Accuracy of Atmospheric Angular Momentum Estimates from Operational Analyses

Abstract

Modern, high resolution atmospheric and geodetic data are finding increasing use in geophysical studies of the angular momentum balance of the earth-atmosphere system. Such studies require knowledge of the level of uncertainty associated with each data set. Here we attempt to assess the accuracy of global atmospheric angular momentum determinations by comparing time series of this quantity derived from both the NMC and ECMWF zonal wind analyses for 1981?85. We find that over this period the rms difference between the two atmospheric series is comparable to the difference between an atmospheric momentum series and the series that is consistent with observed (nontidal) changes in earth rotation. Most of the difference between the NMC and ECMWF global momentum values comes from the region 33°S?16°N. Differences between NMC and ECMWF regional momentum values become vanishingly small in the high latitudes of the Northern Hemisphere, but they remain large throughout the entire Southern Hemisphere. An annual signal of up to 0.5 m s?1 exists in the difference between the NMC and ECMWF tropospheric zonal wind fields. Although seemingly small, this signal is vertically coherent and leads to a marked annual component in the NMC-ECMWF difference momentum values in the tropics and also for the Northern Hemisphere as a whole.