Nontidal oceanic contribution to length‐of‐day changes estimated from two ocean models during 1992–2001

Abstract

Contributions of ocean bottom pressure and currents to changes in the length of day (LOD) during 1992–2001 are evaluated at seasonal timescales by using two different ocean general circulation model products developed at the Scripps Institution of Oceanography and the University of Maryland. The annual amplitudes for the two bottom pressure terms of oceanic angular momentum (OAM) differ by approximately a factor of 3, while their annual phases are similar. Comparing the gravity‐derived nonatmosphere LOD changes with the two OAM bottom pressure terms at annual frequency, we find obvious discrepancies in phase. This result indicates that excitation of LOD changes by other sources, such as hydrology, is also important. The annual amplitudes and phases of the two OAM current terms compare well with those of the observed LOD minus gravity‐derived LOD changes and axial atmosphere angular momentum wind term but only if wind effects between 10 and 0.3 hPa levels are not included in the latter. The two OAM series have similar seasonal amplitudes and phases, but they differ substantially when examined in different latitudinal bands. The annual amplitudes of two OAM‐induced LOD changes are comparable with that of the nonatmosphere LOD changes, but the semiannual amplitudes are far smaller. Our study indicates that the oceanic effects on LOD should continue to be examined. In addition, the effects of zonal winds (10–0.3 hPa) on LOD are more significant than that of atmospheric surface pressures at semiannual frequency, while they play almost the same roles for annual term.