Geodetic Excitation Functions Research Articles

Geodetic residual time series: A combined series by minimization of their internal noise level

This study aims to assess the hydrological effects of polar motion calculated from different combinations of geophysical excitations at decadal, seasonal, and non-seasonal periods. The geodetic residuals GAO, being a difference between observed geodetic excitation function of polar motion Geodetic Angular Momentum (GAM) and atmospheric (Atmospheric Angular Momentum—AAM) plus oceanic excitation functions (Oceanic Angular Momentum—OAM), are compared. Estimating hydrological effects on Earth’s rotation differs significantly, especially when using various oceanic models. Up to now, studies of geophysical excitations of polar motion containing AAM, OAM, and hydrological angular momentum (HAM) have not achieved entire agreement between geophysical (sum of AAM, OAM, and HAM obtained from the models) and geodetic excitation. Many geophysical models of the atmosphere, oceans, and land hydrology can be used to compute polar motion excitation. However, these models are very complex and still have uncertainties in the process descriptions, parametrization, and forcing. This work aims to show differences between various GAO solutions calculated from different mass and motion terms of various AAM and OAM models. Justifying to use one combination of GAO to estimate geodetic residuals is comparing those time series to hydrological excitations computed from Gravity Recovery and Climate Experiment (GRACE) data and the Land Surface Discharge Model (LSDM) model. Especially the quality of each geodetic residual time series is determined by estimating their noise level using a generalized formulation of the “three-cornered hat method” (3CH). This study presents a combined series of geodetic residuals GAO in polar motion (PM), wherein the internal noise level is shortened to a minimum by using the 3CH method. The combined GAO time series are compared with results obtained from the GRACE/GRACE Follow-On (GRACE-FO) solution provided by International Combination Service for Time variable Gravity Fields (COST-G) and the single solution elaborated by the Center for Space Research (CSR) and from the HAM LSDM hydrological model. The results show that higher consistency between GAO and HAM excitations can be achieved by minimizing the internal noise level in the GAO combined excitation series using the 3CH method, especially for the overall broadband and seasonal oscillations. For seasonal spectral bands, an agreement between combined GAO and the best-correlated series of GRACE CSR achieve correlations as high as 0.97 and 0.83 for the χ1 and χ2 equatorial components of PM excitation, respectively. This study’s combined geodetic residual time series slightly improved consistency between observed geodetic polar motion excitations and geophysical ones.

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.

Second-degree Stokes coefficients from multi-satellite SLR

The long wavelength part of the Earth’s gravity field can be determined, with varying accuracy, from satellite laser ranging (SLR). In this study, we investigate the combination of up to ten geodetic SLR satellites using iterative variance component estimation. SLR observations to different satellites are combined in order to identify the impact of each satellite on the estimated Stokes coefficients. The combination of satellite-specific weekly or monthly arcs allows to reduce parameter correlations of the single-satellite solutions and leads to alternative estimates of the second-degree Stokes coefficients. This alternative time series might be helpful for assessing the uncertainty in the impact of the low-degree Stokes coefficients on geophysical investigations. In order to validate the obtained time series of second-degree Stokes coefficients, a comparison with the SLR RL05 time series of the Center of Space Research (CSR) is done. This investigation shows that all time series are comparable to the CSR time series. The precision of the weekly/monthly $$C_{21}$$ and $$S_{21}$$ coefficients is analyzed by comparing mass-related equatorial excitation functions $$\chi ^{\text {mass}}_{1,2}$$ with geophysical model results and reduced geodetic excitation functions. In case of $$\chi ^{\text {mass}}_{1}$$ , the annual amplitude and phase of the DGFI solution agrees better with three of four geophysical model combinations than other time series. In case of $$\chi ^{\text {mass}}_{2}$$ , all time series agree very well to each other. The impact of $$C_{20}$$ on the ice mass trend estimates for Antarctica are compared based on CSR GRACE RL05 solutions, in which different monthly $$C_{20}$$ time series are used for replacing. We found differences in the long-term Antarctic ice loss of $$12.3$$ Gt/year between the GRACE solutions induced by the different $$C_{20}$$ SLR time series of CSR and DGFI, which is about 13 % of the total ice loss of Antarctica. This result shows that Antarctic ice mass loss quantifications must be carefully interpreted.

Asymmetric effects on Earth’s polar motion

Differential equations ruling the Earth’s polar motion are slightly asymmetric with respect to the pole coordinates. This is not only associated with the lack of axial symmetry around the Earth figure axis (triaxiality) but also with the longitude dependency of the pole tide (the main contribution). We propose a consistent handling of both asymmetric contributions, formulating a unique equation in the complex equatorial plane, of which we derive a general solution. Difference with respect to the usual symmetric solution is discussed and found significant in light of the present accuracy of the observed pole coordinates. For the same geophysical excitation, the prograde Chandler wobble is accompanied by a retrograde component up to 2 milliarcseconds (mas), transforming it in a slight elliptic motion. The asymmetric contribution is relatively larger in the geodetic excitation function, for Chandler wobble excitation mixes prograde and retrograde components of comparable level (1 mas).

