Effective Atmospheric Angular Momentum Research Articles

A stochastic excitation model for the study of the chandler wobble

Based on the hypothesis that the Chandler wobble is excited by stochastic motions, a nonlinear, stochastic model is proposed consisting of a random Heaviside step function and an Eulerian centred differential scheme for a linear oscillator with drag. The model is studied both analytically and numerically. From a statistical analysis of the time series of effective atmospheric angular momentum it is shown that the stochastic component can excite 28–40% of the amplitude of the Chandler wobble. Finally the hypothesis of stochastic excitation is discussed.

Dynamical analysis of the atmospheric angular momentum short-term fluctuations. Comparison with the Earth

The general behaviour of geophysical flows (atmosphere, oceans) is mainly nonlinear. Nevertheless, to date most of the studies concerning the atmospheric angular momentum functions and the Earth rotation time series have been devoted to their linear characterization and comparison, assuming their stochastic nature. In order to look for an alternative description of that time series, nonlinear time series analysis is applied to the effective atmospheric angular momentum short-term fluctuations (the pressure term of the equatorial functions and the wind term of the axial function) evaluated from the National Center for Environmental Prospect (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis Project, assuming a possible deterministic and nonlinear nature for these systems. The method consists of embedding the time series into a multidimensional space to recover the `attractor' of the system. It is shown that the dynamics of these processes are indeed nonlinear, deterministic and low-dimensional, thanks to their geometrical and topological description in the embedded space. The existence of a positive Lyapunov adds the property of chaoticity to the underlying system. It is found that the atmospheric processes observed through the angular momentum functions are chaotic and we are able to quantify the associated characterizations (D E=5, 4<D Lyap<5, variations of stability). We can also evaluate a theoretical horizon of prediction for the systems of about 6 days. The results suggest that the atmospheric angular momentum time series should no longer be described as purely stochastic processes. Additionally, the various nonlinear quantities evaluated are of the same order as those previously found for the short-term fluctuations of the Earth's rotation, indicating that the observed chaotic dynamics in the Earth's rotation is induced (through forcing processes) and belongs for a large part to the atmospheric dynamics, as expected.

Regional atmospheric angular momentum contributions to polar motion excitation

We focus on a regional analysis of equatorial components of the effective atmospheric angular momentum (EAAM) functions that measure the excitation of polar motion. These functions are computed from National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis data both globally and in 108 geographic sectors for the period 1968–1997. We investigate the relationship between the regional sector EAAM and the global functions responsible for polar motion excitation. We examine two excitation terms in parallel, with and without the inverted barometer (IB) approximation, which adjusts the atmosphere to account for an isostatic equilibrium response of the ocean to overlying pressure. In the case of pressure terms without IB the largest contributions to the equatorial components of polar motion excitation functions originate in the South Pacific, North Atlantic, and North Pacific regions. Applying the IB correction results in the dominance of Eurasia and North America instead, with nearly all Southern Hemisphere contributions disappearing. When comparing the regional functions to their global sum, such fluctuations are mainly coherent with variations over northern midlatitude land areas. In some sectors, oscillations in these functions tend to occur broadly across two frequency bands: 25–75 and 75–125 days in both prograde and retrograde directions, corresponding to counterclockwise and clockwise polar motion, respectively. Other sectors contain more continuous spectral bands, which are centered at ∼70 days. Coherence and cross‐spectral analyses lead us to identify a region over Eurasia that contributes importantly to exciting polar motion; we also note an eastwardly propagating signal toward this region in these excitation terms.

Possible improvement of Earth orientation forecast using autocovariance prediction procedures

Autocovariance prediction has been applied to attempt to improve polar motion and UT1-UTC predictions. The predicted polar motion is the sum of the least-squares extrapolation model based on the Chandler circle, annual and semiannual ellipses, and a bias fit to the past 3 years of observations and the autocovariance prediction of these extrapolation residuals computed after subtraction of this model from pole coordinate data. This prediction method has been applied also to the UT1-UTC data, from which all known predictable effects were removed, but the prediction error has not been reduced with respect to the error of the current prediction model. However, the results show the possibility of decreasing polar motion prediction errors by about 50 for different prediction lengths from 50 to 200 days with respect to the errors of the current prediction model. Because of irregular variations in polar motion and UT1-UTC, the accuracy of the autocovariance prediction does depend on the epoch of the prediction. To explain irregular variations in x, y pole coordinate data, time-variable spectra of the equatorial components of the effective atmospheric angular momentum, determined by the National Center for Environmental Prediction, were computed. These time-variable spectra maxima for oscillations with periods of 100–140 days, which occurred in 1985, 1988, and 1990 could be responsible for excitation of the irregular short-period variations in pole coordinate data. Additionally, time-variable coherence between geodetic and atmospheric excitation function was computed, and the coherence maxima coincide also with the greatest irregular variations in polar motion extrapolation residuals.

