Moisture for the summer monsoon is provided by the

Southwest Indian Ocean

Explanation:

The summer monsoon appears during the period between April and September, though occasionally can be little bit earlier or later. As the winter ends on the Northern Hemisphere, the heated, moist air masses from the southwestern part of the Indian Ocean that have previously experienced summer are moving toward the southern part of Asia. These monsoons bring in the largest amounts of rainfall on the planet. The countries most affected by it are India, Sri Lanka, Bangladesh, and Myanmar. The enormous amounts of rainfall that these countries experience during the summer monsoons cause lot of problems with floods, landslides, destruction of settlements, erosion, and most tragically loss of lives. Despite the negatives, these monsoons have their positives as well, as they are the ones that fill in the rivers and lakes, providing water for the rest of year, creating fertile deposits that are excellent for agriculture, and providing the conditions for the people in these poor nations to survive.