Polar Motion Excitation Functions Research Articles

The influence of Antarctic ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry

Increasing ice loss of the Antarctic Ice Sheet (AIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Here the contribution of the decreasing AIS to the excitation of polar motion is quantified from precise time variable gravity field observations of the Gravity Recovery and Climate Experiment (GRACE) and from measurements of the changing ice sheet elevation from altimeter satellites. While the GRACE gravity field models need to be reduced by noise and leakage effects from neighboring subsystems, the ice volume changes observed by satellite altimetry have to be converted into ice mass changes. In this study we investigate how much individual gravimetry and altimetry solutions differ from each other. We show that due to combination of individual solutions systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS polar motion excitations can be increased. We investigate the interannual variability of the Antarctic polar motion excitation functions by means of piecewise linear trends. We find that the long-term behavior of the three ice sheet subregions: EAIS (East Antarctic Ice Sheet), WAIS (West Antarctic Ice Sheet) and APIS (Antarctic Peninsula Ice Sheet) is quite different. While APIS polar motion excitations show no significant interannual variations during the study period 2003-2015, the trend of the WAIS and EAIS polar motion excitations increased in 2006 and again in 2009 while it started slightly to decline in 2013. AIS mass changes explain about 45% of the observed magnitude of the polar motion vector (excluding glacial isosatic adjustment). They cause the pole position vector to drift along 59^{circ } East longitude with an amplitude of 2.7 mas/yr. Thus the contribution of the AIS has to be considered to close the budget of the geophysical excitation functions of polar motion.

Reducing filter effects in GRACE-derived polar motion excitations

Polar motion is caused by mass redistribution and motion within the Earth system. The GRACE (Gravity Recovery and Climate Experiment) satellite mission observed variations of the Earth’s gravity field which are caused by mass redistribution. Therefore GRACE time variable gravity field models are a valuable source to estimate individual geophysical mass-related excitations of polar motion. Since GRACE observations contain erroneous meridional stripes, filtering is essential to retrieve meaningful information about mass redistribution within the Earth system. However filtering reduces not only the noise but also smoothes the signal and induces leakage of neighboring subsystems into each other. We present a novel approach to reduce these filter effects in GRACE-derived equivalent water heights and polar motion excitation functions which is based on once- and twice-filtered gravity field solutions. The advantages of this method are that it is independent from geophysical model information, works on global grid point scale and can therefore be used for mass variation estimations of several subsystems of the Earth. A closed-loop simulation reveals that due to application of the new filter effect reduction approach the uncertainties in GRACE-derived polar motion excitations can be decreased from 12–48% to 5–29%, especially for the oceanic excitations. Comparisons of real GRACE data with model-based oceanic excitations show that the agreement can be improved by up to 15 percentage points.

Determining and Evaluating the Hydrological Signal in Polar Motion Excitation from Gravity Field Models Obtained from Kinematic Orbits of LEO Satellites

This study evaluates the gravity field solutions based on high-low satellite-to-satellite tracking (hl-SST) of low-Earth-orbit (LEO) satellites: GRACE, Swarm, TerraSAR-X, TanDEM-X, MetOp-A, MetOp-B, and Jason 2, by converting them into hydrological polar motion excitation functions (or hydrological angular momentum (HAM)). The resulting HAM series are compared with the residuals of observed polar motion excitation (geodetic residuals, GAO) derived from precise geodetic measurements, and the HAM obtained from the GRACE ITSG 2018 solution. The findings indicate a large impact of orbital altitude and inclination on the accuracy of derived HAM. The HAM series obtained from Swarm data are found to be the most consistent with GAO. Visible differences are found in HAM obtained from GRACE and Swarm orbits and provided by different processing centres. The main reasons for such differences are likely to be different processing approaches and background models. The findings of this study provide important information on alternative data sets that may be used to provide continuous polar motion excitation observations, of which the Swarm solution provided by the Astronomical Institute, Czech Academy of Sciences, is the most accurate. However, further analysis is needed to determine which processing algorithms are most appropriate to obtain the best correspondence with GAO.

