Annual Polar Motion Research Articles

Global dust storm signal in the meteorological excitation of Mars' rotation

AbstractThe meteorological excitation of Mars' rotation refers to the change of Mars' orientation parameters caused by meteorological processes, e.g., that portion due to variations of atmospheric circulation and surface pressure, and a cryospheric part due to the variable Martian polar ice caps associated with the CO2 sublimation/condensation effects. Global dust storms (GDSs) are unique meteorological events on Mars that cause changes in surface pressure, polar caps extent, and circulation patterns on a global scale, which in turn alter Mars' rotation under the conservation of Mars' total angular momentum. In this paper, the GDS signal in the meteorological excitation of Mars' rotation is investigated on seasonal, diurnal, and semidiurnal time scales, based on the Martian Climate Database. On seasonal scales, the cryospheric excitation plays a less important role in general, due to its location at high latitudes, than its atmospheric counterpart. The GDS brings (−3%, +15%) changes in the amplitude of annual polar motion and length‐of‐day (LOD) variation and (+22%, −34%) changes in the amplitude of semiannual polar motion and LOD variation. On diurnal and semidiurnal scales, the cryospheric excitation can be fully neglected in comparison to its atmospheric counterpart. A significant GDS signal is found with respect to the diurnal polar motion and semidiurnal LOD excitation variation, because of strong atmospheric tides arising from GDS effects. The amplitude increases in diurnal polar motion and semidiurnal LOD variation during some periods of the Martian year may reach 3–5 times as large as those during the same stage in a typical Martian year.

Excitation of Annual Polar Motion by the Pacific, Atlantic and Indian Oceans

The global oceans play important roles in exciting the annual polar motion besides the atmosphere. However, it is still unclear about how large the regional oceans contribute to the annual polar motion. We investigate systemically the contributions of the Pacific, Atlantic and Indian Oceans to the excitation of the annual polar motion, based on the output data of ocean current velocity field and ocean bottom pressure field from Estimating the Circulation and Climate of the Ocean (ECCO) ocean circulation model over the period 1993-2005. The result shows that due to its particular location and shape, the Atlantic Ocean makes a less significant contribution to the x-component of the annual polar motion excitation than the Pacific and Indian Oceans, while all these three oceans contribute to the y-component of the annual polar motion excitation to some extent.

Normal Morlet wavelet transform and its application to the Earth's polar motion

The normal Morlet wavelet transform (NMWT), which is a mathematical method for harmonic analysis, is defined in this study by resorting to L1 normalization continuous wavelet transform and a normalized simplified Morlet wavelet. The NMWT of a complex time signal enables direct recognition and extraction of the harmonic components of the signal without an inverse transform. We demonstrate theoretically and numerically the benefit of the NMWT in the harmonic analysis. A NMWT analysis of the observed polar motion data shows that except for the prograde annual wobble (PAW), the interseasonal wobbles including the retrograde annual wobble (RAW) became less significant in amplitude by at least 70% after the 1970s, manifesting the accuracy improvement in the polar motion observation. This NMWT analysis proves that the observed Chandler wobble possesses only one instantaneous period at any time point within the 20th century and such instantaneous period is slightly variable around 433.08 days with a standard deviation of 1.57 days from 1950 to 2000. Using the NMWT analysis, we show that at the prograde and retrograde annual scales, the atmospheric pressure and the oceanic current play the most significant and stable role in polar motion excitation. The atmosphere and ocean can explain the phases of the PAW with 15 days discrepancy but only 70% of the amplitude of the PAW. However, they can explain the RAW with less than 10 days in phase discrepancy and less than 10% in amplitude discrepancy. This study reveals that the ocean always reduces the atmospheric effects on the annual polar motion.