Analyses of zonal atmospheric excitation functions and their correlation with polar motion excitation functions

Abstract. The atmospheric influence on the Earth's, rotation can be described by the effective atmospheric angular momentum (EAAM) functions. In this study we focus on the analysis of short period variations of the equatorial components of the zonal EAAM excitation functions χ1 and χ2 and their influence on similar variations of polar motion. The global objective analysis data of the Japanese Meteorological Agency for the period 1986–1992 were used to compute the EAAM excitation functions in different latitude belts. Time- and latitude-variable amplitude spectra of variations of these functions with periods shorter than 150 days, containing pressure, pressure with the inverted barometric correction, and wind terms were computed. The spectra show distinct latitude and time variations of the prograde and retrograde oscillations which reach their maxima mainly in mid-latitudes. Prograde and retrograde oscillations with periods of about 40–60 days and about 110–120 days are seen in the spectra of pressure terms of the equatorial components of the zonal EAAM excitation functions. Additionally, correlation coefficients and cross-spectra between variations of the geodetic polar motion and equatorial components of the zonal EAAM excitation functions were computed to identify the latitude belts of the globe over which atmospheric circulation changes are correlated mostly with short period variations of the polar motion excitation functions. The correlation coefficients vary in time and latitude and reach maximum values in the northern latitudes from 50°N to 60°N. In the cross-spectra between the polar motion excitation functions and pressure terms of the zonal EAAM excitation functions there are peaks of common prograde oscillations with the periods around 20, 30, 40–50, 60 and 80–150 days and of common retrograde oscillations around 20, 30, 40 and 50–70 days.

Analyses of zonal atmospheric excitation functions and their correlation with polar motion excitation functions

The atmospheric influence on the Earth's, rotation can be described by the effective atmospheric angular momentum (EAAM) functions. In this study we focus on the analysis of short period variations of the equatorial components of the zonal EAAM excitation functions χ1 and χ2 and their influence on similar variations of polar motion. The global objective analysis data of the Japanese Meteorological Agency for the period 1986–1992 were used to compute the EAAM excitation functions in different latitude belts. Time- and latitude-variable amplitude spectra of variations of these functions with periods shorter than 150 days, containing pressure, pressure with the inverted barometric correction, and wind terms were computed. The spectra show distinct latitude and time variations of the prograde and retrograde oscillations which reach their maxima mainly in mid-latitudes. Prograde and retrograde oscillations with periods of about 40–60 days and about 110–120 days are seen in the spectra of pressure terms of the equatorial components of the zonal EAAM excitation functions. Additionally, correlation coefficients and cross-spectra between variations of the geodetic polar motion and equatorial components of the zonal EAAM excitation functions were computed to identify the latitude belts of the globe over which atmospheric circulation changes are correlated mostly with short period variations of the polar motion excitation functions. The correlation coefficients vary in time and latitude and reach maximum values in the northern latitudes from 50°N to 60°N. In the cross-spectra between the polar motion excitation functions and pressure terms of the zonal EAAM excitation functions there are peaks of common prograde oscillations with the periods around 20, 30, 40–50, 60 and 80–150 days and of common retrograde oscillations around 20, 30, 40 and 50–70 days.

Connection of the earth's rotation with the atmospheric angular momentum and the strongest earthquakes

Abstract Quick changes of the effective atmospheric angular momentum (AAM) functions are compared with the occurence of the strong earthquakes (with a body-wave magnitude m b ≥ 6.0) and quick changes of the Earth's rotation parameters (ERP). A strong correlation exists between these changes of the AAM functions and the times of earthquakes. The quick variations of the length-of-day (LOD) (up to 0.2 msec) are induced by the variations of the axial component of the AAM functions and strongly correlate with the times of earthquakes too. Delta-like changes of the equatorial components of the AAM functions correlate with the times of earthquakes and do not correlate with quick variations of polar motion. Common periodicities exist both in the AAM functions and earthquake series.

Variability of polar motion oscillations with periods from 20 to 150 days in 1979–1991

Variability of short period oscillations of polar motion with periods ranging from 20 to 150 days were investigated in the period 1979–1991. The new computation method of time variable band pass filter spectra and the Wavelet Transform method were applied. These oscillations are elliptical with variable amplitudes. Modulation periods of amplitude variations of these oscillations of about two and three years were found. Correlations of short period oscillations of polar motion and of effective atmospheric angular momentum (EAAM) excitation functions show annual variations and connections of their increases with El Nino phenomena.

Short periodic variations of polar motion and hemispheric atmospheric angular momentum excitation functions in the period 1984-1992

Abstract. Short periodic oscillations with the periods from 10 up to 110 days of the hemispheric components of effective atmospheric angular momentum (EAAM) excitation function and their correlation with polar motion excitation function have been analyzed. The EAAM data of the Japan Meteorological Agency (JMA) computed for the two hemispheres and the very long baseline interferometry (VLBI) polar motion NGS 92 R01 data (NGS 1992), determined by the National Geodetic Survey were applied. The distinct oscillations with periods of about 28, 35-55 and 60-80 days were detected in the χy-component of both polar motion excitation function and northern EAAM excitation functions containing wind and pressure, with and without inverted barometric correction terms. The χy-component of the polar motion excitation function is significanly correlated (correlation coefficient equal to 0.55-0.75) with the χy-components of the northern EAAM excitation functions mentioned above, which are mostly induced by the atmospheric circulation over lands. No meaningful correlation between polar motion excitation function and the southern EAAM excitation functions was found. The χx-components of the EAAM and polar motion excitation functions are not significantly correlated. The strong short periodic variation of the length of day (LOD) and χy in the early 1988 seems to be caused by the above-mentioned 35-55 days oscillations of the northern hemisphere atmosphere. This variation can be related to the rapid passing from the El Niño to the La Niña phenomenon or from the minimum to the maximum in the Southern Oscillation Index in 1987-1989.

New indications of the 4-month oscillation in solar activity, atmospheric circulation and Earth's rotation

The 4-month oscillation, detected earlier by the same authors in geophysical and solar data series, is now confirmed by the analysis of other observations. In the present results the 4-month oscillation is better emphasized than in previous results, and the analysis of the new series confirms that the solar activity contribution to the global atmospheric circulation and consequently to the Earth's rotation is not negligeable. It is shown that in the effective atmospheric angular momentum and Earth's rotation, its amplitude is slightly above the amplitude of the oscillation known as the Madden-Julian cycle.