Hydrological signals in polar motion excitation – Evidence after fifteen years of the GRACE mission

We investigate the contribution of continental hydrology signals to polar motion excitation resulting from changes in mass distribution associated with terrestrial water and snow/ice based on a variety of datasets. The impact of the former is less well known than that of other geophysical fluids, namely atmospheric and oceanic angular momentum signals, especially for oscillations over shorter-than-annual periods. The geodetic excitation function of polar motion can be determined from observations, while the hydrological excitation function can be estimated either from global models of the land-based hydrosphere or from indirect observations of variations in the Earth’s gravity field. With more than 15 years of data, the pair of GRACE satellites has provided valid datasets for studying interannual polar motion excitation. Recent results are promising and indicate agreement between GRACE-derived interannual hydrological excitations and the corresponding geodetic observations. Our work focuses on the non-seasonal spectral range, and compares hydrological excitations based on recent GRACE solutions from different centres with signals from climate models, and as a residual from geodetic observations.

Mass-related excitation of polar motion: an assessment of the new RL06 GRACE gravity field models

The new Release-06 (RL06) Gravity Recovery and Climate Experiment (GRACE) gravity field solutions are evaluated by converting them into equatorial effective angular momentum functions (so-called excitation functions) for polar motion and comparing these to respective time series based on space-geodetic observations (geodetic excitation). The same is performed for the older RL05 solutions using identical processing. Maps of equivalent water heights derived from both releases show that the signal-to-noise ratio is significantly improved in RL06. The derived polar motion excitation functions from RL05 and RL06 differ by about 15%. An analysis of the contributions of different Earth subsystems revealed that the release update mainly influenced the hydrological (12%) and oceanic excitations (17%), but it has a relatively small impact on the cryospheric excitations related to Antarctica (4%) and Greenland (1%). The RL06 data from different GRACE processing centers are more consistent among each other than the previous RL05 data. Comparisons of the GRACE-based excitation functions with the geodetic and model-based oceanic excitations show that the latest release update improved the agreement by about 2 to 15 percentage points.

Hydrological excitation of polar motion by different variables from the GLDAS models

Continental hydrological loading by land water, snow and ice is a process that is important for the full understanding of the excitation of polar motion. In this study, we compute different estimations of hydrological excitation functions of polar motion (as hydrological angular momentum, HAM) using various variables from the Global Land Data Assimilation System (GLDAS) models of the land-based hydrosphere. The main aim of this study is to show the influence of variables from different hydrological processes including evapotranspiration, runoff, snowmelt and soil moisture, on polar motion excitations at annual and short-term timescales. Hydrological excitation functions of polar motion are determined using selected variables of these GLDAS realizations. Furthermore, we use time-variable gravity field solutions from the Gravity Recovery and Climate Experiment (GRACE) to determine the hydrological mass effects on polar motion excitation. We first conduct an intercomparison of the maps of variations of regional hydrological excitation functions, timing and phase diagrams of different regional and global HAMs. Next, we estimate the hydrological signal in geodetically observed polar motion excitation as a residual by subtracting the contributions of atmospheric angular momentum and oceanic angular momentum. Finally, the hydrological excitations are compared with those hydrological signals determined from residuals of the observed polar motion excitation series. The results will help us understand the relative importance of polar motion excitation within the individual hydrological processes, based on hydrological modeling. This method will allow us to estimate how well the polar motion excitation budget in the seasonal and inter-annual spectral ranges can be closed.

Hydrological Excitations of Polar Motion Derived from Different Variables of Fgoals − g2 Climate Model

Abstract The hydrological contribution to decadal, inter-annual and multi-annual suppress polar motion derived from climate model as well as from GRACE (Gravity Recovery and Climate Experiment) data is discussed here for the period 2002.3-2016.0. The data set used here are Earth Orientation Parameters Combined 04 (EOP C04), Flexible Global Ocean-Atmosphere-Land System Model: Grid-point Version 2 (FGOAL-g2) and Global Land Data Assimilation System (GLDAS) climate models and GRACE CSR RL05 data for polar motion, hydrological and gravimetric excitation, respectively. Several Hydrological Angular Momentum (HAM) functions are calculated here from the selected variables: precipitation, evaporation, runoff, soil moisture, accumulated snow of the FGOALS and GLDAS climate models as well as from the global mass change fields from GRACE data provided by the International Earth Rotation and Reference System Service (IERS) Global Geophysical Fluids Center (GGFC). The contribution of different HAM excitation functions to achieve the full agreement between geodetic observations and geophysical excitation functions of polar motion is studied here.