Excitation of non-atmospheric polar motion by the migration of the Pacific Warm Pool

Changes in the annual variation of the Earth’s polar motion are found to be largely caused by the variation of the atmospheric angular momentum (AAM). Recent simulation results of oceanic general circulation models further suggest global oceanic effects on the annual polar motion in addition to the atmosphere. In comparison with previous model studies of global oceanic effects, this research particularly singles out a large-scale ocean anomaly and investigates its effect on the annual polar motion, determined from satellite observations of the movement of the Western Pacific Warm Pool (WPWP). Although the scale of the warm pool is much smaller than that of the solid Earth, analysis of the non-atmospheric polar motion excitation has shown that the WPWP contributes non-negligibly to the annual polar motion. The analysis consists of over 30 years of WPWP data (1970–2000) and shows values of polar motion excitation for the x-component of (2.5 mas, −79°) and for the y-component of (0.6 mas, 173°). Comparison of this result with the total geodetic non-atmospheric polar motion excitation of (10.3 mas, 59°) for the x-component and (10.6 mas, 62°) for the y-component shows the significance of the WPWP. Changes in the Earth’s polar motion have attracted significant attention, not only because it is an important geodetic issue, but also because it has significant value as a global measure of variations within the hydrosphere, atmosphere, cryosphere, and solid Earth, and hence global changes.

Oceanic and soil moisture contributions to seasonal polar motion

Oceanic contributions to the annual and semi-annual wobble of polar motion have been evaluated by Wünsch (Wünsch, J., 2000. Oceanic influence on the annual polar motion. J. Geodynamics 30, 389–399) using three different ocean circulation models: (a) the Parallel Ocean Climate Model (Semtner, A.J., Chervin, R.M., 1992. Ocean circulation from a global eddy-resolving model. J. Geophys. Res. 97, C4, 5493–5550; (b) the model used by Ponte et al. (Ponte, R.M., Stammer, D., Marshall, J., 1998. Oceanic signals in observed motions of the Earth's pole of rotation. Nature 391, 476–479); (c) the Hamburg Ocean Model for Circulation and Tides used by Thomas and Sündermann [Thomas, M., Sündermann, J., 1998. Zur simultanen Modellierung von allgemeiner Zirkulation und Gezeiten im Ozean und Auswirkungen auf bestimmte Erdrotations parameter. In: Freeden, W. (Ed.), Progress in Geodetic Science. Aachen, pp. 144–151]. That result is extended by considering oceanic refinements as well as time variable soil moisture and snow load. Five soil moisture models were used. The snow load according to Chao et al. (Chao, B.F., O'Connor, W.P., Chang, A.T.C., Hall, D.K.,Foster, J.L., 1987. Snow-load effect on the Earth's rotation andgravitational field 1979-1985. J. Geophys. Res. 92, 9415–9422) was added to this in an attempt to close the balance. The NCEP/NCAR reanalysis atmosphere+Ponte et al. (1998) ocean+Chao and O'Connor (Chao, B. F., O'Connor, W. P., 1988. Global surface-water-inducedseasonal variations in the Earth's rotation and gravitationalfield. Geophys. J. 94, 263–270) (rain+snow) nearly close the annual balance of polar motion excitation, i.e. it is very close to the geodetic excitation functioncomputed from the time series of the IERS (International Earth Rotation Service). The other models of soil moisture differ in theirpolar motion contribution from the Chao and O'Connor (1988) estimates. Further improvements in this work await results from the space gravity missions CHAMP, GRACE and GOCE.

Oceanic influence on the annual polar motion

Oceanic contributions to the seasonal wobbles of polar motion are evaluated using three different ocean circulation models: (a) the Parallel Ocean Climate Model; mass term only, i.e. tensor of inertia changes; (b) the model used by Ponte et al. (1998) (Ponte R.M., Stammer D., Marshall J., 1998. Oceanic signals in observed motions of the Earth’s pole of rotation, Nature 391, 476–479 (mass term+motion term); (c) the model by Thomas and Sündermann (1998) (Thomas M., Sündermann J., 1998. Zur simultanen Modellierung von allgemeiner Zirkulation und Gezeiten im Ozean und Auswirkungen auf bestimmte Erdrotationsparameter. In: Freeden, W., (Ed.), Progress in Geodetic Science, Aachen, pp. 144–151) (mass term+motion term). The equatorial excitation components χ 1, and χ 2 describe an ellipse as the superposition of prograde and retrograde circular components at each frequency. The excitation ellipses are given at annual, semi-annual and thrice-yearly periods. These ellipse parameters are compared to differences: (excitation from observations) – (atmospheric part). The model by Ponte et al. nearly closes the balance.

