The Earth's Rotation and Atmospheric Circulation —II. The Continuum

(Received 1974 January 24)* Summary In an earlier paper (Lambeck & Cazenave, Part I) we investigated the seasonal variations in the Earth’s rotation and their relation to the global wind circulation. In this paper we consider mainly the high frequency part of the length-of-day spectrum. A month-by-month evaluation of the zonal wind excitation function shows that these rotational variations are all of zonal wind origin for frequencies up to at least 6 cycles per year. The zonal winds also contribute to frequencies greater than 6 cpy in the Earth’s rotation and will degrade the results for the Love numbers k2 estimated from the tidal terms near 13 and 26 cpy. Rapid changes of as much as during 5 days have been observed in the length of day on several occasions and these are also caused by meteorological disturbances. Any study of other possible excitation functions will require that these meteorological contributions are first evaluated. There does not appear to be any single wind pattern that is responsible for the observed high frequency variations in the length of day and it does not seem possible to use these observations as an indication of the nature of the zonal wind pattern other than as a global zonal wind index. There is some evidence that the zonal winds will contribute to those long-period variations in the Earth‘s rotation that are usually attributed to core-mantle coupling, In a previous paper (Lambeck & Cazenave 1973; referred to hereafter as Paper I) we discussed the relation between the annual, semi-annual and biennial variations in the Earth’s rate of rotation and the atmospheric circulation. We concluded that these observed periodic fluctuations in the rotation are largely due to the zonal winds and, on the basis of the astronomical evidence, we were able to draw some conclusions concerning the extent and variability of the downward propagation and the period of the global quasi-biennial circulation. We concluded, for example, that for the years 1965-1969 the period of this oscillation was closer to 3 years than to two but that by 1970 the period was again close to 2 years. The recent time-height zonal wind section at the Canal Zone and Kawajalein given by Wallace (1973) confirms such changes in the period. In this paper we are primarily concerned with the non-seasonal changes in the Earth’s rotation and their relation to variations in the zonal wind circulation. Short duration irregular variations in the length of day (lod) have been observed since