What causes the low-pressure region that prevails over the tropics?

Answer 1

The sun heats the air in equatorial regions causing it to expand. This creates higher pressure high in the troposphere so air moves away from the equator towards the poles. The loss of air above creates a low pressure area at the surface.

Warm humid air is also less dense than colder drier air and this encourages vertical motion in equatorial regions but the initial cause of the low pressure is the upper level movement towards the poles.

The air moving away creates high pressure at about 30° where it sinks back to Earth creating the sub-tropical ridge.