called a Rossby wave. These waves develop due to temperature contrasts and the Coriolis effect, following a 3D curved path.
  • Mechanism: Rossby waves meander more when temperature contrasts and the Coriolis effect are weaker.
  • Genesis: These waves form when polar air moves towards the equator and tropical air moves poleward.
  • Significance: Rossby waves explain the formation of low-pressure cyclones and high-pressure anticyclones.
• Seasonal changes: Jet streams extend farther equatorward in winter and have seasonal shifts with the sun’s movement.
• Aggressive in winter: During winter, increased thermal contrast intensifies the speed and formation of jet streams.
• High-velocity winds: Jet streams can reach speeds of 400-500 km/hr due to strong pressure gradients caused by thermal contrasts.
• Direction: Jet streams flow from west to east in both hemispheres.
• Wide coverage: Jet streams can stretch hundreds of kilometers in width and thousands of kilometers in length.
  • Width: 10 to 12 km.
  • Depth: 2 to 3 km.
  • Length: 3000 km.
  • Altitude: Below the tropopause.

Influencing Factors on Jet Stream Flow:

• Coriolis Effect: The Earth’s rotation accentuates the flow of the jet stream, contributing to its directional pattern.
• Landmasses: Landmasses interrupt jet streams, causing friction and temperature differences. As jet streams interact with land, they create a continuous state of change.
• Temperature: Winter stratospheric temperatures affect the strength and position of jet streams. Cooler polar stratospheric temperatures encourage stronger polar/tropical jet streams.
• Irregular periodic variation: Land and ocean temperature anomalies (e.g., El Niño and Southern Oscillation) can alter the strength and amplitude of jet streams.

Jet Streams and Indian Climate:

1. Seasonal Migration of Subtropical Jet Stream (STJ):

• Seasonal migration: In winter, the STJ flows along the southern slopes of the Himalayas. As summer approaches, the jet stream shifts northward, reaching the northern edge of the Tibetan Plateau by July-August.
• Indication of monsoon movements: The periodic movement of the jet stream often indicates the onset and withdrawal of the monsoon. The STJ moves south of the Himalayas in winter and northward as summer begins.
• Onset of the monsoon: The northward movement of the subtropical jet stream is one of the first signals of the monsoon’s onset over India.

2. STJ in Summer:

• Beginning of summer: The STJ begins its northward march, making the weather hot and dry in northern India. This is due to increased solar radiation and hot winds like the loo.
  • In India, the weakening of the Inter-Tropical Convergence Zone (ITCZ), which moves northward, leads to the buildup of monsoon conditions.
• End of May: The southern jet breaks, and it is diverted north of the Tibetan Plateau, causing the sudden burst of the monsoons as the pressure zone over northwest India moves northward, facilitating the south-west monsoon winds.
• Tibetan Plateau gets heated up: As the Tibetan Plateau heats up, a clockwise cyclic origin occurs in the troposphere, generating winds from the plateau after it becomes too hot.
  • One flow of air moves as the Easterly Jet Stream towards the equator, while another flow moves as the Westerly Jet Stream towards the poles.
• Tibetan Plateau and Central Asia become hot: The easterly winds in the upper troposphere become active, leading to the strengthening of the south-west monsoon winds.
  • In India: The Westerly Jet Stream helps form a high-pressure belt over the Arabian Sea and also contributes to creating the south-west monsoon over the Indian subcontinent.

3. STJ in Winters:

• In winter, the Subtropical Westerly Jet Stream blows from the west to the east across West and Central Asia.
• Bifurcation: The jet stream splits due to the Himalayan ranges and Tibetan Plateau:
  • One branch flows northwards along the plateau.
  • The second branch flows southward across the Himalayas at about 25° north latitude.
• Arrival of Western Disturbances: These westerly winds bring cold disturbances into the Indian subcontinent, particularly in winter, resulting in cold weather.

4. North-Eastern Monsoon:

• The cold westerly jet stream brings cold waves to India’s north and western parts, resulting in high pressure over these regions. These winds blow towards the low-pressure area over the Bay of Bengal.
  • Dry winds from the northwestern region move towards the Bay of Bengal, cooling the atmosphere in Northern India.
  • These winds, after reaching the Bay of Bengal, turn westerly, guided by the Ferrel Cell, forming the North-Eastern Monsoon.
  • When these winds reach Tamil Nadu’s coast, they bring moisture from the Bay of Bengal, causing rainfall.