Three-Dimensional Atmospheric Angular Momentum Simulated by the Japan Meteorological Agency Model for the Period of 1955-1994

Abstract

Axial and equatorial atmospheric angular momentum (AAM) functions for the rotational dynamics of the Earth are calculated monthly from ensemble mean data of three independent 40-year simulations during 1955-1994 by the global model of the Japan Meteorological Agency (JMA) forced by observed near-global sea surface temperature (SST) conditions. The model results are compared with those from the reanalysis data of the National Centers for Environmental Prediction (NCEP) and the operational objective analysis data of JMA and with the functions inferred from the observed length of day (LOD) and polar motion. The annual term of the simulated axial wind AAM function (dimensionless relative angular momentum of atmosphere due to zonal wind) during 1984-1994 agrees well with those from the two analysis data sets and roughly with the inferred function from LOD, while the semi-annual term is considerably over-estimated, suggesting an incompleteness in the simulated subtropical zonal winds. The annual term of the simulated equatorial pressure AAM function (dimensionless atmospheric inertia products due to atmospheric mass redistribution) is considerably over-estimated with respect to those from the two analysis data sets, presumably due to the large simulated redistribution of atmospheric mass between the Eurasian continent and the North Pacific Ocean. For interannual variations during 1955-1994, only the axial wind AAM function is reasonably simulated and shows good agreement with that from NCEP data as well as the Southern Oscillation Index. The above results lead to an understanding that the SST-forced AGCM simulates reasonably the atmospheric axial modes exciting LOD change but not the equatorial (non-axial) modes exciting the polar motion.