Anna Korbacz 1, Aleksander Brzeziński 1 and Maik Thomas 2. Journées Systèmes de référence spatio-temporels.

Transcription

1 Geophysical excitation of LOD/UT1 estimated from the output of the global circulation models of the atmosphere - ERA-4 reanalysis and of the ocean - OMCT Anna Korbacz 1, Aleksander Brzeziński 1 and Maik Thomas 2 1 Space Research Centre, Polish Academy of Sciences, Warsaw, Poland akorbacz@cbk.waw.pl 2 GeoForschungsZentrum, Potsdam, Germany presented at Journées Systèmes de référence spatio-temporels Paris, France, September 27 A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

2 Overview 1 Introduction 2 Data description and analysis 3 Results 4 Conclusions A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

3 Introduction Motivation Earlier investigation of excitation of the equatorial components of Earth rotation have revealed significant differences between effective angular momentum functions derived from ERA-4 reanalysis and nontidal OMCT and from other atmospheric and oceanic models (Korbacz et al., 27). These differences have been found smaller at shorter periods while increasing towards the longer periods. Current research is an extension of the previous work by considering the axial component of rotation expressed as changes of the length of day (LOD). The combination ERA-4 + OMCT will be compared to other available models of the atmospheric and oceanic excitation and to the ervations of LOD over a broad band of frequencies: from decadal through interannual and seasonal to intraseasonal. Korbacz A., et al., 27, Atmospheric and nontidal oceanic excitation of polar motion estimated from the output of the models ERA-4 and OMCT, Geophysical Research Abstracts, Vol. 9, 9625, 27, European Geosciences Union. A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

4 Introduction General considerations According to the principle of conservation of angular momentum, changes in the LOD are excited by mass redistributions and angular momentum exchanges between the solid Earth and geophysical fluids: atmosphere, oceans, land hydrosphere and fluid core. It has been proven that variations with decadal and longer periods are excited mainly by the interactions between the core and the mantle. Changes with shorter periods are driven mostly by the dynamically coupled system atmosphere-oceans. Comparison between geophysical excitation data and geodetic ervations of LOD is much simpler than in case of polar motion - there is a linear relationship between χ 3 component of the atmospheric and oceanic angular momenta (AAM, OAM) and LOD. A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

5 Introduction Objectives of the work estimate atmospheric and oceanic excitation of LOD using a new consistent set of AAM and OAM time series: AAM: from ERA-4 reanalysis model OAM: from OMCT simulation driven by ERA-4 output fields compare to the estimates based on the alternative AAM and OAM data sets and to the excitation inferred from the Earth rotation ervations A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

6 Data description and analysis Atmospheric angular momentum (AAM) estimates AAM and AAMIB - NCEP-NCAR reanalysis data ftp://ftp.aer.com/pub/anon collaborations/sba/aamf.ncep.reanalysis temporal resolution: 6 hours data span: AAM - ERA-4 reanalysis data temporal resolution: 6 hours data span: Note: AAM series are based on the full variability of the pressure fields AAMIB series assume an isostatic ocean response to atmospheric pressure fluctuations and thus only include effects of variability in the average atmospheric pressure over the ocean A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

7 Data description and analysis Oceanic angular momentum (OAM) estimates OAM code ocean model forcing atmosphere model temporal resolution data span T6 OMCT ERA-4 3 min P1 MOM + assimil. COADS /NCEP 1 month G5 ECCO-MITgcm NCEP 1 days G3 ECCO-JPL NCEP 1 day Pas ECCO-Scripps + assimil. NCEP 1 day Note: all OAM series except T6 are referred to the inverted barometer (IB) ocean model, therefore should be combined with AAMIB when computing the aggregated oceanic-atmospheric excitation (important for PM, much less important for LOD) T6 Thomas, M., et al., 21, Geophys. Res. Lett., 28(12), P1 Ponte, R. M., 21, unpublished manuscript. G5 Gross, R. S., et al., 25, J. Geophys. Res., 11, B945. G3 Gross, R. S., et al., 23, J. Geophys. Res., 18, B8. Pas Ponte, R. M., et al., 21, Geophys. Res. Lett., 28(9), A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

8 Data description and analysis LOD data COMB22 solution - combination of Earth rotation ervations from different techniques, derived by Gross (23) data span: temporal resolution: 12 hours - time series of LOD from the COMB22 solution Gross, R. S., 2, Combinations of Earth Orientation Measurements: SPACE97, COMB97 and POLE97. J. Geodesy, vol. 73, A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

