ATMOSPHERIC CONTRIBUTIONS TO NONSEASONAL VARIATIONS IN THE LENGTH OF DAY

Atmospheric wind and pressure contributions to non-seasonal variations in the length of day (LOD) determined by the International Radio Interferometry Surveying axe evaluated during the years of 1984-1991 using Japan Meteorological Agency data that include winds up to 10mb. We confirm that intra-annual LOD variations are the result of tropospheric zonal wind changes, which include a pronounced quasi-seven month oscillation in addition to the well-known intra-seasonal oscillation. It is also shown that inter-annual LOD variations on time scales of two to five years are the superposed contributions from both a stratospheric quasi-biennial oscillation and tropospheric low frequency variations of two-to-five year time scales in zonal wind. The residual LOD after removing these atmospheric contributions shows a decadal-like variation, which may be caused by a core - mantle coupling torque.