Atmospheric Angular Momentum Time Series Research Articles

Interdecadal oscillations in Atmospheric Angular Momentum variations

Interdecadal oscillations in Atmospheric Angular Momentum variationsGlobal Atmospheric Angular Momentum (AAM) is an intrinsic index for describing processes that affect the atmospheric circulation on time scales ranging from intraseasonal to secular. It is associated with length-of-day (LOD) variability through conservation of global angular momentum in planet Earth and thus is of considerable importance for quantifying how the Earth acts as a system. The availability of lengthy AAM time series computed from the recent 20th Century atmospheric reanalyses (1870-2008), complemented by the NCAR-NCEP reanalysis in the overlapping period of 1948-2008 allows the investigation of the role of decadal and interdecadal cycles as well as the recent overall trend in AAM. Thus, we extend to the entire 20th century (and prior, back to 1870) results concerning decadal time scales and a secular positive trend detected over recent decades by different authors. In addition, we also note that AAM has features of interdecadal time scales that modulate the lower frequency variability. These interdecadal time signals oscillate with periods of about 30-50 years, and we found an indication of an 80-90 year period. Short term signals interact with the long-term (secular) trend. Particularly over the years 1950-1985 the global positive trend in AAM appears to result from a conjunction of constructive positive slopes from all lower frequency signals (interdecadal short-term trends and the long-term positive secular trend). Since the mid 1980s, however, the interdecadal oscillation short-term trend contribution decreases, as does the total signal in global AAM. These oscillations appear as two interdecadal modes originating within the Pacific (associated principally with the Pacific Decadal Oscillation and also ENSO) from which they propagate poleward, with differing characteristics in each hemisphere.

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.

Atmospheric Angular Momentum Time-Series: Characterization of their Internal Noise and Creation of a Combined Series

The atmosphere induces variations in Earth rotation. These effects are classically computed using the “angular momentum approach”. In this method, the variations in Earth rotation are estimated from the variations in the atmospheric angular momentum (AAM). Several AAM time-series are available from different meteorological centers. However, the estimation of atmospheric effects on Earth rotation differs when using one atmospheric model or the other. The purpose of this work is to build an objective criterion that justifies the use of one series in particular. Because the atmosphere is not the only cause of Earth rotation variations, this criterion cannot rely only on a comparison of AAM series with geodetic data. Instead, we determine the quality of each series by making an estimation of their noise level, using a generalized formulation of the “three-cornered hat method”. We show the existence of a link between the noise of the AAM series and their correlation with geodetic data: a noisy series is usually less correlated with Earth orientation data. As the quality of the series varies in time, we construct a combined AAM series, using time-dependent weights chosen so that the noise level of the combined series is minimal. To determine the influence of a minimal noise level on the correlation with geodetic data, we compute the correlation between the combined series and Earth orientation data. We note that the combined series is always amongst the best correlated series, which confirms the link established before. The quality criterion, while totally independent of Earth orientation observations, appears to be physically convincing when atmospheric and geodetic data are compared

A spectral search for the inner core wobble in Earth's polar motion

The inner core wobble (ICW) is a normal mode of rotation of the inner core relative to the mantle predicted theoretically. While many papers have been devoted to its theoretical formulation, no attempt of its detection has been published in the literature. In this work, we first give a preliminary formulation for the variations of gravity and geopotential coefficients associated with the ICW that complements the formulation for the polar motion. The results show that the ICW is most likely detectable using polar motion data. Then we attempt to detect the ICW by comparing the spectrum of an Earth orientation parameter time series with that of the sum of an oceanic angular momentum time series and an atmospheric angular momentum time series. We found no firm evidence of the ICW using our approach to a resolvability of a few milliarc seconds. At such a low amplitude in polar motion, the ICW is far below the current detectability of the variations of gravity measured by absolute and superconducting gravimeters and of the variations of the geopotential coefficients obtained from the Gravity Recovery and Climate Experiment (GRACE).