Combination of gravimetric and altimetric space observations for estimating oceanic polar motion excitations

Global dynamic processes cause variations in the Earth's rotation, which are monitored by various geometric observation techniques such as Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Very Long Baseline Interferometry (VLBI) with millimeter accuracy. The integral effect on Earth rotation of mass displacements and motion is therefore precisely known, but the separation of contributions from particular geodynamic processes remains a challenge. Here we show that the oceanic mass effect on Earth rotation can be derived from both time variable gravity field solutions from the Gravity Recovery And Climate Experiment (GRACE) and sea level anomalies (SLA) observed from satellite altimeter missions. The GRACE solutions require filtering and the application of an ocean mask, whereas the SLA need to be corrected for the steric effect as polar motion is only affected by mass redistributions. We assess the accuracy of our oceanic polar motion excitations by using GRACE and SLA solutions from different processing centers. In addition, we compare polar motion excitations from GRACE, satellite altimeter data and their combinations with excitations estimated from ocean models. We show that the combination of gravimetric and altimetric solutions reduces systematic errors of the individual solutions. The combined solutions are about 2 times more accurate than ocean model results and about 3 times more accurate than the so‐called reduced geodetic excitation functions. We anticipate our analysis to be valuable input for improved modeling of oceanic mass redistributions.

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.

The Earth’s variable Chandler wobble

Aims. We investigated the causes of the Earth’s Chandler wobble variability over the past 60 years. Our approach is based on integrating of the atmospheric and oceanic angular momentum computed by global circulation models. We directly compared the result of the integration with the Earth’s pole coordinate observed by precise astrometric, space, and geodetic techniques. This approach differs from the traditional approach in which the observed polar motion is transformed into a so-called geodetic excitation function, and compared afterwards with the angular momentum of the external geophysical fluid layers.Methods. In the time domain, we integrated the atmospheric angular momentum time series from the National Center for Environmental Prediction/National Center for Atmospheric Research Reanalysis project and the oceanic angular momentum data from the ECCO consortium. We extracted the Chandler wobble from this modeled polar motion by singular spectrum analysis, and compared it with the Chandler wobble extracted from the observed polar motion given by the International Earth Rotation and Reference Systems Service data.Results. We showed that the combination of the atmosphere and the oceans explains most of the observed Chandler wobble variations, and is consistent with results reported in the literature and obtained with the traditional approach. Our approach allows one to appreciate the separate contributions of the atmosphere and the oceans to the various bumps and valleys observed in the Chandler wobble. Though the atmosphere explains the Chandler wobble amplitude variations between 1949 and 1970, the reexcitation of the Chandler wobble that begins in the 1980s, after a minimum around 1970, and that reaches its maximum in the late 1990s is due to the oceans, while the atmospheric contribution remains stable within the same period.

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.

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.

Atmospheric quasi-14 month fluctuation and excitation of the Chandler wobble

A quasi-14 month fluctuation in the atmospheric excitation function for the Earth’s wobble is discussed by using the re-analysis data of the European Center for Medium-range Weather Forecast for the 14 years between 1980 and 1993. The spectrum of the atmospheric wind excitation function shows a striking peak near the 14-month period. As a result, the atmospheric (wind plus pressure) excitation function shows exactly the same power as that of the geodetic excitation function inferred from the observed wobble at the Chandler wobble frequency (about 14 months), suggesting that the atmosphere excited the Chandler wobble between 1980 and 1993. The wind fluctuation comes mostly from the tropospheric wind.

El Nino-related variations in atmosphere–polar motion interactions

The influence of El Nino phenomena on the correlation between atmospheric and geodetic excitation functions of polar motion during four decades, from 1962 to 2000, is studied. These correlations are computed in three different spectral ranges of polar motion variations, namely 90–150, 150–230 and 230–450 days, which include the 120-day, semiannual, and annual oscillations, respectively. These correlation coefficients are variable in all spectra ranges. They are the most stable in the case of the annual oscillation, reaching maxima of about 0.9. In the case of the semiannual oscillation they are more variable, especially before 1970, and they reach maximum values of 0.8–0.9. In the case of the 120-day oscillation, correlation coefficients are the most variable, though more stable after 1980. The disturbances of correlation coefficients between atmospheric and geodetic excitation functions of polar motion are highly correlated with the epochs of El Nino/La Nina phenomena in the most cases, suggesting that during these events, non-atmospheric effects, such as those in the ocean, also have an influence on polar motion.

