Diurnal and Semidiurnal Tides in Global Surface Pressure Fields

Aiguo Dai  and Junhong Wang

J.  Atmos. Sci.,  in press, 1999.


Global surface pressure data from 1976 to 1997 from over 7500 stations
and COADS marine reports have been analyzed using harmonic and zonal
harmonic methods. We find 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. S1 is also strong (~1.1 mb) over high terrain such 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. S1 peaks around 0600-0800 LST in low latitudes and around 1000-1200
LST over most of midlatitudes, while S2 peaks around 1000 and 2200 LST over
low-and mid-latitudes. S1 is much stronger over the land than over the ocean and
its amplitude distribution is strongly influenced by land masses, 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 can not 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 non-migrating tides
are substantially stronger than previously reported. The amplitudes of both the
migrating and non-migrating tides decrease rapidly poleward with a slower pace at
middle and high latitudes.