Fully reprocessed ERS-1 altimeter data from 1992 to 1995: Feasibility of the detection of long term sea level change

M. Anzenhofer, T. Gruber
1998 Journal of Geophysical Research  
Global mean sea level observations are necessary to answer the urgent questions about climate changes and their impact on socio-economy. At GeoForschungsZentrum/Geman Processing and Archiving Facility ERS altimeter data is used to systematically generate geophysical products such as sea surface topography, high-resolution geoid and short-and long-period sea surface height models. On the basis of this experience, fully reprocessed ERS-1 altimeter data is used to generated a time series of
more » ... sea surface height models from April 1992 to April 1995. The reprocessing consists of improved satellite ephemerides, merging of Grenoble tidal model, and application of range corrections due to timing errors. With the new data set the TOPEX/POSEIDON prelaunch accuracy requirements are fulfilled. The 3-year time series is taken to estimate the rate of change of global mean sea level. A careful treatment of seasonal effects is considered. A masking of continents, sea ice, and suspect sea surface heights is chosen that is common for all sea surface height models. The obtained rate of change is compared to external results from tide gauge records and TOPEX/POSEIDON data. The relation of sea level changes and sea surface temperature variations is examined by means of global monthly sea surface temperature maps. Both global wind speed and wave height maps are investigated and correlated with sea surface heights and sea surface temperatures in order to find other indicators of climate variations. The obtained rate of changes of the various global maps is compared to an atmospheric CO2 anomaly record, which is highly correlated to El Nifio events. The relatively short period of 3 years, however, does not allow definite conclusions with respect to possible long-term climate changes. the oceans [Church et al., 1991; Meier, 1984] . However, results presented further below will show that the sea level rise has regional tendencies; that is, there are areas, that are much more affected than others. Within such a future scenario, the socioeconomic consequences are virtually unpredictable [Broecker, 1996]. Before the advent of satellite altimetry the sea level could only be observed through tide gauges. The latter has three major disadvantages. (1) As tide gauges are located on the shores of continent, and islands exclusively, their global distribution is necessarily uneven (see Plate 8). (2) Tide gauge measurements reflect relative motions between the ground the instrument stands on and the sea surface, which, for example, could be shifted systematically due to postglacial rebound or tectonic uplift toward other stations. (3) A modification of the ocean circulation pattern may also influence the local sea level and hence tide gauge measurements. With altimeter data it was the first time that the sea surface could be monitored in a continuous and repeated manner in an unique reference datum. However, because of unknown altimeter drifts and systematics of Seasat, Geos-3, and Geosat [Allan, 1983; Bonavito et al., 1975; Cheney et al., 1991; Horai, 1982 ] and missing overlapping data with other altimeter missions, only data of the European satellites ERS-1 and ERS-2 and the U.S./French satellite TOPEX/POSEIDON can be used to measure the sea level change with a sufficient accuracy. This means that the sea level change can be investigated from 1991 onward. It should, however, not be neglected that recent efforts have demonstrated that the use of tide gauges to link Geosat to TOPEX/POSEIDON and ERS-1 has proven useful and sea level estimates have been obtained. An uncertainty with tide gauges, however, remains and the estimates are difficult to prove. The problems of linking tide gauges and altimeter data, as described above, should not degrade the usefulness and importance of tide gauge measurements. They play an important role in calibrations and help to link ocean and 8O89 8090 ANZENHOFER
doi:10.1029/97jc02566 fatcat:eyhdk23pe5gqta52j5txypkg4a