Himalayas acted on the monsoon in two ways. The Tibetan plateau, heated up during summer and thereby established an atmospheric circulation that was conducive for the monsoon. The vast mountain range also acted as a tall barrier, preventing cold, dry air in the northern latitudes from entering the subcontinent and subduing the warm, moisture-laden winds from the oceans that drive the monsoon.

Role of Himalayas and Tibetan highlands in South west monsoon:-

• The Tibetan Plateau is an enormous block of highland acting as a formidable barrier. Due to its protruded height it receives 2-3°C more insolation than the neighboring areas.
• The plateau affects the atmosphere in two ways: (a) as a mechanical barrier, and (b) as a high- level heat sources.
• At the beginning of June the subtropical jet stream is completely withdrawn from India and occupies a position along 40° N (to the north of Tibetan Plateau).
• The plateau accentuatesthe northward displacement of the jet stream. Hence the burst of monsoon in June is prompted by the Himalayas and not by the thermally induced low pressure cell over Tibet. (Tibetan plateau is responsible for south-west monsoons. But it is the subtropical jet stream that facilitates sudden outburst of monsoons with its sudden northward migration)
• In the middle of October the plateau proves to be the most important factor in causing the advance of the jet south of the Himalayas or bifurcating it into two parts.
• The winter Tibetan Plateau cools rapidly and produces a high pressure cell. (Cyclonic condition over Tibet ceases and an anticyclonic condition is established). The high pressure cell over Tibet strengthens N-E monsoons.
• Tibet gets heated in summer and is 2°C to 3°C warmer than the air over the adjoining regions. Because the Tibet Plateau is a source of heat for the atmosphere, it generates an area of rising air (convergence)(intense low pressure cell).
• During its ascent the air spreads outwards in upper troposphere (divergence) and gradually sinks (subsidence) over the equatorial part of the Indian Ocean.
• It finally approaches the west coast of India as a return current from a south-westerly direction and is termed as equatorial westerlies.
• It picks up moisture from the Indian Ocean and causes rainfall in India