Assessment of the Global and Regional Land Hydrosphere and Its Impact on the Balance of the Geophysical Excitation Function of Polar Motion

The impact of continental hydrological loading from land water, snow and ice on polar motion excitation, calculated as hydrological angular momentum (HAM), is difficult to estimate, and not as much is known about it as about atmospheric angular momentum (AAM) and oceanic angular momentum (OAM). In this paper, regional hydrological excitations to polar motion are investigated using monthly terrestrial water storage data derived from the Gravity Recovery and Climate Experiment (GRACE) mission and from the five models of land hydrology. The results show that the areas where the variance shows large variability are similar for the different models of land hydrology and for the GRACE data. Areas which have a small amplitude on the maps make an important contribution to the global hydrological excitation function of polar motion. The comparison of geodetic residuals and global hydrological excitation functions of polar motion shows that none of the hydrological excitation has enough energy to significantly improve the agreement between the observed geodetic excitation and geophysical ones.

Chandler wobble parameters from SLR and GRACE

AbstractThe period and quality factor Q of the Chandler wobble are functions of the internal structure and dissipation processes of the Earth. Better estimates of the period and Q of the Chandler wobble can therefore be used to better understand these properties of the Earth. Here the period and Q of the Chandler wobble are estimated by finding those values that minimize the power in the Chandler frequency band of the difference between observed and modeled polar motion excitation functions. The observations of the polar motion excitation functions that we used are derived from both space‐geodetic polar motion observations and from satellite laser ranging (SLR) and Gravity Recovery and Climate Experiment (GRACE) observations of the degree‐2 coefficients of the Earth's time‐varying gravitational field. The models of the polar motion excitation functions that we used are derived from general circulation models of the atmosphere and oceans and from hydrologic models. Our preferred values for the period and Q of the Chandler wobble that we estimated using this approach are 430.9 ± 0.7 solar days and 127 (56, 255), respectively.

Separation of atmospheric, oceanic and hydrological polar motion excitation mechanisms based on a combination of geometric and gravimetric space observations

The goal of our study is to determine accurate time series of geophysical Earth rotation excitations to learn more about global dynamic processes in the Earth system. For this purpose, we developed an adjustment model which allows to combine precise observations from space geodetic observation systems, such as Satellite Laser Ranging (SLR), Global Navigation Satellite Systems, Very Long Baseline Interferometry, Doppler Orbit determination and Radiopositioning Integrated on Satellite, satellite altimetry and satellite gravimetry in order to separate geophysical excitation mechanisms of Earth rotation. Three polar motion time series are applied to derive the polar motion excitation functions (integral effect). Furthermore we use five time variable gravity field solutions from Gravity Recovery and Climate Experiment to determine not only the integral mass effect but also the oceanic and hydrological mass effects by applying suitable filter techniques and a land–ocean mask. For comparison the integral mass effect is also derived from degree 2 potential coefficients that are estimated from SLR observations. The oceanic mass effect is also determined from sea level anomalies observed by satellite altimetry by reducing the steric sea level anomalies derived from temperature and salinity fields of the oceans. Due to the combination of all geodetic estimated excitations the weaknesses of the individual processing strategies can be reduced and the technique-specific strengths can be accounted for. The formal errors of the adjusted geodetic solutions are smaller than the RMS differences of the geophysical model solutions. The improved excitation time series can be used to improve the geophysical modeling.

