Atmospheric, hydrological, and ocean current contributions to Earth's annual wobble and length‐of‐day signals based on output from a climate model

Abstract

The atmospheric, hydrologic, and ocean current contributions to the annual wobble and length‐of‐day change are investigated, based upon 10 years of monthly values simulated by a coupled ocean‐atmosphere general circulation model (COAGCM). Also used are time series of geodetic excitation inferred from the observed wobble in the “SPACE97” data and, for reference, atmospheric angular momentum (AAM) functions from operational objective analysis data of Japan Meteorological Agency (JMA) for the period 1988–1997. The simulated annual variation in equatorial AAM function, which includes pressure and wind contributions, agrees well with that from the JMA operational analysis for 90°E component, but disagrees for 0° component due to differences in both pressure and wind contributions over ocean hemisphere. In the seasonal cycle the simulated hydrologic and ocean current excitation functions plus the AAM function from the JMA operational analysis tend to be in better agreement with the geodetic excitation function than those from the COAGCM output themselves. Because of the small magnitude of the simulated ocean current effects, the simulated hydrological cycle is found to be an important contributor to the nonatmospheric annual variations in polar motion. This confirms the importance of hydrology as a source of the annual wobble.