During the investigation period, we have conducted studies on data assimilation techniques using eddy-resolving OGCMs. The Princeton Modular Ocean Model (MOM) has been implemented to run on the UT Cray computer and on a CSR RS-6000 workstation. Although MOM is a rigid-lid model, which does not conserve mass, we are using it as a tool to study data assimilation techniques. In this study, the model has been initialized by using climatological data. The initial conditions for the model were zero currents and the climatological temperature and salinity fields developed by Levitus. The model was also forced with climatological wind stress developed by Hellerman. The implementation used a 1° horizontal resolution, 15 vertical levels, real topography, with a second order horizontal mixing scheme with constant eddy coefficients for momentum and for heat transport and diffusion. Sea surface climatological temperature was used as a boundary condition. The three-year model initiation run was used to obtain a climatological distribution of ocean currents. As a first step to satellite data assimilation in the model, ERS-1 scatterometer vector wind measurements, and the altimeter sea surface topography measurements available from the TOPEX/ Poseidon altimeter were used. We first used the ERS-1 computed wind stresses as boundary conditions. The data were filtered and subjectively analyzed. Using the 3-year spun up model, the ocean model was then forced with monthly averaged scatterometer wind fields (October 1992-March 1993). A comparison of the model output with the in situ wind measurements recorded in the Western Tropical Pacific for the same period indicated good agreement. Since MOM is not a free-surface model, we used the sea surface heights observed from TOPEX/Poseidon altimetry in the form of temperature fields to update the solution at various time steps. The ECMWF wind fields are now being prepared for use in the model and to perform further comparison studies. We expect to conduct further data assimilation studies using an improved version of MOM (MOM2) which is a free-surface model.

At present, we are actively involved in analyzing a wide range of existing satellite data, which include pre-EOS sensor data. These include altimeter data from Seasat, Geosat, TOPEX/Poseidon, ERS-1, ERS-2, and other pre-EOS sensor data including Lageos-class satellite laser ranging data, and AVHRR thermal images from the NOAA polar-orbiting satellites. We will collect and analyze data from SeaWifs when it is launched. We are also analyzing an existing set of tracking data, both radiometric from the Doris and GPS systems, and satellite laser ranging data collected over other cannonball geodetic satellites. We are involved in analyzing data from tide gauge measurements and a wide range of other data sets collected under the WOCE and TOGA programmatic activities. Finally, we are analyzing a wide range of meteorological and geophysical data and output from GCMs, including NMC (reanalysis), ECMWF, and GEOS for Atmospheric Angular Momentum and other studies.

Specific research activities during the next two-year period will be focused on the following:

• A major portion of NMC's goal to reanalyze 40 years of daily weather observations will be realized. The preliminary results reported above indicate that these re-analyses will help improve previous estimates of AAM. We have arranged for AAM and torque statistics to be included in the Reanalysis archives, and we plan to use these data to reassess the quality achievable in AAM estimates during the pre-EOS era.
• Because the NMC reanalyses extend into the upper stratosphere, it is timely to reconsider the role of the stratosphere in contributing to fluctuations in AAM on subseasonal through interannual time scales. Independent values of stratospheric AAM based on satellite retrievals of geopotential heights in the upper atmosphere are available and can be used to help estimate the uncertainties in statistics calculated for this remote region of the global atmosphere.
• The torque balance for the SC model will be addressed, with focus on the bottom pressure torque and its relation to the friction torque. These studies are integral to understanding the overall mechanisms responsible for changes in oceanic angular momentum.
• The study of the ocean's role in exciting variability in Earth rotation and satellite orbits based on output from the barotropic ocean model will continue. In particular, the influence of altimeter data assimilation on model results will be assessed. Time scales of interest range from several days to years.
• The SC ocean model output, from runs using daily wind forcing for several years, will be examined with regard to angular momentum quantities. Comparisons with appropriate barotropic model runs will be attempted.
• Ocean modeling runs will continue using the Princeton MOM2 model. Altimeter and scatterometry assimilation efforts will continue. Output from OGCM (Los Alamos and Naval Postgraduate School modes) will be used to conduct studies of the effects of oceanic mass variations on the Earth rotation.
• Further work with the more than five-year NASA GEOS reanalysis to study the atmospheric momentum budget and relate it to that of the whole Earth system will be performed.
• A full budget using the ERS-1 and ERS-2 derived scatterometry will be completed including localized convergences and divergences of momentum.
• We will continue to extend the ERS-1 vector wind time series for both stress torque calculation, and for model assimilation efforts.
• We will continue to provide analysis of multiple altimetric (TOPEX/Poseidon, ERS-1, and Geosat) global mean sea level measurements to study the potential for the use of satellite altimetry for accurate monitoring of mean sea level variations and to conduct cross-calibration of the altimeter instruments.
• We will continue to interpret the current long period tide and secular zonal harmonic variations solutions, and to work on the study of interior properties of the solid Earth by application of these estimates as constraints.
• Geocenter variation time series from Lageos-1 and Lageos-2 will be intercompared with those obtained using TOPEX/Poseidon tracking data and global GPS measurements. Geophysical interpretation of the geocenter variations in terms of global mass redistribution and its impact on sea level measurements will be performed.