ANGULAR-MOMENTUM EXCHANGE AMONG THE SOLID EARTH, ATMOSPHERE, AND OCEANS - A CASE-STUDY OF THE 1982-1983 EL-NINO EVENT

The 1982-1983 El Nino/Southern Oscillation (ENSO) event was accompanie d by the largest interannual variation in the Earth's rotation rate on record. In this study we demonstrate that atmospheric forcing was the dominant cause for this rotational anomaly, with atmospheric angular momentum (AAM) integrated from 1000 to 1 mbar (troposphere plus strato sphere) accounting for up to 92% of the interannual variance in the le ngth of day (LOD). Winds between 100 and 1 mbar contributed nearly 20% of the variance explained, indicating that the stratosphere can play a significant role in the Earth's angular momentum budget on interannu al time scales. Examination of LOD, AAM, and Southern Oscillation Inde x (SOI) data for a 15-year span surrounding the 1982-1983 event sugges ts that the strong rotational response resulted from constructive inte rference between the low-frequency (similar to 4-6 year) and quasi-bie nnial (similar to 2-3 year) components of the ENSO phenomenon, as well as the stratospheric Quasi-Biennial Oscillation (QBO). Sources of the remaining LOD discrepancy (similar to 55 and 64 mu s rms residual for the European Centre for Medium-Range Forecasting (EC) and U.S. Nation al Meteorological Center (NMC) analyses) are explored; noise and syste matic errors in the AAM data are estimated to contribute 18 and 33 mu s, respectively, leaving a residual (rms) of 40 (52) mu s unaccounted for by the EC (NMC) analysis. Oceanic angular momentum contributions ( both moment of inertia changes associated with baroclinic waves and mo tion terms) are shown to be candidates in closing the interannual axia l angular momentum budget.