1. The Earth Rotational Excitations in a Coherent Geophysical Fluids System Jianli Chen Center for Space Research, University of Texas at Austin, USA http://www.csr.utexas.edu/personal/chen Email: chen@csr.utexas.edu 2006 WPGM, July 24 - 27, Beijing, China G41A-05 Thu. 9:30 AM I sincerely apologize for being absent due to an unexpected urgency, and am grateful to Richard for his kind help. - Jianli Chen

2.  Atmospheric Contribution to Length-of-day (LOD)  Wind and pressure effects  Topographic effects  Upper wind effects   Oceanic and Hydrological Contributions  Ocean current and bottom pressure  Terrestrial water storage change  Global Mass Balance  Mass conservation of the ocean model  Mass balance between land and ocean  Mass balance between atmosphere and land/ocean Objectives

3. Global Mass Balance Atmosphere Land Ocean Precipitation Evaporation Evapotranspiration Runoff Snow/ice sheet melting

4. About LOD Excitations  Atmospheric Angular Momentum (AAM) Change  Dominant contributor, 90% of the observed LOD change;  Upper winds (above 10 mb) appear important as well;  Oceanic Angular Momentum (OAM)  Hydrological Angular Momentum (HAM)  At period of a few years or shorter, LOD = AAM + OAM + HAM or OAM + HAM = LOD - AAM  Can we close the budget yet ?

5. About LOD Excitations (Cont.)  Unlike polar motion X and Y, LOD is particularly sensitive to zonal wind circulation in the atmosphere -  Errors in wind fields  Upper winds (not included in typical atmospheric models)  LOD is also particularly sensitive to mass balance among the atmosphere, land, and ocean -  Mass conservation issue of individual models (e.g., OGCMs);  Mass conservation of the entire earth system;  How to coherently combine the atmosphere, ocean, and land

6. About LOD Excitations (Cont.)  LOD excitations can be computed as [Eubanks 1993],  Based on the definition of spherical harmonics, the above mass-term excitation can be rewritten as [see details in Chen 2005 - JGR, 110, B08404 10.1029/2004JB003474],

7.  Atmospheric Excitations  NCEP reanalysis atmospheric model  Winds of 17 layers from 1000 mb to 10 mb  Surface pressure  Daily intervals, Gaussian grids (~1.904  x 1.875  )  Jan. 1948 to the present  Topography effects are applied - integration from the real surface to the top of the model (at 10 mb). Data and Processing

8.  Oceanic Excitations  ECCO data assimilating ocean general circulation model  Ocean current and bottom pressure  1993 - present, telescoping meridional grids, 46 layers  Ocean bottom pressure (OBP) at 12-hourly intervals  Currents at 10-day intervals  Hydrological Excitations  CPC land data assimilation system  Terrestrial water storage change  Monthly, 1980 to present, 1  x 1  grids Data and Processing (cont.)

9.  Water Mass Balance  Step 1: Conserving the total mass of the ECCO model  Step 2: Balancing land and ocean - adding a uniform layer over the oceans that is equal to the total water storage change over land.  Step 3: Balancing atmosphere and land/ocean - adding a uniform layer over the land/ocean that is equal to the total mass change of the atmosphere.  Different strategy in Step 3 - uniform or Gaussian redistribution of total atmospheric mass change. Data and Processing (cont.)

10. Results  Observed LOD Variations  AAM Contributions  OAM Contributions  HAM Contributions

26. Main Conclusions  Global water mass balance plays an important role in estimating oceanic and hydrological excitations of LOD change.  When total atmospheric mass change is compensated by land and ocean, the combined seasonal oceanic and hydrological contribution is much smaller than before (when full mass balance is not enforced).  Remaining LOD variations unaccounted for by the atmosphere (i.e., LOD - AAM) are more likely caused by errors in atmospheric wind fields.  A full mass balance (or conservation) of the Earth system is mandatory in order to close the LOD budget.

27. Additional Notes  This research was supported by NASA's Solid Earth and Natural Hazards Program (under grants NNG04G060G, NNG04GP70G).  Results presented here have been published in Chen, J.L., Global Mass Balance and the Length-of-day Variation, J. Geophys. Res., 110, B08404 10.1029/2004JB003474, 2005. Reprints are available at: http://www.csr.utexas.edu/personal/chen/publication.html Or email request to: chen@csr.utexas.edu Thanks !