NOTE: This directory contains a 1-year set of high resolution sea level anomaly grids from TOPEX/POSEIDON and ERS-1/2 data. A complete set of similar grids for 1992 through 2002, as well as the alongtrack data itself for the same time period, will placed in a separate DVD-ROM as a contibution from AVISO and CLS to WOCE.
This series of files correspond to maps of Sea Level Anomaly (MSLA) obtained from a complete reprocessing of TOPEX/POSEIDON and ERS-1/2 data. There is one map every 7 days for a period of almost 10 years (October 1992 to February 2002). T/P maps are available for the whole time period but there are no T/P+ERS combined maps between January 1994 and March 1995 (ERS-1 geodetic phase). From June 1996 to February 2002, ERS-2 data are used.
TOPEX/POSEIDON M-GDR distributed by AVISO were used (AVISO/Altimetry, 1996). The M-GDR tidal correction was replaced, however, by the more recent GOT99 tidal correction (Ray, 1999) and we used an inverse barometer correction with a variable mean pressure (e.g. Dorandeu and Le Traon, 1999). Radiometer drift and TOPEX-A / TOPEX-B/ POSEIDON relative biases were also corrected for.
ERS-1/2 are the OPRs distributed by CERSAT (CERSAT, 1996). Tidal and inverse barometer corrections were updated to be homogenous with T/P. Additional corrections (radiometer, electromagnetic bias, SPTR/OSU) were also applied as recommended, in particular, in the CLS OPR validation reports (CLS, 1996). Orbits were updated to use the DEOS DGM04 precise orbits (Scharroo and Visser, 1998) . A global adjustment using T/P as a reference was finally performed to correct for ERS-1/2 orbit error (Le Traon and Ogor, 1998).
Sea Level Anomalies (SLA) were computed using conventional repeat-track analysis. The CLS Mean Sea Surface was used to correct for cross-track geoid gradient errors. SLA are relative to a 7-year mean (January 1993 to January 199). A specific processing is performed to get an ERS-1/2 mean consistent with T/P mean.
SLA maps were obtained using the mapping method detailed in Le Traon et al. (1998) and applied to more than five years of TP and ERS data by Ducet et al. (2000) . This is a global sub-optimal space/time objective analysis which takes into account along-track correlated errors. Several improvements were made compared to the version used by Ducet et al. (2000) . In particular, an improved statistical description of sea level variability, noise and long wavelength errors is used. Covariance functions including propagation velocities that depend on geographical position were thus used. For each grid point, the zonal and meridional spatial scales, the time scale and the zonal and meridional propagation velocities were adjusted from five years of TP+ERS combined maps. In addition to instrumental noise, a noise of 10% of the signal variance was used to take into account the small scale variability which cannot be mapped and should be filtered in the analysis (see discussion in Le Traon et al., 2001) . Long wavelengths errors (LWE) due to residual orbit errors but also tidal or inverse barometer errors and high frequency ocean signals (e.g., Tierney et al., 2000) were also derived from an analysis of TP and ERS data.
The maps are provided on a MERCATOR 1/3° grid. Resolutions in kilometers in latitude and longitude are thus identical and vary with the cosine of latitude (e.g. from 37 km at the equator to18.5 km at 60°N/S). Units are centimeters. The files are in a NetCDf format and filenames are as follows : hh_msla_oer_tp (or merged) _h_date.nc (e.g. T/P map for January 23, 2002 = hh_msla_oer_tp_h_19015.nc). Mapping errors (in percentage of signal variance) are provided on separate files.
Formats as well as reading softwares are described in the SSALTO/DUACS (CLS/CNES near real time altimeter processing system) handbook.available at :
http://www.jason.oceanobs.com/documents/donnees/duacs/handbook_duacs_uk.pdf (a copy has been placed in this local file)
Reading softwares for the NetCDF maps (as well as a sample program
and a sample NetCDF file with ASCII dump) are available on the AVISO ftp site
ftp://ftp.cls.fr/pub/oceano/AVISO/DUACS/SOFTWARE/PublicReadGridDelivery.tar.gz (a copy of the contents of this archive has been placed in this local directory)
When using these data, please cite "The altimeter products were produced by the CLS Space Oceanography Division as part of the Environment and Climate EU ENACT project (EVK2-CT2001-00117) and with support from CNES”.
Contact : Pierre-Yves Le Traon
CLS Space Oceanograhy Division
e-mail : firstname.lastname@example.org
š AVISO/Altimetry 1996, "AVISO User Handbook for Merged TOPEX/POSEIDON products", AVI-NT-02-101, Edition 3.0.
š CERSAT, 1996: Altimeter and Microwave Radiometer ERS Products User manual, C2-MUT-A-01-IF.
š CLS, 1996: Validation of ERS-1/2 OPR cycles, CLS.OC/NT/96.010 (ERS-1) and CLS.OC/NT/96.011 (ERS-2) (one report per cycle).
š Ducet, N., P.-Y. Le Traon, and G. Reverdin, 2000: Global high resolution mapping of ocean circulation from TOPEX/Poseidon and ERS-1 and -2. J. Geophys. Res., 105, 19477-19498.
š Le Traon, P.-Y. and F. Ogor, 1998: ERS-1/2 orbit improvement using TOPEX/POSEIDON: the 2 cm challenge. J. Geophys. Res., 103, 8045-8057.
š Le Traon, P.-Y., F. Nadal, and N. Ducet, 1998: An improved mapping method of multisatellite altimeter data. J. Atmos. Oceanic Technol., 15, 522-534.
š Le Traon, P.-Y., G. Dibarboure, and N. Ducet, 2001: Use of a High-Resolution Model to Analyze the Mapping Capabilities of Multiple-Altimeter Missions. J. Atmos. Oceanic Technol., 18, 1277-1288.
š Ray, R., 1999: A Global Ocean Tide model from TOPEX/Poseidon Altimetry, GOT99.2. NASA Tech. Memo. NASA/TM-1999-209478, 58 pp. Goddard Space Flight Center, NASA Greenbelt, MD, USA.
š Scharroo, R. and P. Visser, 1998: Precise orbit determination and gravity field improvement for the ERS satellites. J. Geophys. Res., 103, 8113-8127.
š Tierney, C., J. Wahr, F. O. Bryan, and V. Zlotnicki, 2000: Short-period oceanic circulation: implications for satellite altimetry. Geophys. Res. Lett., 27, 1255-1258.