On the annual variation of tropospheric zonal winds observed above Christmas Island in the central equatorial Pacific

Abstract

Observations of tropospheric winds have been made nearly continuously since early 1986 in the altitude range 2–18 km using a 50 MHz VHF wind profiler at Christmas Island (2°N, 157°W). Tropospheric zonal winds observed at Christmas Island show a pronounced annual variation in the middle to upper troposphere with westerly winds typically observed during northern winter. The annual variation is strongest during La Niña and weakest during El Niño. The amplitude of the cycle has a broad peak in the upper troposphere. Empirical orthogonal function (EOF) analysis of the vertical structure of the annual variation shows that the principal mode explains nearly 90% of the variance in the 90‐ to 365‐day frequency band. We attribute the annual variation in zonal winds to the annual variation in deep tropical heating associated with deep convection. The annual variation in tropical convection gives rise to annual variations in the surface pressure and pressure difference between Darwin and Tahiti. The pressure difference is greatest during northern hemisphere winter months. Other measures of the annual variation in tropical convection are provided by the tropical tropopause height and by highly reflective clouds (HRC). The tropical tropopause is highest during the northern hemisphere winter months when convection is most active over the maritime continent/western Pacific warm pool region and lowest in northern summer when convection is most active over the land masses of the northern hemisphere. The HRC show an annual migration in the centers of active convection. The annual latitudinal migration of the active centers of convection over the western Pacific with respect to the equatorial waveguide appears to be an important factor in determining the annual variation evident in the remote response of the zonal winds observed over Christmas Island.