Interannual signals in length of day and atmospheric angular momentum

International audienceAtmospheric angular momentum (AAM) and length of day (LOD) series are investigated for their characteristics on interannual time scales during the half-century period 1949 to 1998. During this epoch, the interannual variability in LOD can be separated naturally into three bands: a quasi-biennial, a triennial-quadrennial and one at six-seven years. The atmosphere appears to excite the first two bands, while it does not contribute to the last. Considering the quasi-biennial (QB) band alone, the atmosphere appears to excite most of its signal in LOD, but it arises from separate fluctuations with stratospheric and tropospheric origin. Thus, although close in frequency, stratospheric and tropospheric processes differ in their amplitude and phase variability. The time shift can be noted especially during the strong El Niño events of 1982-83 and 1997-98 when both processes have positive phase and thus combine to help produce particularly strong peak in AAM and LOD. In addition, we have reconfirmed the downward propagation in the stratosphere and upward propagation in the troposphere of AAM observed in earlier studies for other variables. In the triennial-quadrennial (TQ) band, time-variable spectral analyses reveal that LOD and AAM contain strong variability, with periods shorter than four years before 1975 and longer thereafter. This signal originates mainly within the troposphere and propagates upwards from the lower to the higher layers of the troposphere. According to a zonal analysis, an equatorial poleward mode, strongly linked to the SOI, explains more than 60% of the total variability at these ranges. In addition, this study also indicates that an equatorward mode, originating within polar latitudes, explains, on average, more than 15% of the triennial-quadrennial oscillation (TQO) variability in AAM, and up to 30% at certain epochs. Finally, a six year period in LOD noted in earlier studies, as well as in lengthier series covering much of the century, is found to be absent in atmospheric excitations, and it is thus likely to arise from mantle/core interactions

The Combined Solution C04 for Earth Orientation Parameters Consistent with International Terrestrial Reference Frame 2005

The Earth Orientation Center of the IERS, located at Paris Observatory, SYRTE, has the task to provide to the scientific community the international reference time series for the Earth Orientation Parameters (EOP), referred as ”IERS C04 ” (Combined 04), resulting from a combination of operational EOP series, each of them associated with a given geodetic technique. The procedure developed to derive the C04 solution was recently upgraded back to 1993. The main objective is to insurre its consistency with respect to the newly release ITRF 2008. Due to the separate determination of both terrestrial reference frames and EOP, there has been a slow degradation of the overall consistency since the least ITRF release in 2005, and discrepancies at the level of 50 micoarseconds for x pole coordinate exists between the current IERS C04 and the ITRF realization. We have taken this opportunity to upgrade the numerical combination procedure. Now there are better estimates of the errors of combined values. Individual EOP series have been reprocessed since 1993. Pole coordinates are now fully consistent with ITRF. The new C04 solution, referred as 08 C04, updated two times per week became the official C04 solution since february 2010

Long-term Earth Orientation Monitoring Using Various Techniques

AbstractA continuous composite series of polar motion components extending from 1846 until now called EOP (IERS) C01 is available at the Earth Orientation Section of the Central Bureau of the IERS. This series is the basis of the IERS system. It relies on different series derived from optical astrometry until 1972 and geodetic techniques since. It is given at 0.1 year intervals (1846–1889) and 0.05 year intervals (1890-now). Its accuracy has dramatically improved from 100 mas in 1846 to about 0.2 mas at present.Now the IERS combined solutions involve mainly the contributions of VLBI, GPS and SLR techniques. It is regularly recomputed to take advantage of the improvement of the various recent individual contributions and of the refinement of the analyses procedures.The objective of this paper is to describe this long-term polar motion series and to present the evolution and the state of the art of the multi-technique EOP combined solutions and the predictions regularly computed at the IERS/CB.</jats:p

Monitoring Earth Orientation Using Various Techniques: Current Results and Future Prospects

AbstractUntil 1972, astrometry based on a network of optical instruments was the only technique able to monitor the Earth orientation (polar motion, Universal Time and nutation). Since various techniques have shown their capability to give all or a part of these parameters: Doppler observations of navigation satellites, laser ranging to the Moon and to dedicated artificial satellites, Very Long Baseline Interferometry (VLBI) and more recently GPS and DORIS. The different Earth Orientation Parameters (EOP) series obtained by the individual techniques are inhomogeneous in time span, quality, time resolution – which supports the concept of combined solutions taking advantage of the various contributions.The main task of IERS is the maintenance of both a conventional terrestrial reference system based on observing stations and a conventional celestial reference system based on extragalactic radio sources and also the matrix product allowing the transformation between these two systems which takes into account precession-nutation, polar motion and Earth rotation.The objective of this paper is to present the evolution, the state of the art and future prospectives concerning the multi-technique EOP combined solution made at IERS/CB.</jats:p