Gas convects from the interior to the surface, and then circulates back down inside. As the gas breaches the upper layers of the atmosphere, half the material falls back down on the equator-side of the convection cell, and half settles back down on the polar-side of the cell. Because the planet is rotating, the coriolis force deflects the polar-side gases easterly, and the equator-side gases westerly in opposite directions. The sense is reversed in the opposite hemisphere.