SEASONAL-VARIATIONS IN LENGTH OF DAY AND ATMOSPHERIC ANGULAR-MOMENTUM

The seasonal variations of the Earth's rotation are still not sufficie ntly well explained in terms of their causes. Quantitative estimates o f the variability of the oscillations in length of day (LOD) and atmos pheric angular momentum (AAM) have been applied very seldom. Therefore , the problem is re-examined. In particular, the axial AAM component l abelled chi(3) is related to changes in LOD. For this reason, the foll owing series of data at one-day intervals is used in this study: (a) L OD from the series EOP (IERS) C04 from 1962 to 1996, (b) chi(3) (W), c hi(3) (P) and chi(3) (P + IB) from the series AAM (NMC) from 1976 to 1 995 and (c) chi(3) (W), chi(3) (P) and chi(3) (P + IB) from the series AAM (JMA) from 1983 to 1995. Here, chi(3) (W) is the wind term, chi(3 ) (P) the pressure term and chi(3) (P + IB) the pressure term with inv erted-barometer response. First, the seasonal oscillations are separat ed from the various time-series by filtering. To illustrate their char acteristics, the amplitudes, periods and phases of the annual and semi -annual oscillations are then derived and presented in terms of their temporal variability. The discrepancies between the magnitudes of the annual and semi-annual components of LOD without the tidal effects Sa and without Ssa and of chi(3) (W), chi(3) (W) + chi(3) (P) and chi(3) (W) + chi(3) (P + IB) show to what extent uncertainties are present in the data, which portions of AAM originate from chi(3) (W), chi(3) (P) and chi(3) (P + IB), and whether another excitation source contribute s to seasonal LOD variations. At the annual frequency, the wind term f rom the upper stratosphere that is neglected is evidently responsible for the imbalance between the LOD and AAM data. However, at the semi-a nnual frequency, the discrepancy is not fully explained by the missing stratospheric wind term, and a contribution from the global surface w ater redistribution is likely.