FLUCTUATIONS IN THE EARTHS ROTATION SINCE 1830 FROM HIGH-RESOLUTION ASTRONOMICAL DATA

Fluctuations in the Earth's rotation since 1830, as evidenced by changes in the length of the day, are derived from astronomical data having subannual resolution. Before 1955.5, timings of linar occultations are used; after 1955.5, the data are taken from the time series TAI-UT1. Although the data in the earliest period, 1830-90, display decade fluctuations in the length of the day, they are not accurate enough to reveal interannual variations. In this regard, also, the results from 1890-1925 are somewhat dubious. The quality of the data after 1925, though, is such that the temporal behaviour of the interannual fluctuations in the length of the day can be traced with confidence. We present plots of the interannual fluctuations in the period 1890-1987 and the longer-term decade fluctuations in the period 1830-1983. The interannual fluctuations in the length of the day since 1925 are compared with an index of the El Nino/Southern Oscillation (ENSO) phenomenon in the ocean-atmosphere system and are subjected to spectral analysis. The results support the conclusions reached by other authors that these fluctuations are linked to circulation changes in the atmosphere associated with ENSO, and in part to the quasi-biennial oscillation in the equatorial stratosphere's zonal winds. A spectral analysis of our 62 yr series of length of day values since 1925 reveals two significant peaks in the interannual range 2-4 yr. One is roughly biennial and the other is about twice this period, broadly supporting results obtained previously from shorter records. Our analysis of high-resolution data, therefore, contributes to ongoing efforts to establish a close relationship between the length of the day and aspects of the global climate system in the period before modern data became available in 1955.5.