The Coriolis Effect
Winds generally blow out from the subtropics towards the equator and subpolar regions, and from the polar regions to the subpolar latitudes. Complicating matters is that the rotation of the Earth causes the winds to rotate as they move (the Coriolis effect). The rotation of the Earth causes an object to deflect towards the right (as viewed by a stationary observer) in the Northern Hemisphere, which results in a clockwise motion, and to deflect towards the left (as viewed by a stationary observer) in the Southern Hemisphere, which results in a counterclockwise motion. These rotations combined with the zonal distribution result in enormous, nearly ocean-scale major cells or gyres of surface winds.