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 (S-1) is comparable to the semidiurnal pressure oscillation (S-2) in magnitude over much of the globe except for the low-latitude open oceans, where S-2 is about twice as strong as S-1. Over many land areas, such as the western United States, the Tibetan Plateau, and eastern Africa, S-1 is even stronger than S-2. This is in contrast to the conventional notion that S-2 predominates over much of the globe. The highest amplitudes (similar to 1.3 mb) of S-1 are found over northern South America and eastern Africa close to the equator. Here S-1 is also strong (similar to 1.1 mb) over high terrain such as the Rockies and the Tibetan Plateau. The largest amplitudes of S-2 (similar to 1.0-1.3 mb) are in the Tropics over South America, the eastern and western Pacific. and the Indian Ocean. Here S-1 peaks around 0600-0800 LST at low latitudes and around 1000-1200 LST over most of midlatitudes. while S-2 peaks around 1000 and 2200 LST over low- and midlatitudes. Here S-1 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 S-2 are small. The spatial variations of S-1 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 S-1. Although S-2 is much more homogeneous zonally than S-1, there are considerable zonal variations in the amplitude of S-2 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 hearing in convective precipitation are needed to explain the observed regional and zonal variations in S-2. The migrating rides S-1 and S-2 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.