Computations of atmospheric angular momentum, with emphasis on the merit period

Three-dimensional wind and pressure fields used to calculate effective angular momentum functions (EAMFs) for earth rotation/polar motion are produced by a complex 3-step system at the U.S. National Meteorological Center. The resulting series of functions have errors which can be estimated by comparisons with similar series produced by the European Center for Medium Range Weather Forecasts. For the axial wind component, related to length of day (lod) changes, the small differences between the two series of atmospheric angular momentum (AAM) are confined to the tropics and the higher latitudes of the Southern Hemisphere. In addition, differences between the seasonal components of the AAM series and those of lod are found to be significantly reduced by including the entire stratosphere's momentum. Comparisons of the EAMFs for 8 calendar years indicate substantial interannual variability exists in the annual amplitudes of these forcing terms. In particular, the axial AAM showed quite different behavior from normal in 1983 and 1984, the years of the MERIT campaign. These two years showed very high and low annual amplitudes for AAM, respectively, which can be attributed to the effect of the tropical Pacific sea surface warming (El Nino) and its aftermath.