Oceanic excitation of the continuous oscillations of the Earth

Globally distributed atmospheric pressure data, and a few deep ocean bottom pressure data of continuous recording more than a year long were examined. At most sites, the annual mean power spectrum of the atmospheric pressure has a nearly constant decay rate as a function of frequency below about 5 mHz and shows variability above this frequency. The spectra from the deep ocean bottom near Japan show persistent spectral shape with a constant decay up to 2 mHz and flat or slightly increasing up to around 10 mHz, and quick decay beyond. A very similar spectral shape is reported for recordings near the East Pacific Rise. Above the sharp cutoff at about 10 mHz, the power at the ocean bottom is consistently larger than the surface, especially in the flat portion between 2 and 10 mHz by one or two order of magnitude. The observed power spectra of ground motion of the Earth (Peterson 1993, New Low Noise Model) also has a structure with constant decrease as frequency increases below 2 mHz and flat portion up to about 10 mHz and decrease beyond. The structural resemblance between the ocean bottom pressure and ground motion, as well as the larger power, indicates the pressure at the ocean bottom might be responsible for the continuous oscillations of the Earth. The sharp cutoff near 10 mHz is well explained if we assume the pressure fluctuation source is the surface gravity waves of the ocean. We found that the seasonal variation of atmospheric pressure correlates with the local mean wind speed and pressure power at the ocean bottom. We also found that, at many sites, the wind speed, hence the pressure power, is larger in winter time than in summer, contradicting the atmospheric hypothesis in which atmospheric pressure disturbances maintained by a local energy balance between solar flux and turbulent dissipation. Because of the meridional imbalance of solar influx, the energy transport from the equatorial area to the polar regions causes a planetary-scale wind system. The large wind energy in DC component is efficiently converted into higher normal mode frequency by the wind and ocean wave interaction over the sea. A strong sustained wind blows in the circum-Antarctic ocean in winter and we suggest this to be the potential excitation source in the summer time in the northern hemisphere.