Aleksander Brzeziński

INFLUENCE OF THE ATMOSPHERE ON EARTH ROTATION: WHAT NEW CAN BE LEARNED FROM THE RECENT ATMOSPHERIC ANGULAR MOMENTUM ESTIMATES?

The interaction between the atmosphere and the underlying solid mantle is oneof the most important sources of changes in all three components of theEarths rotation vector on different time scales. In this paper the NCEP/NCARreanalysis time series of four times daily atmospheric effective angularmomentum (EAM) estimates is used to investigate some selected aspects of theatmospheric influence on Earth rotation. Emphasis is placed on thecontroversial features which were difficult or impossible to study using theoperational EAM data, such as excitation of the free oscillations in polarmotion, the Chandler wobble (CW) and the free core nutation (FCN), or theroles of diurnal and semidiurnal atmospheric tides and atmospheric normalmodes in the rotational dynamics of the Earth.

The use of the precise observations of the celestial ephemeris pole in the analysis of geophysical excitation of Earth rotation

Perturbations of Earth rotation are currently observed as time variations of universal time UT1-UTC and as changes in the terrestrial and the celestial orientation of the axis of the celestial ephemeris pole (CEP) as expressed by five Earth orientation parameters (EOP). On the other hand, dynamical theories of Earth rotation usually describe perturbations of the instantaneous rotation vector. A great improvement in the spatial and the temporal resolution of the EOP determination also raises the problem of the distinction between high-frequency polar motion and nutation. Here we derive the time domain relations between polar motion of the rotation axis and the observed changes of the CEP as well as between the spatial and the terrestrial motion of the CEP. These relations were discussed by Brzezinski (1992a). Here they are presented in a much simpler and direct manner using only matrix transformations between the conventional Terrestrial Reference System and Celestial Reference System in the form proposed by Capitaine et al. (1986) and Capitaine (1990), based upon the concept of the nonrotating origin (Guinot, 1979). These relationships are first obtained in a strict form that enables us to estimate the order of magnitude of errors arising from various approximations. The final linear expressions are used in an algorithm that transforms arbitrary differential equations describing geophysical excitation of polar motion to the form using only the observed quantities as variables.

Oceanic excitation of the chandler wobble

We estimate the oceanic contribution to the excitation of the Chandler wobble using an 11-year time series of ocean angular momentum. The time domain comparison of this series with the non-atmospheric excitation inferred from the polar motion and atmospheric angular momentum data shows a high correlation over most of the period of 1985 to 1996 when they overlap. The frequency domain comparison yields a high coherence in the vicinity of the resonant Chandler frequency. In terms of the free wobble excitation power, we compute the ocean contribution to be 11.9 mas2/cpy, which is approximately the same amount as supplied by the atmosphere. The combined action of the atmosphere and oceans provides about 80% of the power needed to maintain the wobble observed during 1985 to 1996, with the remaining deficit being at the level of one standard deviation of the excitation power estimated from a 100-year record of polar motion data.

Oceanic excitation of the Chandler wobble using a 50-year time series of ocean angular momentum

A 50-year time series of ocean angular momentum (OAM) is used to estimate the oceanic contribution to the excitation of the Chandler wobble. Our estimate of the oceanic excitation power, 18 mas2/cpy, is in good agreement with the residual excitation derived from a simultaneous use of polar motion and atmospheric angular momentum data, 21 mas2/cpy. Direct comparison of the OAM series and the inferred non-atmospheric excitation yields lower correlation and lower coherence at the Chandler frequency than in the study of Brzezinski and Nastula (2001) based on the shorter OAM series of Ponte et al. (1998). Differences in coherence levels are partly related to significant differences found in the two OAM series, indicating substantial dependence of OAM results on model and data assimilation procedures.

