Diurnal and semidiurnal surface wind variations over the tropical Pacific Ocean

To advance our understanding of the large-scale atmospheric circulation, the daily cycle of the surface wind field over the tropical Pacific Ocean was examined. Hourly wind measurements collected from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N-8°S, 165°E-180°W) from 1993 through 2004 were used to document the diurnal and semidiurnal harmonics of surface wind variability. The zonal wind component is dominated by a semidiurnal variation, with a near-uniform amplitude (~0.15 m/s) and phase (~0300 and 1500 local time) across the tropical Pacific. Semidiurnal zonal wind signals are reduced over the eastern equatorial Pacific where the difference in sea surface temperature and air temperature is minimal, indicative of stable conditions in the boundary layer. The semidiurnal cycle shows no pronounced seasonal variability and is generally well explained by atmospheric thermal tidal theory. The meridional wind, on the other hand, exhibits primarily diurnal variability (amplitudes ~0.20 m/s) with opposite phase on either side of the equator, particularly in the western Pacific (the maximum northward component ~0600-0900 local time in the Northern Hemisphere, 1800-2100 local time in the Southern Hemisphere). The diurnal cycle in meridional wind induces a diurnal cycle of surface wind divergence near the equator, with enhanced convergence in the late evening (~2100 local time). The diurnal cycle of the equatorial wind divergence may contribute to diurnal variability in convection and rainfall in the tropical Pacific. The diurnal meridional wind variations in the eastern equatorial Pacific exhibit seasonal variability, with higher amplitudes during boreal summer and autumn. The southward phase propagation of these winds (from ~1000 local time at 8°N to ~1800 local time at 8°S) may be associated with gravity waves emanating from a diurnally pulsating heat source in the intertropical convergence zone.