A tropical cyclone is the generic term for a hurricane, typhoon, or tropical storm. Tropical cyclones derive their energy from evaporation of water at the ocean surface which ultimately recondenses when it cools to the saturation point. The rotation of a tropical cyclone is caused by the Coriolis effect.

What is unclear to me is: Why do these massive bodies of warm, moist air clump together over thousands of kilometers? What physical process causes them to be drawn in towards the center of the storm?

*image taken from the Wikipedia article on tropical cyclones