Christian Bizouard

Atmospheric torque on the Earth and comparison with atmospheric angular momentum variations

The purpose of this paper is to compute atmospheric torques on the Earth, including the oceans, with an emphasis on the equatorial components. This dynamic approach is an alternative method to the classical budget-based angular momentum method for viewing atmospheric effects on Earths orientation in space. The expression of the total torque interaction between the atmosphere and the Earth is derived from the angular momentum balance equation. Such a torque is composed of three parts due to pressure, gravitation, and friction. Each of these torque components is evaluated numerically by a semi-analytical approach involving spherical harmonic approximations, and their orders of magnitude are intercompared. For the equatorial components the pressure and gravitational torques have far larger amplitudes than that of the friction torque; these two major torques have the same order of magnitude but opposite signs, and the value of the sum of the torques is shown to be close to the equatorial components of the atmospheric angular momentum time derivative s, as would be expected in a consistent model-based analysis system. The correlation between the two time series is shown to be very good at low frequency and decrease slowly with increasing frequency. The correlation is still significant (≥ 0.7) up to 0.5 cycle per day, but the correlation coefficient reduces to 0.5 at the diurnal frequency band, indicating the difficulty of calculating rapidly changing model-based torques within an atmospheric analysis system.

Lunar influence on equatorial atmospheric angular momentum

This study investigates the relationship between the equatorial atmospheric angular momentum oscillation in the nonrotating frame and the quasi-diurnal lunar tidal potential. Between 2 and 30 days, the corresponding equatorial component, called Celestial Atmospheric Angular Momentum (CEAM), is mostly constituted of prograde circular motions, especially of a harmonic at 13.66 days, a sidelobe at 13.63 days, and of a weekly broadband variation. A simple equilibrium tide model explains the 13.66 day pressure term as a result of the O1 lunar tide. The powerful episodic fluctuations between 5 and 8 days possibly reflect an atmospheric normal mode excited by the tidal waves Q1 (6.86 days) and σ1 (7.095 days). The lunar tidal influence on the spectral band from 2 to 30 days is confirmed by two specific features, not occurring for seasonal band dominated by the solar thermal effect. First, Northern and Southern Hemispheres contribute equally and synchronously to the CEAM wind term. Second, the pressure and wind terms are proportional, which follows from angular momentum budget considerations where the topographic and friction torques on the solid Earth are much smaller than the one resulting from the equatorial bulge. Such a configuration is expected for the case of tidally induced circulation, where the surface pressure variation is tesseral and cannot contribute to the topographic torque, and tidal winds blow only at high altitudes. The likely effects of the lunar-driven atmospheric circulation on Earths nutation are estimated and discussed in light of the present-day capabilities of space geodetic techniques.

Reconstruction of prograde and retrograde Chandler excitation

Abstract Observed polar motion consists of uniform circular motions at both positive (prograde) and negative (retrograde) frequencies. Generalized Euler–Liouville equations of Bizouard, taking into account Earths triaxiality and asymmetry of the ocean tide, show that the corresponding retrograde and prograde circular excitations are coupled at any frequency. In this work, we reconstructed the polar motion excitation in the Chandler band (prograde and retrograde). Then we compared it with geophysical excitation, filtered out in the same way from the series of the Oceanic Angular Momentum (OAM) and Atmospheric Angular Momentum (AAM) for the period 1960–2000. The agreement was found to be better in the prograde band than in the retrograde one.

Elliptic polarisation of the polar motion excitation

Because of its geophysical interpretation, Earth’s polar motion excitation is generally decomposed into prograde (counter-clockwise) and retrograde (clockwise) circular terms at fixed frequency. Yet, these later are commonly considered as specific to the frequency and to the underlying geophysical process, and no study has raised the possibility that they could share features independent from frequency. Complex Fourier Transform permits to determine retrograde and prograde circular terms of the observed excitation and of its atmospheric, oceanic and hydrological counterparts. The total prograde and retrograde parts of these excitations are reconstructed in time domain. Then, complex linear correlation between retrograde and conjugate prograde parts is observed for both the geodetic excitation and the matter term of the hydro-atmospheric excitation. In frequency domain, the ratio of the retrograde circular terms with their corresponding conjugate prograde terms favours specific values: the amplitude ratio follows a probabilistic gamma distribution centred around 1.5 (maximum for 1), and the argument ratio obeys a distribution close to a normal law centred around

Remaining error sources in the nutation at the submilliarc second level

2 \alpha = 160^{\circ }

Low-frequency excitation of length of day and polar motion by the atmosphere

2α=160∘. These frequency and time domain characteristics mean an elliptical polarisation towards

Regional atmospheric influence on the Chandler wobble

\alpha ={\sim } 80^{\circ }