Atmospheric, hydrological, and ocean current contributions to Earth's annual wobble and length‐of‐day signals based on output from a climate model

The atmospheric, hydrologic, and ocean current contributions to the annual wobble and length‐of‐day change are investigated, based upon 10 years of monthly values simulated by a coupled ocean‐atmosphere general circulation model (COAGCM). Also used are time series of geodetic excitation inferred from the observed wobble in the “SPACE97” data and, for reference, atmospheric angular momentum (AAM) functions from operational objective analysis data of Japan Meteorological Agency (JMA) for the period 1988–1997. The simulated annual variation in equatorial AAM function, which includes pressure and wind contributions, agrees well with that from the JMA operational analysis for 90°E component, but disagrees for 0° component due to differences in both pressure and wind contributions over ocean hemisphere. In the seasonal cycle the simulated hydrologic and ocean current excitation functions plus the AAM function from the JMA operational analysis tend to be in better agreement with the geodetic excitation function than those from the COAGCM output themselves. Because of the small magnitude of the simulated ocean current effects, the simulated hydrological cycle is found to be an important contributor to the nonatmospheric annual variations in polar motion. This confirms the importance of hydrology as a source of the annual wobble.

Seasonal oscillations in regional and global atmospheric excitation of polar motion

Time variable spectra of seasonal oscillations of global and regional atmospheric excitation functions of polar motion for the period 1948–1999 are computed from the NCEP/NCAR (National Centres for Environmental Prediction/ National Centre for Atmospheric Research) reanalysis Atmospheric Angular Momentum (AAM) data, namely from the pressure and the pressure with the inverted barometric (IB) correction. Amplitudes of the time variable spectra of seasonal oscillations of the global and regional atmospheric excitation functions of polar motion are variable. Contributions of atmospheric excitation function of polar motion from lands and especially from the Eurasia are the most important in the seasonal spectrum range of polar motion excitation function. The maps of the mean amplitudes of seasonal oscillations of atmospheric excitation functions of polar motion are computed for the fifty years period in 108 equal-weight sectors. They show the regions in which these oscillations are the most energetic. Correlations between the global and regional atmospheric excitation functions of polar motion and the geodetic excitation functions are investigated. The correlation coefficients computed between the global/regional atmospheric and geodetic excitation functions of polar motion became more stable and achieved the highest values of about 0.8 after 1970, 1975 and 1985 for annual, semi-annual and 120 days oscillations, respectively. The correlation coefficients in the Asia region are the closest to those of the global data.

El Niño impact on atmospheric polar motion excitation

We study the influences of El Niño phenomena on correlations between short‐period variations of atmospheric and geodetic excitation functions of polar motion during 1980–1998. The correlation between these two excitation functions varies with time, with correlation coefficients computed for 1 year spans reaching maxima of 0.7–0.8. Series of these correlation coefficients themselves correlate with El Niño and Southern Oscillation Index (SOI) data, indicating the influence of El Niño and SOI on atmospheric and geodetic excitation functions of polar motion and on polar motion itself. The maximum correlation coefficients for Niño 4 sea surface temperature data is equal to 0.8 in the case of El Niño events in 1983 and 1987. Impacts of El Niño on polar motion have an impulsive character causing irregular variations of polar motion during the El Niño epochs.

The AAM data sources and the excitation of Δ LOD: YU Nan-hua & ZHENG Da-wei Shanghai Observatory, CAS, Shanghai 200030

Time variable spectra of seasonal oscillations of global and regional atmospheric excitation functions of polar motion for the period 1948–1999 are computed from the NCEP/NCAR (National Centres for Environmental Prediction/ National Centre for Atmospheric Research) reanalysis Atmospheric Angular Momentum (AAM) data, namely from the pressure and the pressure with the inverted barometric (IB) correction. Amplitudes of the time variable spectra of seasonal oscillations of the global and regional atmospheric excitation functions of polar motion are variable. Contributions of atmospheric excitation function of polar motion from lands and especially from the Eurasia are the most important in the seasonal spectrum range of polar motion excitation function. The maps of the mean amplitudes of seasonal oscillations of atmospheric excitation functions of polar motion are computed for the fifty years period in 108 equal-weight sectors. They show the regions in which these oscillations are the most energetic. Correlations between the global and regional atmospheric excitation functions of polar motion and the geodetic excitation functions are investigated. The correlation coefficients computed between the global/regional atmospheric and geodetic excitation functions of polar motion became more stable and achieved the highest values of about 0.8 after 1970, 1975 and 1985 for annual, semi-annual and 120 days oscillations, respectively. The correlation coefficients in the Asia region are the closest to those of the global data.