When the air is warm it rises and goes upwards it expands and thus it becomes lighter and hence moves out and the air that cools and falls becomes dense and hence develops a higher pressure where the air from warm places converges.
As the lighter moves from the ITCZ towards the poles the polar air mass reaches downwards to the equator and hence, creates a tricellular circulation system and which impacts the global circulation pattern of the planet.
This pattern is affected by differential heating of the surface of the earth and rotational motion of the earth on its axis plus the tilted angles it foams with the sunlight.
