Semi-diurnal wind variation in the friction layer above the tropical ocean

Conventional theory predicts that in the layer of frictional influence, the semi-diurnal forcing by pressure-gradient oscillations should produce responses in air motion which show reduced amplitude and phase-leading, in comparison with frictionless conditions in the free troposphere. Wind-profile data obtained by anemometers on a floating mast during the ‘1965-Atlantic Meteor Expedition’ at an anchor station in the SE-trade-wind regime near the equator showed that the amplitude of the west component at the 4-m level practically equalled the fr ictionless value, while other measurements gave evidence of frictional effects, such as phase-leading, and vertical shear from 1 to 10 m. The observed wind response is explained by dynamic coupling as a consequence of forcing by the horizontal pressure gradient oscillation and resulting semi-diurnal variation of surface stress and therefore of eddy diffusivity.