Effects of Huge Earthquakes on Earth Rotation and the length of Day

We calculated the co-seismic Earth rotation changes for several typical great earthquakes since 1960 based on Dahlen’s analytical expression of Earth inertia moment change, the excitation functions of polar motion and, variation in the length of a day (ΔLOD). Then, we derived a mathematical relation between polar motion and earthquake parameters, to prove that the amplitude of polar motion is independent of longitude. Because the analytical expression of Dahlen’s theory is useful to theoretically estimate rotation changes by earthquakes having different seismic parameters, we show results for polar motion and ΔLOD for various types of earthquakes in a comprehensive manner. The modeled results show that the seismic effect on the Earth’s rotation decreases gradually with increased latitude if other parameters are unchanged. The Earth’s rotational change is symmetrical for a 45° dip angle and the maximum changes appear at the equator and poles. Earthquakes at a medium dip angle and low latitudes produce large rotation changes. As an example, we calculate the polar motion and ΔLOD caused by the 2011 Tohoku-Oki Earthquake using two different fault models. Results show that a fine slip fault model is useful to compute co-seismic Earth rotation change. The obtained results indicate Dahlen’s method gives good approximations for computation of co-seismic rotation changes, but there are some differences if one considers detailed fault slip distributions. Finally we analyze and discuss the co-seismic Earth rotation change signal using GRACE data, showing that such a signal is hard to be detected at present, but it might be detected under some conditions. Numerical results of this study will serve as a good indicator to check if satellite observations such as GRACE can detect a seismic rotation change when a great earthquake occur.

Oceanic excitation of polar motion: Identification of specific oceanic areas important for polar motion excitation

In this paper regional values of the oceanic excitation function of polar motion are computed from bottom pressure and oceanic current fields from the ECCO/JPL data-assimilating model kf080 for the period 1993–2009. The influence of different geographic regions of the ocean on the excitation of polar motion is determined by calculating correlations and covariances between these regional excitations and either the global non-atmospheric excitation or the global oceanic excitation. The non-atmospheric excitation is estimated by subtracting the atmospheric signal from the excitation computed from geodetic observations of polar motion; the global oceanic excitation function is equivalent to the sum of the oceanic excitation function computed in every grid point. Our attention focuses on the regional distribution of the oceanic polar motion excitation for two time scales: the seasonal spectral band and the band around the Chandler period. We identified the southern Indian Ocean and the South Pacific Ocean as important regions for non-atmospheric polar motion excitation. The maximum of variability over the southern Indian Ocean is especially important in the case of annual oscillation. The Atlantic Ocean makes less significant contribution to the non-atmospheric polar motion excitation than the Pacific and Indian Ocean in both considered spectral ranges. Inland seas like the Mediterranean and the Sea of Japan have high covariance with the global signals.

Comparison of the geophysical excitations of polar motion from the period: 1980.0–2009.0

In this study we compared contributions to polar motion excitation determined separately from each of three kinds of geophysical data: atmospheric pressure, equivalent water height estimated from hydrological models, and harmonic coefficients of the Earth gravity field obtained from Gravity Recovery and Climate Experiment (GRACE). Hydrological excitation function (Hydrological Angular Momentum — HAM) has been estimated from models of global hydrology, based on the observed distribution of surfacewater, snow, ice, and soil moisture. In our considerationwe used several global models of land hydrosphere and models ofAtmospheric Angular Momentum (AAM) and Oceanic Angular Momentum (OAM). All of themwere compared with observed Geodetic Angular Momentum (GAM). The spectra of the following excitation functions of polar motion: GAM, AAM+OAM, AAM+OAM+HAM, GAM-AAM-OAM residual geodetic excitation function, and HAM were computed too. The time variable spectra of geodetic, gravimetric, and the sum of atmospheric, oceanic, and hydrological excitation functions are also presented. Phasor diagrams of the seasonal components of polar motion excitation functions of all HAM excitation functions as well as of two GRACE solutions: Center for Space Research (CSR), Centre National d’Etudes Spatiales/Groupe de Recherche en Geodesie Spatiale (CNES/GRGS) were determined and discussed.

