Combining GEOSAT and TOPEX/Poseidon Data by Means of Data Assimilation

In order to assess the sea level rise for periods longer than the continuous observations by TOPEX/Poseidon two assimilation experiments were performed. Both use the GEOSAT mission data additionally and bridge the gap between these observations in accordance with standard ocean observations. The first experiment (GETO) showed that without continuous records of the sea level no reliable estimates can be made. Therefore additional observations are used in the second experiment (GETORC), that are provided by sea level time series at tide gauges. From these data global maps of sea level anomalies were constructed using an EOF technique. GETORC then provides the answer to the total sea level rise and to its temporally and spatially varying contributions. Ocean warming accounts for one third of global sea level rise by steric expansion. Half of it is confined to the upper 700 m of the ocean. The other half of oceanic thermosteric expansion is found in the deep ocean below. In addition to warming of the ocean we find an inflow of fresh water with amplitude close to 2 mm/year.