A Climatology of Diurnal and Semidiurnal Surface Wind Variations over the Tropical Pacific Ocean Based on the Tropical Atmosphere Ocean Moored Buoy Array

Abstract

Abstract Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E–95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for the meridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (∼0.14 m s−1) and phase [maximum westerly wind anomalies ∼0325/1525 local time (LT)] across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator (maximum southerly wind anomalies ∼0700 LT at 5°N and ∼1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (June–November). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ∼0700 LT (opposite conditions ∼1900 LT); the amplitude of the divergence diurnal cycle is ∼5 × 10−7 s−1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force.