Comparison of polar motion with oceanic and atmospheric angular momentum time series for 2-day to Chandler periods

During a 4-year period starting in July 1996 and using intervals ranging from 3 days to 4 years, four precise polar motion (PM) series have been compared to excitation by atmospheric angular momentum (AAM) augmented with oceanic angular momentum (OAM) data. The first three series (C03, C04 and Bulletin A) are multi-technique combinations generated by the International Earth Rotation and Reference Systems Service (IERS) and the fourth combined series (IGS00P02) is produced by the International GPS Service (IGS) using only GPS data. The IGS PM compared the best with the combined excitations of atmosphere and oceans (AAM+OAM) at all intervals, showing high overall correlation of 0.8–0.9. Even for the interval of only three days, the IGS PM gave a significant correlation of about 0.6. Moreover, during the interval of February 1999 – July 2000, which should be representative of the current precision of the IGS PM, a significant correlation (>0.4) extended to periods as short as 2.2 days and 2.5 days for the x p and y p PM components, respectively. When using the IERS Bulletin B (C04) PM and an interval of almost 6 years, starting in November 1994, the combined OAM+AAM accounted for practically all the annual, semi-annual and Chandler wobble (CW) PM signals. When only AAM was used, either the US National Centers for Environment Prediction reanalysis data, which were used throughout this study, or the Japanese Meteorological Agency data, two large and well-resolved amplitude peaks of about 0.1 mas/day, remained at the retrograde annual and CW periods.

Isolation of atmospheric effects on rapid polar motion through Wiener filtering

SUMMARY Rapid polar motions are irregular low amplitude fluctuations of the rotation pole of the Earth that occur over timescales of days to months. They have been attributed primarily to variations in atmospheric angular momentum (AAM), with a contribution from oceanic angular momentum as well. Monte Carlo experiments conducted by Stieglitz & Dickman suggest that, given the observed correlations between AAM and rapid polar motion, only two-thirds of the atmospheric excitation is actually being transferred to the solid earth's rotation. Consequently, the traditional procedure for correcting polar motion for atmospheric effects, by simply subtracting the original AAM time-series, is inefficient. Following a suggestion by Stieglitz & Dickman, we have attempted to optimally correct polar motion for atmospheric effects using the technique of Wiener filtering. With the atmospheric excitation as input and geodetic excitation (produced by deconvolving polar motion) as desired output, we use the Wiener filter to produce an atmospheric time-series that relates as meaningfully as possible to the geodetic excitation. The filter can also account for cross-component excitation of rapid polar motion by AAM, such as might result from atmospheric forcing of the oceans or groundwater. We find that the Wiener filter acts to screen non-geodetic power (mostly reddish noise) from AAM across the entire frequency spectrum. The resulting Wiener filtered AAM (WAAM) is reduced in amplitude from the original AAM by at least half and shows higher correlation and coherence with geodetic excitation. Additionally, correcting polar motion data with WAAM reveals the presence of tidal signals not seen in previous studies and allows long-period tide models to be tested.

Unusual Behavior in Atmospheric Angular Momentum during the 1965 and 1972 El Niños

Abstract The global atmospheric angular momentum (AAM) is known to increase with tropical eastern Pacific sea surface temperature (SST) anomalies during El Nino events. Using a reanalysis dataset, the ratio of the monthly AAM anomaly to El Nino SST anomaly (based on the Nino-3.4 index) is found to be approximately 1 angular momentum unit (=1025 kg m2 s−1) per degree Celsius for most post-1975 El Ninos. This ratio is much smaller, however, during the 1965/66 and 1972/73 El Ninos, raising the possibilities that either the early reanalysis data are in error due to sparse observations, or the atmospheric response to the two early El Ninos was unusual. The possibility of a severe data problem in the reanalysis is ruled out by cross-validating the AAM time series with independent measurements of length of day. The latitudinal structures of the zonal wind anomalies in 1965/66 and 1972/73 are examined for both the reanalysis and a set of general circulation model (GCM) simulations. Multiple GCM runs with specifie...

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.