Variations of the Earth's figure axis from satellite laser ranging and GRACE

[1] Satellite laser ranging (SLR) data were used to determine the variations in the Earth's principal figure axis represented by the degree 2 and order 1 geopotential coefficients: C21 and S21. Significant variations at the annual and Chandler wobble frequencies appear in the SLR time series when the rotational deformation or “pole tides” (i.e., the solid Earth and ocean pole tides) were not modeled. The contribution of the ocean pole tide is estimated to be only ∼8% of the total annual variations in the normalized coefficients: / based on the analysis of SLR data. The amplitude of the nontidal annual variation of is only ∼ 30% of from the SLR time series. The estimates of the annual variation in from SLR, the Gravity Recovery and Climate Experiment (GRACE) and polar motion excitation function, are in a good agreement. The nature of the linear trend for the Earth's figure axis determined by these techniques during the last several years is in general agreement but does not agree as well with results predicted from current glacial isostatic adjustment (GIA) models. The “fluid Love number” for the Earth is estimated to be ∼0.9 based on the position of the mean figure axis from the GRACE gravity model GGM03S and the mean pole defined by the IERS 2003 conventions. The estimate of / from GRACE and SLR provides an improved constraint on the relative rotation of the core. The results presented here indicate a possible tilt of the inner core figure axis of ∼2° and ∼3 arc sec displacement for the figure axis of the entire core.

Patterns of atmospheric excitation functions of polar motion from high‐resolution regional sectors

In this paper regional values of equatorial components of the atmospheric excitation function of polar motion, which are proportional to the equatorial components of angular momentum (AAM), were computed at high spatial resolution in 3312 equal‐area sectors from the surface pressure fields of the NCEP‐NCAR reanalyses in order to improve our knowledge of patterns of atmospheric excitation functions of polar motion. The inverted barometer (IB) model of oceanic isostatic adjustment is applied to readjust the effective atmospheric pressure fields. To identify the influence of different geographic regions of the atmosphere on the excitation of polar motion, correlations and covariances between these regional excitations values and either the global geodetic excitation function, determined from the geodetic observational data, or the atmospheric excitation functions, equivalent to the sum of such excitations in all regions, were computed in different spectral bands, from subseasonal to interannual period ones. The covariances show the regional contribution to the global excitation, whereas the correlations show an overall similar relationship. Furthermore, these analyses determine regional sources of polar motion excitation by surface pressure variations in the atmosphere in different spectral bands. The broad regions over Eurasia and North America were identified as principal atmospheric excitation sources for polar motion. The results revealed that atmospheric excitation over land regions in the northern middle latitudes, especially over Asia, is exceptionally strong, a point generally noted in previous studies, though without identifying detailed features. Over the Himalayan Mountains the covariance of local with global atmospheric excitation in the annual band forms a small minimum surrounded by larger values in a ring‐like formation. For other terms, maxima of atmospheric excitation have a longitudinal formation mostly across the Eurasian continent. The atmospheric excitation of polar motion over the ocean is weak when subject to the IB correction, but it contains nevertheless a clear annual signal and is highly correlated with the annual oscillation of the global geodetic excitation function of polar motion.

The Dynamics of Atmospherically Driven Intraseasonal Polar Motion

Abstract The atmospheric dynamical processes that drive intraseasonal polar motion are examined with National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data and with pole position data from the International Earth Rotation Service. The primary methodology involves the regression of different atmospheric variables against the polar motion excitation function. A power spectral analysis of the polar motion excitation function finds a statistically significant peak at 10 days. Correlation calculations show that this peak is associated with the 10-day, first antisymmetric, zonal wavenumber 1, normal mode of the atmosphere. A coherency calculation indicates that the atmospheric driving of polar motion is mostly confined to two frequency bands, with periods of 7.5–13 and 13–90 days. Regressions of surface pressure reveal that the 7.5–13-day band corresponds to the 10-day atmospheric normal mode and the 13–90-day band to a quasi-stationary wave. The regressions of pole position and the various torques indicate not only that the equatorial bulge torque dominates the mountain and friction torques but also that the driving by the equatorial bulge torque accounts for a substantial fraction of the intraseasonal polar motion. Furthermore, although the 10-day and quasi-stationary wave contributions to the equatorial bulge torque are similar, the response in the pole position is primarily due to the quasi-stationary wave. Additional calculations of regressed power spectra and meridional heat fluxes indicate that the atmospheric wave pattern that drives polar motion is itself excited by synoptic-scale eddies. Regressions of pole position with separate torques from either hemisphere show that most of the pole displacement arises from the equatorial bulge torque from the winter hemisphere. Together with the above findings on wave–wave interactions, these results suggest that synoptic-scale eddies in the winter hemisphere excite the quasi-stationary wave, which in turn drives the polar motion through the equatorial bulge torque.