Excitation of annual polar motion by atmosphere and ocean

The quantitative result of annual polar motion excitation by the ocean is presented for the first time. The atmospheric excitation amounts to more than double of the oceanic excitation. The sum of atmospheric and oceanic excitations approximates more to the observed annual polar motion excitation, compared with atmospheric excitation only. This suggests that the atmosphere and ocean are the main excitation sources of annual polar motion.

A Markov chain probability model for the annual polar motion series: Yang Ben-you, Sun Chang-an & Tao Gui-qin Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008

A Markov chain probability model for the annual polar motion series: Yang Ben-you, Sun Chang-an & Tao Gui-qin Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008

Chandler wobble and annual polar motion from 1965.0 to 1985.0

A new method of deconvolution in Fourier transformation is proposed: the data is first truncated with the Hanning window, after the transform, the effect of the window is eliminated by deconvolution. This method has a high resolution in the frequency domain and can give the periods, amplitudes and phases of the spectral peaks from a short time series. The polar motion data of BIH from 1962.0 to 1988.0 are analyzed by this method. The amplitudes of both the Chandler wobble and the annual polar motion are found to be not stable and a pseudo-annual excitation for maintaining the Chandler wobble is indicated.

Polar motion caused by variation in air mass distribution

We present formulae for calculating the effects on the polar motion by excitations with different periods. It is shown that the free Chandler wobble and the forced polar motion behave similarly near the Chandler frequency. From an analysis of the excitation function of air mass distribution, it is shown that any variation in air mass distribution not only produces the predominant effect on the annual polar motion, but also produces important effects on the Chandler wobble.

Computation of water storage contributions to polar motion

We have undertaken a computation of water storage fluctuations and their contribution to polar motion excitation for the period 1900–1982. The calculation is based mainly upon monthly mean observations of precipitation at individual stations around the globe. Seasonal average variations are computed for over 500 individual drainage basins, permitting a comparison with observed annual polar motion.

Meteorological excitation of the annual polar motion

Summary. Numerous studies have indicated that the annual term in the polar motion cannot be explained in any detail by meteorological/hydrological excitation and no reasonable alternative excitations have been put forward. Part of the problem has been that the hydrostatic adjustment of the oceans to the atmospheric pressure changes has traditionally been computed using the inverse barometer approach. This approach does not properly model the gravitational interaction between the atmosphere and oceans, and the inverse barometer theory is modified in this paper to account for this properly. The information necessary to compute the ocean tide and polar excitation caused by any change in the atmospheric pressure pattern is presented. The results of the application of this theory to two global atmospheric pressure data sets are examined and compared to results of other workers. It is concluded that the atmosphere is observed well enough to answer the question of the annual excitation of polar motion and it is argued that the ground water excitation is the component with the largest error and remains the chief obstacle to the successful solution of this problem.

An analysis of the homogeneous ILS polar motion series

Summary A new reduction of the International Latitude Service Observations for the years 1899–1977 has recently been completed under the direction of Dr S. Yumi and the International Astronomical Union Commission 19. This paper examines the annual, Chandler frequency, and long period motion of the Earth's pole implied by this new homogeneous data set. The estimated annual polar motion differs slightly from earlier results. The new value for the Chandler frequency is also very close to earlier results, but the estimated Q is somewhat larger, as might be expected by a reduction in noise level. Analysis of the long period motion shows an apparent drift and, in agreement with earlier studies using other data, there is some evidence of an approximately 30-yr oscillation in the pole position.