What climatic patterns allowed sailors to sail southwest from China and Southeast from India to meet in the Straits of Malacca?

The wind allowed them to sail

Explanation:

Sailing ship is mostly being pushed by wind or some other climatic pattern.

People were using the monsoon route and the monsoon is transferring air mass from the ocean to the land.

Straits of Malacca are connecting Indian and Pacific ocean and those are opposite directions so for this kind of sailing sailors need to wait for the right wind directions, but surely the most important thing that they need is the wind.