The correct option is 1 only
Explanation
The mechanism of the Indian Monsoon is heavily influenced by upper-air circulation, specifically the seasonal movement of Jet Streams. The interplay between the Sub-Tropical Westerly Jet Stream (STWJ) and the Tropical Easterly Jet Stream (TEJ) plays a critical role in the onset and progression of the monsoon.
Statement 1 is Correct:
The Sub-Tropical Westerly Jet Stream flows south of the Himalayas over the North Indian plains during winter. As summer approaches, the apparent movement of the sun towards the Tropic of Cancer causes the Inter-Tropical Convergence Zone (ITCZ) to shift northward. This thermal and pressure shift forces the Westerly Jet Stream to withdraw from the Indian plains and move north of the Himalayas. This withdrawal is a necessary condition for the onset of the monsoon.
Statement 2 is Incorrect:
The Tropical Easterly Jet Stream (TEJ) does not set in before the withdrawal of the Westerly Jet Stream. The TEJ is generated due to the intense heating of the Tibetan Plateau during summer. This heating and the subsequent formation of the TEJ occur only after the Westerly Jet Stream has shifted north of the Himalayas. The withdrawal of the STWJ is the trigger that allows the TEJ to establish itself around 15°N latitude.
Statement 3 is Incorrect:
The 'burst' of the monsoon (sudden onset of heavy rainfall) is not solely caused by the Westerly Jet Stream. While the withdrawal of the STWJ allows the monsoon winds to enter, the burst is closely associated with the establishment of the Tropical Easterly Jet Stream and the strengthening of the low-pressure trough over India. The TEJ steers tropical depressions into India, contributing significantly to the rainfall distribution and intensity.
Key Takeaway:
The withdrawal of the Sub-Tropical Westerly Jet Stream from the south of the Himalayas is the immediate trigger for the onset of the Southwest Monsoon, paving the way for the establishment of the Tropical Easterly Jet Stream.