Wind field differences between three meteorological reanalysis data sets detected by evaluating atmospheric excitation of Earth rotation

Abstract

We compared atmospheric excitations of the varying Earth rotation calculated from three different meteorological reanalysis data sets (NCEP/NCAR, NCEP-DOE and ERA-40) over 23 years, from 1979 to 2001. Atmospheric excitations are evaluated by atmospheric angular momentum (AAM) functions. We found that differences in AAM functions are primarily caused by differences in the wind data, especially the northward wind data, between the three reanalysis data sets. Differences between the NCEP-DOE and ERA-40 AAM functions are larger than those between the NCEP/NCAR and NCEP-DOE ones. The northward wind differences that have a large effect on the equatorial AAM functions originate from the upper troposphere at the tropics and midlatitudes in the southern hemisphere, especially in three regions: east Pacific off Latin America, Africa and the Indian Ocean. The eastward wind differences are seen at midlatitudes in the southern hemisphere. These differences are not confined to a single frequency. The difference spectra show a reddish frequency dependence with some peaks in seasonal bands and tidal diurnal bands. Since the eastward and northward winds over the tropics can most efficiently change the Earth's spin rate and polar motion, respectively, the wind accuracy over the tropics directly determines the accuracy in evaluations of the atmospheric excitations. Copyright 2008 by the American Geophysical Union.