9 Data description and analysis Data analysis perform linear transformation AAM, OAM χ 3 LOD, split up the excitation functions AAM, OAM and erved: model (first order polynomial + sum of the sinusoids with periods 1, 1/2 and 1/3 year), residuals = total excitation model, compare amplitudes and phases of the seasonal harmonics, computed from the geophysical model and ervations split up residual series into four components: decadal: T > 8 years interannual: 2 < T < 8 years seasonal: 1/2 < T < 2 years intraseasonal: 2 days < T < 1/2 year compare the estimated oceanic and atmospheric excitations to the erved one in each of the selected spectral bands A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées Journées / 19 ()

10 Data description and analysis Data sets 3 AAM LOD wind (ms) AAM LOD pressure (ms) OAM LOD currents (ms).2.2 OAM LOD mass (ms) Figure: Atmospheric and oceanic contributions to LOD estimated from the models ERA-4 and OMCT. Variations with period shorter than 2 days have been removed by smoothing. Scale is the same in each panel. A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées 27 1 / 19 ()

11 Data description and analysis Data sets.5 AAM LOD wind (ms).5.2 AAM LOD pressure (ms).2.2 OAM LOD currents (ms).2.2 OAM LOD mass (ms) Figure: Same as in the previous figure but after removal of the model containing the first order polynomial and sum of the sinusoids with periods 1, 1/2 and 1/3 year. A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

12 Results Seasonal harmonics LOD cos (ms) T6 G3 Annual zoom AAM OAM LOD sin (ms) LOD cos (ms) T6 G3 Semiannual zoom OAM AAM LOD sin (ms) LOD cos (ms) T6 G3 Terannual.4.2 zoom LOD sin (ms) Figure: Seasonal harmonics of the atmospheric and oceanic excitation of LOD. Analysis was done over the period Note: in case of T6 shown are additionally separate contributions from AAM and OAM (dotted line) A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

13 Results Seasonal harmonics LOD cos (ms) T6 G5 P1 Annual zoom AAM OAM LOD sin (ms) LOD cos (ms) T6 G5 P1 Semiannual zoom OAM AAM LOD sin (ms) LOD cos (ms) T6 G5 P1 Terannual.2.1 zoom LOD sin (ms) Figure: Seasonal harmonics of the excitation of LOD. Analysis was done over the period A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

14 Results AAM+OAM vs. LOD: decadal component (T > 8 yrs) LOD (ms) Time (years) G5 P1 T6 Corr. with Var. reduct. (%) Standard deviation (µs) Series ( ) wind pres. curr. mass P G T A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

15 Results AAM+OAM vs. LOD: interannual component (2 yrs < T < 8 yrs).4 LOD (ms).2.2 G5 P1 T Time (years) Corr. with Var. reduct. (%) Standard deviation (µs) Series ( ) wind pres. curr. mass P G T A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

16 Results AAM+OAM vs. LOD: seasonal band (1/2 yr < T < 2 yrs) Note: Seasonal harmonics have been removed prior to this analysis..4 LOD (ms).2.2 G3 T Time (years) Corr. with Var. reduct. (%) Standard deviation (µs) Series ( ) wind pres. curr. mass G T A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

17 Results AAM+OAM vs. LOD: intraseasonal oscillations (2 d < T < 1/2 yr).4 LOD (ms).2.2 Pas G3 T Time (years) Corr. with Var. reduct. (%) Standard deviation (µs) Series ( ) wind pres. curr. mass Pas G T A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

18 Conclusions Conclusions Unlike equatorial component of Earth rotation, the axial component of T6 combination AAM+OAM does not differ significantly from other geophysical models. In case of annual and terannual harmonics in LOD there is an excellent agreement in amplitude and phase between models and ervations. For semiannual oscillation there is a high agreement in amplitude, but large phase difference ( 3 o ) occurs (land hydrology?). At decadal periods the AAM and OAM are much smaller than erved LOD, as could be expected from earlier works. For periods shorter than 8 years the atmospheric/oceanic excitation plays an important role with the highest agreement with ervations in the seasonal band. The dominating contribution to the excitation of LOD at periods shorter than 8 years comes from zonal winds. Variations of the atmospheric pressure and the ocean play a minor role. A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()

19 Acknowledgements Conclusions Anna Korbacz would like to express her sincere thanks for the travel grant provided by the Descartes-Nutation consortium. This work has been supported by the Polish Ministry of Science and Higher Education under grant No. N /3972. A. Korbacz et al. Geophysical excitation of LOD/UT1 Journées 27 Journées / 19 ()