Atmospheric angular momentum and the length of day: a common fluctuation with a period near 50 days

Abstract

Semiannual, annual and some longer period changes in the rotation of the Earth have long been attributed to meteorological causes^1–3. More recently some higher frequency variations in the length of day (l.o.d.) were shown to be strongly correlated with variations of the polar component of the angular momentum of the atmosphere^4,5. However, fluctuations in l.o.d. with periods near 50 days were not definitely identified, because either the sampling was too coarse⁴ or it extended over too short an interval⁵. Fluctuations in tropical wind speeds with periods in this range have, nevertheless, been studied^4,6,7, but their counterparts, although expected⁴, had not been identified in the Earth's rotation. Last year, four astronomical measures of changes in l.o.d. obtained in 1979 were shown to exhibit nearly the same ∼50-day fluctuation⁸. To find out whether this fluctuation was persistent and of meteorological origin we analysed lunar laser ranging (LLR) observations from the McDonald Observatory in Texas and zonal (east-west) wind data deduced from sources distributed over the globe. The relevant data were available for the common period 1976–79. We describe these analyses in turn and then compare the results.