Geography – Influence of Jet Streams on the South West Monsoon

Jet Streams significantly influence the South West Monsoons (SWMs) in the Indian subcontinent. This relation between the Jet Streams and SWMs can be summarised as below:

• Sub-tropical Westerly Jet Stream
  • It is responsible for the onset of monsoon.
  • In the initial position, that is until the end of May, this jet stream supplies the wind to the Gangetic plain which is heated up intensely as the sun is directly above the Tropic of Cancer (during the summer in Northern hemisphere). These winds supplied by this jet stream prevent the formation of a Low Pressure area in the Gangetic plain. This is as shown in the figure below.

Sub-tropical Westerly Jet Stream

• By the property of this jet stream by which when it moves towards north by the end of May, it no longer provides the winds to the Gangetic plain. This results in the formation of a Low Pressure area in the Gangetic plain, which starts attracting the South Easterlies (planetary winds) of the southern hemisphere. When they enter the northern hemisphere, they bend by 90 degrees due to the change in direction of the Coriolis force in the northern hemisphere. This marks the on set of the South West Monsoon. This is shown in the figure below

Sub-tropical Westerly Jet stream after moving northwards

• Tropical Easterly Jet Stream
  • This jet stream is oriented along the Kolkata-Bangalore axis in that direction.
  • It controls the spread of the South West Monsoon. When it is strong, it opposes the spread of the South West monsoon and when it is weak, it allows the spread of South West monsoon.
• Somalia Low-level Jet Stream
  • Its orientation is as shown in the figure below.
  • When it is weak, it results in formation of a monsoonal break called the “lull” in the monsoon and when it is strong, the monsoon will also be strong.

Somalia low level Jet Stream

Thus, jet streams play a significant role in the Indian monsoons.