Diurnal and semidiurnal tides in global surface pressure fields

Global surface pressure data from 1976 to 1997 from over 7500 land stations and the Comprehensive Ocean-Atmosphere Data Set have been analyzed using harmonic and zonal harmonic methods. It is found that the diurnal pressure oscillation (S₁) is comparable to the semidiurnal pressure oscillation (S₂) in magnitude over much of the globe except for the low-latitude open oceans, where S₂ is about twice as strong as S₁. Over many land areas, such as the western United States, the Tibetan Plateau, and eastern Africa, S₁ is even stronger than S₂. This is in contrast to the conventional notion that S₂ predominates over much of the globe. The highest amplitudes (~1.3 mb) of S₁ are found over northern South America and eastern Africa close to the equator. Here S₁ is also strong (~1.1 mb) over high terrain such as the Rockies and the Tibetan Plateau. The largest amplitudes of S₂ (~1.0-1.3 mb) are in the Tropics over South America, the eastern and western Pacific, and the Indian Ocean. Here S₁ peaks around 0600-0800 LST at low latitudes and around 1000-1200 LST over most of midlatitudes, while S₂ peaks around 1000 and 2200 LST over low- and midlatitudes. Here S₁ is much stronger over the land than over the ocean and its amplitude distribution is strongly influenced by landmasses, while the land-sea differences of S₂ are small. The spatial variations of S₁ correlate significantly with spatial variations in the diurnal temperature range at the surface, suggesting that sensible heating from the ground is a major forcing for S₁. Although S₂ is much more homogeneous zonally than S₁, there are considerable zonal variations in the amplitude of S₂, which cannot be explained by zonal variations in ozone and water vapor. Other forcings such as those through clouds' reflection and absorption of solar radiation and latent heating in convective precipitation are needed to explain the observed regional and zonal variations in S₂. The migrating tides S₁¹ and S₂² predominate over other zonal wave components. However, the nonmigrating tides are substantially stronger than previously reported. The amplitudes of both the migrating and nonmigrating tides decrease rapidly poleward with a slower pace at middle and high latitudes.