Spectral Characteristic of Polar Motion in the 2005-2006 and 1999-2000 Winters Seasons

Spectral Characteristic of Polar Motion in the 2005-2006 and 1999-2000 Winters SeasonsThe Earth's pole moves on the Earth's surface along the spiral curve known as "polhody". Polhody computed from the IERS C04 pole coordinates were compared with those computed after removing successively oscillations with periods shorter than 150, 30, 10, 2 days. The comparison of these polhody show that the loops in the winter seasons of 2005-2006 and 1999-2000 are caused by the oscillations of polar motion with periods shorter than 30 days. These short periodical oscillations of geodetic excitation function of polar motion were correlated with those of the atmospheric and oceanic excitation functions. There are high correlations with coefficients equal to 0.8-0.9 during epochs when loops occur.

Revised atmospheric excitation function series related to Earth's variable rotation under consideration of surface topography

The atmospheric angular momentum is closely related to variations in the Earth rotation. The atmospheric excitation function (AEF), known also as the atmospheric effective angular momentum function, is introduced in studying the atmospheric excitation of the Earth's variable rotation. It may be separated into two portions, i.e., the “wind” terms due to the atmospheric motion relative to the mantle and the “pressure” terms due to the variations of atmospheric mass distribution evident through surface pressure changes. The AEF wind terms during the period of 1948–2004 are reprocessed from the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP/NCAR) reanalysis 6‐hourly wind and pressure fields. Some previous calculations were approximate, in that the wind terms were integrated from an isobaric lower boundary of 1000 hPa. To consider the surface topography effect, however, the AEF is computed by integration using the winds from the Earth's surface to 10 hPa, the top atmospheric model level, instead of from 1000 hPa. For these two cases, only a minor difference, equivalent to ∼0.004 ms in length‐of‐day variation, exists with respect to the axial wind term. However, considerable differences, equivalent to 5∼6 milliseconds of arc in polar motion, are found regarding equatorial wind terms. We further compare the total equatorial AEF (with and without the topography effect) with the polar motion excitation function (PMEF) during the period of 1980–2003. The equatorial AEF gets generally closer to the PMEF, and improved coherences are found between them when the topography effect is included.

Modeling and forecast of the polar motion excitation functions for short-term polar motion prediction

Short-term forecast of the polar motion is considered by introducing a prediction model for the excitation function that drives the polar motion dynamics. The excitation function model consists of a slowly varying trend, periodic modes with annual and several sub-annual frequencies (down to the 13.6-day fortnightly tidal period), and a transient decay function with a time constant of 1.5 days. Each periodic mode is stochastically specified using a second-order auto-regression process, allowing its frequency, phase, and amplitude to vary in time within a statistical tolerance. The model is used to time-extrapolate the excitation function series, which is then used to generate a polar motion forecast dynamically. The skills of this forecast method are evaluated by comparison to the C-04 polar motion series. Over the lead-time horizon of four months, the proposed method has performed equally well to some of the state-of-art polar motion prediction methods, none of which specifically features forecasting of the excitation function. The annual mode in the χ2 component is energetically the most dominant periodicity. The modes with longer periods, annual and semi-annual in particular, are found to contribute more significantly to forecast accuracy than those with shorter periods.

Statistical property of the Chandler wobble excitation function

The Chandler wobble excitation function (hereinafter, geodetic excitation) is obtained by removing the seasonal components and low frequency components with periods from several years to decades from the polar motion excitation function derived from SPACE2002 series. The geophysical excitation functions of the individual AAM, OAM, HAM, and two combined excitations of the AAM + OAM and AAM + OAM + HAM at 1d, 5d, 1m and 3m intervals are statistically tested for the hypothesis of the normality, and then tested for the hypotheses of identical distribution between the geodetic and the geophysical excitations. The results show that, among the total 16 components of the two combined excitation functions at 1d, 5d, 1m and 3m intervals, most follow random normal processes, the hypotheses of identical distribution between the geodetic excitation and the two combined excitation are acceptable, while most of the hypotheses of identical distribution between the geodetic excitation and the individual excitations of the AAM, OAM, and HAM are rejectable.