Conservation of Angular Momentum
When the distance from the axis of rotation decreases, the velocity of rotation must increase to conserve angular momentum. As a parcel of air moves north (or south) from the equator, its distance from the Earth's axis of rotation decreases. To conserve angular momentum, the parcel must increase its west-to-east speed, creating a subtropical jet stream.

The same force that allows ice skaters to spin faster produces Earth's subtropical jet stream.