RAS question
Which of the following statements about the subtropical westerly jet stream and the Indian monsoon is correct?
Correct answer: (B) The jet stream shifts north of the Himalayas in summer, allowing the monsoon to advance.
In summer, the subtropical westerly jet shifts north of the Himalayas, removing an upper-air westerly barrier and helping the Indian monsoon advance.
Explanation
The subtropical westerly jet is not a year-round feature over southern India, nor does it strengthen over India during the monsoon. In winter, it flows south of the Himalayas over northern India. By early June, heating of the Tibetan Plateau pushes this jet north of the Himalayas, removing the upper-air westerly barrier and allowing the monsoon to burst over India. IMD's official account supports the same mechanism: over India, the Tropical Easterly Jet appears just after the Sub Tropical westerly Jet shifts north of the Himalayas, and the duration and intensity of Tibetan Plateau heating directly affect Indian monsoon rainfall. That is why option B captures the causal link.
Why the other options are wrong
- (A) The westerly jet does not strengthen over India in the monsoon season; it withdraws north of the Himalayas and is followed by the Tropical Easterly Jet.
- (C) The jet stream is connected to the monsoon because its withdrawal removes the upper-air westerly barrier that otherwise blocks monsoon advance.
- (D) The jet stream cannot remain over southern India throughout the year because the monsoon mechanism depends on its summer shift north of the Himalayas.
Concept
This tests the upper-air circulation control of the Indian monsoon, especially the seasonal movement of jet streams. RAS repeatedly asks it because monsoon onset is not explained by surface winds alone; the Himalayan-Tibetan upper-air set-up is central.