The Annual and Semiannual Cycles of the Tropical Wind Field

Abstract

The annual and semiannual harmonics of the wind field within 25° of the equator at six tropospheric pressure levels are described, based on data from a network of rawinsonde stations. The annual cycle in the zonal wind component is most prominent in the upper troposphere at subtropical latitude with maximum westerlies occurring late in each hemisphere's winter season. Significant amplitudes of the annual cycle in the meridional wind component are found to be confined to layers about 150 mb thick at the bottom and top of the troposphere. The semiannual cycle is significant only in the zonal wind component. At stations near the equator, the semiannual cycle in zonal wind has larger amplitude than the annual cycle.