India declares onset as up to 280mm of rain falls in 72 hours in Kerala, while downpours hit south-west Thailand The monsoon season has officially begun in parts of Asia, marking the start of a period of enhanced rainfall vital to the region’s economy. The south-west monsoon begins each year as a consequence of a growing temperature difference between the Asian land mass and the Indian Ocean. Through spring, the land heats up more rapidly than the surrounding sea, creating a pressure difference that draws moisture-laden ocean air inland.