Comparison between polar motion excitation functions estimated from different models of geophysical fluids

Abstract

The motion of the Earth's pole is excited by processes internal to the Earth's system – continually changing mass distribution in the geophysical fluids, i.e., atmosphere, ocean, and land hydrology. The mass redistribution and its movements within the Earth system excite the Earth's rotational changes mainly at seasonal or shorter timescales. The importance of atmospheric and oceanic angular momentum (AAM, and OAM, respectively) signals for polar motion excitation at seasonal and interannual timescales is well known. However, previous studies showed that the AAM, computed from different models of atmospheric pressure changes and winds, slightly differ from each other, especially in ­χ1 component. The discrepancies between various representations of ocean bottom pressure and currents from different OAM models are apparent too.An essential technique for understanding Earth's rotational changes is comparing the sum of mass and motion terms of AAM and OAM based on different geophysical models.Up to now, studies of geophysical excitations of polar motion containing AAM, OAM, and hydrological angular momentum (HAM) have not achieved entire agreement between geophysical (sum of AAM, OAM, and HAM obtained from the models) and geodetic (GAM, geodetic angular momentum; obtained from geodetic measurements of polar motion) excitation. There are many geophysical models of the atmosphere, oceans, and land hydrology that can be used to compute polar motion excitation. However, these models are very complex and still suffer from uncertainties in the process descriptions, parametrization, and forcing.Until now, no studies have shown that selecting one particular combination of AAM+OAM models provides the best correlation with GAM. The choice of AAM and OAM time series is usually entirely arbitrary and the only criterion considered is that the AAM model should be combined with the OAM model in which the forcing data is taken from the  AAM model used. This analysis of the most recent AAM and OAM series highlights that hydrological signals in polar motion differ significantly. The main goal of this presentation is to demonstrate that using different combinations of mass and motion terms of both AAM and OAM may have a considerable influence on the geophysical excitation of polar motion and its consistency with GAM. In this study, we extend the present understanding of the problem of inconsistency of mass and especially motion terms of different AAM and OAM models at seasonal and non-seasonal time scales.