UTCSR TOPEX/POSEIDON Gridded Sea Level Anomalies

Last Update: August 9, 1994

The data are here.


These data are from the University of Texas at Austin Center for Space Research (UT/CSR) monthly dynamic ocean topography data base anonymous ftp site (ftp.csr.utexas.edu). This data base contains gridded dynamic topography heights observed by the TOPEX/POSEIDON (T/P) altimeter measurements and the corresponding sea level anomalies. These solutions, in the form of sea surface anomalies in the equatorial Pacific, appear monthly in the Climate Diagnostics Bulletin published by U.S. Department of Commerce NOAA/NWS/NMC.

Please note that the maps contained in this directory will differ slightly from the solutions presented in the Bulletin. Starting in June, 1994, the ocean topography anomaly maps published in the Bulletin were averaged over 10-days, the time for TOPEX/POSEIDON to repeat its groundtrack. The dynamic topography and topography anomaly maps in the directoy month_dat/ are averaged over a month, while the maps in the directory cycle_dat/ are averaged over a 10-day repeat period. To see what dates a T/P repeat cycle refer to, please refer to the file CYCLE.START_TIMES. Note that all the data has been compressed using the unix command compress. To use these files, upload them to your directory, then uncompress the file.

The monthly maps contain all of the altimeter data which is available within any given month. Because we like to wait until we receive all the data for a month before uploading them to this site, the maps in this directory may have a one-month real-time lag. Cycle averaged maps will also be uploaded at the begining of each month.

One situation affecting the data in the solutions is the fact that the POSEIDON altimeter is on for a complete repeat cycle approximately every 100 days. Because POSEIDON data availability lags behind TOPEX data availability, this could cause delays as long as several months. To allay these problems, we will provide monthly maps using just TOPEX data initially, and then update them as soon as the POSEIDON data are available and processed. Both solutions will be available, and a naming convention has been adopted (described below) so that you can be aware when a monthly solution is incomplete and contains only TOPEX altimeter data.


The dynamic ocean topography heights are determined from TOPEX/POSEIDON altimeter data in which all media and instrument corrections (ionosphere, wet and dry troposphere, and electromagnetic bias) and geophysical corrections (tides and inverted barometer) have been applied to the measurements. The altimeter measurements are then reduced to sea surface topography heights using a precise orbit computed at UT/CSR using the available Satellite Laser Ranging and DORIS doppler tracking data.

The altimeter measurements used are equivalent to the Geophysical Data Record (GDR) with the exception of the precise orbit and a preliminary ocean tide correction computed using 1 year of TOPEX altimeter data. This preliminary ocean tide model produces more consistent topographies and reduces altimeter residuals a great deal. An altimeter bias is also removed from the data during this process.

The topography heights were computed using 10-second altimeter normal points and consist of surface spherical harmonics complete to degree and order 25. All the topography heights in the grids are referenced to the geoid computed from the JGM-3 gravity field model and corrected to refer to the mean ocean geoid. [Note: this is a change of the previous data storedat this site, which was referred to the JGM-2 geoid. ALL data is now referred to the JGM-3 geoid.]

The dynamic ocean topography heights are referenced to the mean JGM-3 geoid. The dynamic ocean topography height anomalies are with respect to a long-term mean sea surface. The current mean sea surface model was determined using TOPEX altimeter data from cycles 2-64, or 620 days worth of altimeter measurements.

For more information about availability of other relevant data, please send email to C. K. Shum at shum@csr.utexas.edu, or Don Chambers at chambers@csr.utexas.edu.