Oceanic excitation of polar motion from intraseasonal to decadal periods

We study the oceanic excitation of polar motion by using six different time series of the non-tidal oceanic angular momentum which are either publicly available or more experimental in nature. The oceanic excitation series are compared to each other and to the difference between the excitation inferred from the polar motion data and the corresponding atmospheric contributions. Comparisons show good agreement between the observed residual excitation and most of the ocean series. By performing computations separately for the seasonal sinusoids and four non-overlapping spectral components decadall, interannual, seasonal and intraseasonal) we demonstrate that the quality of the ocean products depends on the considered frequency band. We also discuss the importance of using data-constrained ocean models in future efforts to improve ocean angular momentum estimates.

The CEP and Geophysical Interpretation of Modern Earth Rotation Observations

The definition of the Celestial Ephemeris Pole (CEP) which is the pole of reference for precession and nutation, should be revised taking into account recent advances in observation and theory. This paper reviews the current realization of the CEP and discusses possible extensions of both the conceptual definition and the realization of the CEP. Attention is focused on the corresponding connections between the Earth orientation parameters describing rotational variations and the related excitation parameters expressing dynamics of the geophysical fluids.

High Frequency Oscillations of the Celestial Ephemeris Pole by Variations of the Effective Angular Momentum Function

The motion of the CEP is conventionally described by four among five the so-called Earth Orientation Parameters (EOP). Two of them, x, y, express displacement of the CEP from the terrestrial pole and the other two, dψ, de, express the celestial offset of the CEP with respect to the position predicted by the conventional IAU precession/nutation models. These parameters are routinely determined by modern space techniques with spatial and temporal resolution approaching the level of 0.1 mas (milliarcsecond) and a few hours, respectively. At the same time we observe also a significant improvement of quality of the related geophysical data, example being the regular estimates of the atmospheric EAM function χ (IERS Annual Reports).

On Polar Motion Equations Applied for Analysis of the Short Term Atmospheric Excitation

The atmospheric excitation data for earth rotation studies are published recently as the so-called “effective angular momentum” functions introduced by Barnes et al (1983). In applications, however, the corresponding polar motion equation is usually simplified to the form of the classical equation given by Munk and MacDonald (1960). The difference between these two equations is small only for perturbations which are slow compared to the diurnal cycle.

Diurnal and Sub-diurnal Terms of Nutation

Abstract : This paper discusses the short-period terms of nutation that are included in the recent theories of nutation for a rigid Earth. The author argues that these terms should be expressed as terrestrial perturbations (i.e., polar motion), and describes how such a transformation can be accomplished in practice. Based on the available rigid-Earth amplitudes, he estimates this lunisolar effect in polar motion for an elastic Earth with liquid core, and compares it to the oceanic and atmospheric perturbations at similar frequencies.

Excitation of Nutation as Deduced from Results of the Recent Atmospheric Reanalysis Project

Nearly diurnal variations in the equatorial components of the atmospheric angular momentum are expected to excite minor but well measurable nutational motions of the earth’s pole. These motions include the free core nutation (FCN) of variable amplitude between 100 and 500 microarcseconds (µas), as well as a contribution to the amplitudes of some important constituents of the lunisolar nutation. We investigate this problem using a 29-years long homogeneous series of the 4-times daily EAM (effective angular momentum) estimates based on results of the common U.S. NCEP/NCAR reanalysis project. The most important atmospheric contributions are found for the following nutation constituents: prograde annual (77 µas), retrograde annual (53 µas), prograde semiannual (45 µas), and for the constant offset of the pole (dψ sin e0 = -86 µas, de = 77 µas); among them only the prograde semiannual component is driven mostly by the wind term of the EAM function while in all other cases the pressure term is dominating. Comparison with the VLBI corrections to the IAU 1980 nutation model and taking into account the ocean tide contribution, yields a good agreement for the prograde annual and semiannual nutations, that is our estimation receives an important observational confirmation. We also investigated time variability of the atmospheric contribution to the nutation amplitudes by performing the sliding window least squares analysis of both the atmospheric excitation and VLBI nutation data.