From AMS Glossary
They comprise the equatorward side of the midlatitude westerlies, but are found at upper levels rather than at the surface. The antitrades are best developed in the winter hemisphere and also above the eastern extremities of the subtropical highs. Farther west their base is higher and their appearance less regular. The antitrades were formerly regarded as return currents carrying away, to higher latitudes, the air that rises in the intertropical convergence zones, the westerly component being due to the conservation of angular momentum as the air moves into higher latitudes. Bjerknes (1935) showed that this explanation is incomplete: The antitrades are dynamical in origin and constitute an essential part of the atmosphere's primary circulation.
Bjerknes, J. 1935. La circulation atmosphérique dans les latitudes sous-tropicales. Scientia. 57. 114–123.