Phase Space Structure, Attractor Dimension, Lyapunov Exponent and Nonlinear Prediction from Earth's Atmospheric Angular Momentum Time Series

—On a short time scale, Atmospheric Angular Momentum (AAM) has been demonstrated to be essentially the sole excitation source of LOD variations. The LOD variation, therefore, merely reflects the AAM variation (LOD as proxy for AAM). The study of the nonlinear nature of AAM variability (e.g., its orbital complexity, dimensionality and extreme sensitivity to the initial conditions) may provide a physical premise for theoretical modelling of the earth-atmosphere-ocean system. Analysis of the high quality of detailed daily LOD/AAM variations time series, spanning the period of 1962–1992, reveals a non-zero and low positive Lyapunov exponent value which suggests possible evidence of deterministic chaos in the underlying dynamics. Application of modern nonlinear prediction techniques capable of distinguishing chaos and random fractals to the data set, further support the above findings and render a predictive time limit of approximately 12–15 days. A low dimensional strange attractor and a low average Lyapunov exponent suggest a low level of unpredictability and stability in the system dynamics. It is argued here that a possible source of the raised entropy in LOD/AAM systems possibly stems from a conceivable nonlinear interaction between the seasonal cycle and inter- or intra-annual fluctuations due to thermodynamics properties of the atmosphere-ocean system.

The influence of global warming in Earth rotation speed

Abstract. The tendency of the atmospheric angular momentum (AAM) is investigated using a 49-year set of monthly AAM data for the period January 1949-December 1997. This data set is constructed with zonal wind values from the reanalyses of NCEP/NCAR, used in conjunction with a variety of operationally produced AAM time series with different independent sources and lengths over 1976-1997. In all the analyzed AAM series the linear trend is found to be positive. Since the angular momentum of the atmosphere-earth system is conserved this corresponds to a net loss of angular momentum by the solid earth, therefore decreasing the Earth rotation speed and increasing the length of day (LOD). The AAM rise is significant to the budget of angular momentum of the global atmosphere-earth system; its value in milliseconds/century (ms/cy) is +0.56 ms/cy, corresponding to one-third of the estimated increase in LOD (+1.7 ms/cy). The major contribution to this secular trend in AAM comes from the equatorial Tropopause. This is consistent with results from a previous study using a simplified aqua-planet model to investigate the AAM variations due to near equatorial warming conditions. During the same time interval, 1949-1997, the global marine + land-surface temperature increases by about 0.79 °C/cy, showing a linear correspondence between surface temperature increase and global AAM of about 0.07 ms per 0.1 °C. These results imply that atmospheric angular momentum may be used as an independent index of the global atmosphere's dynamical response to the greenhouse forcing, and as such, the length of day may be used as an indirect indicator of global warming.Key words. Meteorology and atmospheric dynamics (general circulation) · Geodesy

Orographically forced oscillations in a dynamical model of the Martian atmosphere

A single-layer shallow-water model with realistic topography and idealized zonal forcing is used to analyze barotropic, orographically forced, intraseasonal oscillations in the Martian atmosphere. A robust oscillation with a mean period near 85 Martian days (sols) is observed in a time series of the model global atmospheric angular momentum (AAM). Its robustness with respect to changes of the model parameters is verified. This oscillation is still present when an annual component is added to the forcing to account for seasonal variations in the Sun's declination and for the redistribution of atmospheric mass in connection with condensation and sublimation of carbon dioxide in the polar caps. Composites of the surface height anomaly field keyed to filtered AAM time series are examined to identify the spatial features associated with the 85-sol oscillation and its relation to the spatial distribution of the most prominent aspects of the Martian mountain field. The oscillation has a standing zonal component, with centers of action located upstream and downstream of salient orographic features, and a global meridional component. A resemblance is detected between its northern hemisphere (NH) features and those of the terrestrial 40-day oscillation. This resemblance is attributed to the similarity between the two planets' NH orographic covers. It is noted that interferences between the 85-sol oscillation and shorter-timescale phenomena which cause a redistribution of the Martian atmosphere's heat and momentum content, such as global dust storms, will have to be examined with a more complete model and/or with long records from the imminent Mars Observer mission.

Seth Carlo Chandler, Jr.: Discoveries in polar motion

On January 1, 1988, the new International Earth Rotation Service (IERS) will become operational. IERS will encompass the services provided by the International Latitude Service (ILS), the International Polar Motion Service (IPMS), and the Bureau International de l'Heure (BIH) for nearly a century and will provide improved services that will be particularly useful to scientific researchers. For example, the IERS will establish and maintain a conventional celestial reference frame defined by a selected set of extragalactic radio sources, establish and maintain a conventional terrestrial reference frame defined by a set of globally distributed stations marked by monuments, determine improved constants of precession and nutation, and collect and distribute ancillary data important to understanding earth rotation, such as atmospheric angular momentum time series.