Zonal winds over the equator

Abstract

SUMMARYThe dynamics of mean zonal flow over the Equator are described using a momentum budget. First the budget is drawn up for the troposphere, and the estimates of the mechanisms involved are shown to be consistent with the observed zonal wind/height profile. The technique is then applied to the stratosphere up to 30 km where the approximately $6-month oscillation in zonal winds is observed. The necessity for a similar oscillation in the momentum convergence due to large-scale horizontal eddies is deduced, and this oscillation is verified from wind observations. T h e vertical velocity/height profile associated with the zonal wind oscillation is derived and shown to be constant in time. The downward motion above 20 krn and upward motion below is shown to be consistent with observed ozone and water vapour observations. Finally, types of synoptic systems are suggested which could be associated with the observed pattern of zonal flow and momentum convergence.

the present status of the 26-month oscillation1,2

“…The momentum balance has been treated recently by Tucker [36] who finds that a cyclic variation in the divergence of the horizontal eddy flux of momentum is primarily responsible for the zonal wind oscillation. However, analysis by Reed [46] of the wind data from tropical stations with the longest records, using power spectrum techniques, fails to give convincing evidence of a 26-month cycle in the eddy flux.…”

Time Sequence of Power Spectra of Disturbances in the Equatorial Lower Stratosphere in Relation to the Quasi-Biennial Oscillation

“…Correlation between the zonal and meridional transport of momentum. Tucker (1964Tucker ( , 1965) and Wallace and Newell (1964) showed that the horizontal transport of momentum due to the disturbances with periods less than a month exhibits a quasi-biennial periodicity in the low and middle latitudes, and that the divergence of momentum fluxes contributes to the time change of the mean zonal flow. But they have not mentioned wind components is a measure of the horizontal Fig.…”

Stratospheric Wave Disturbances Propagating over the Equatorial Pacific

“…The 26-month (or quasi-biennial) oscillation has been fully documented (see Reed, 1965). Several authors have suggested to relate the time change of the stratospheric mean zonal wind to the horizontal convergence of the eddy transport of momentum into the equatorial belt (Reed, 1962;Tucker, 1964). However, one still lacks observational evidence of large-scale eddies in the tropical stratosphere, although Riehl and Higgs (1960) noted a large shear line moving across the Caribbean Sea area during January 1960 at 110,000 f t (33.5 km) height.…”