Recent estimates of earth-atmosphere interaction torques and their use in studying polar motion variability

Abstract

The use of Earth-atmosphere interaction torques is a potential but generally less addressed alternative to the classical angular momentum approach for modeling variations in Earth rotation. We present an update on this subject for the purpose of explaining seasonal and intraseasonal polar motion variability based on the output of the most recent meteorological reanalysis systems of the ECMWF (European Centre for Medium-Range Weather Forecasts) and NASA's Global Modeling and Assimilation Office. The agreement of both models in terms of the three prime torque constituents is shown to be far superior to that of the conventionally deployed wind term of atmospheric angular momentum (AAM). A sufficiently good closure of the equatorial AAM budget equation within the ECMWF reanalysis provides additional endorsement for the use of atmospheric torques as excitation measures. When used as such, polar motion residuals after reduction of the AAM pressure term, as well as oceanic and hydrological excitation, are considerably better modeled by the torque-based quantities than by the standard wind term of AAM, in particular at intraseasonal periodicities. This finding is obtained by means of a newly proposed, hybrid excitation formalism, which derives the AAM counterparts of torque terms from inversion of the AAM budget equation in the frequency domain.