What drives the wet monsoon season of southern and eastern Asia?

The wet (or summer) monsoon season in southern and eastern Asia is driven by differential heating between the land and ocean surfaces known as the land-sea breeze effect. During May, the land over southern and eastern Asia heats quickly when the sun is directly overhead. This heating initiates a region of low pressure on the continent. Conversely, the Indian and Pacific Oceans maintain relatively cool temperatures, as the high heat capacity of water slows the temperature increase of the ocean. This temperature gradient creates a region of high pressure over the oceans.

As warm air rises from the heated landmass, it creates an area of low pressure. The air over the ocean is cooler and denser, so it moves to replace the rising warm air. When air moves from a region of high pressure to a region of low pressure, this is called a monsoon wind. The moisture that evaporates from the warm ocean waters condenses at higher altitudes, forming clouds and eventually rain. This moist air is what brings heavy rainfall to the countries in southern and eastern Asia during the wet monsoon season.