DIURNAL AND SEMIDIURNAL VARIATIONS OF THE SURFACE WIND-FIELD OVER THETROPICAL PACIFIC-OCEAN

The climatological large-scale patterns of diurnal and semidiurnal nea r-surface wind variations over the tropical Pacific Ocean are document ed using 3 yr of hourly measurements from the Tropical Atmosphere-Ocea n moored buoy array. Semidiurnal variations account for 68% of the mea n daily variance of the zonal wind component, while diurnal variations account for 82% of the mean daily variance of the meridional wind com ponent. The spatially uniform amplitude (0.15 m s(-1)) and phase (0300 LT) of the semidiurnal zonal wind variations are shown to be consiste nt with atmospheric thermal tidal theory. The diurnal meridional wind variations on either side of the equator are approximately out of phas e. This pattern results in a diurnal variation of wind divergence alon g the equator, with maximum divergence in the early morning (similar t o 0800 LT). The average amplitude of the diurnal cycle in zonal mean d ivergence is 0.35 x 10-h s(-1), which corresponds to a day-night chang e of 45% relative to the daily mean. The relative day-night changes in near-surface equatorial wind divergence are larger in the western Pac ific (78%) than in the eastern Pacific (31%) due mainly to differences in the daily mean divergence. The diurnal amplitude of equatorial win d divergence changes seasonally and interannually in proportion to the strength of the mean divergence. It is suggested that diurnal heating of the sea surface may contribute to the zonally symmetric diurnal cy cle of equatorial wind divergence.