Explanation:

The trade winds from the Northern and Southern Hemispheres converge into a narrow belt close to the equator, nowadays generally referred to as the Intertropical Convergence Zone (ITCZ). The convergence
of the trade winds results in rising motion of the colliding air masses . This region is also known as the doldrums, where the weather is generally cloudy and periods of light winds are frequently interrupted by squalls and
hard rain, making for a troubled and uncertain sea voyage. Shift of the ITCZ will create low pressure in northern India and will drag the southwest monsoon towards Indian's mainland. Hence shift of ITCZ is prerequisite
for monsoon.