While reading the topic "Monsoon" in Aguado's Understanding Weather and Climate, I read this line

During winters... Air flows down the southern slope of the Himalayan Mountains due to more rapid cooling over land than over the Indian Ocean. This flow is enhanced by a portion of the jet stream flowing along the southern flank of the mountain range. Interactions between the jet stream and mountains cause convergence in the upper troposphere, which leads to sinking air that gets forced southward.

It is talking about the Subtropical Westerly Jet Stream's southern branch flowing along the southern Himalayas, but how does it cause convergence in upper troposphere? And how the convergence in upper troposphere leads to sinking of air?

In the same topic, after few lines, while explaining the situation in summer season it is mentioned that

The situation changes abruptly during late spring or early summer....Aloft is an easterly jet stream, with often divergent motion promoting uplift.

Why this Easterly jet stream has divergent motion, how it differs from the SubTropical Westerly Jet Stream in this characteristic? And again the same question, how does divergent motion in upper troposphere cause uplift?

Since I couldn't find anything useful related to Jet Streams and convergence/divergence it would be helpful if someone could provide exact mechanism.


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