Coastal sea level trends from TOPEX-Poseidon satellite altimetry and tide gauge data in the Mediterranean Sea during the 1990s

A joint analysis of TOPEX-Poseidon satellite altimetry and tide gauge data is used to estimate sea level trends along the Mediterranean Sea coast from 1993 January to 2002 February. A generalized least squares (GLS) method allows us to take into account the space-time distribution of the data and their respective error budgets. Nearly optimal parameters of the covariance functions involved in the GLS method are estimated at different scales of analysis, using a trial and error method. Combined data time series are then reconstructed at 153 locations along the coast. The basin-wide averaged linear trend for the coastal area shows a sea level rise of 4.54 ± 0.30 mm yr
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compared to 4.28 ± 0.30 mm yr
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for the open ocean only, i.e. a 0.26 ± 0.16 mm yr
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difference (the error is smaller because the systematic error does not take part in the error budget) which is quite significant. At local scales, the coastal linear trends ranges from −8.5 to +13.5 mm yr
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which is much narrower than ranges found by other open sea studies ([−24.0 to −10.0; +16.0 to +29.0] mm yr
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). However, a detailed error budget shows that the largest estimation errors come from the spatial extrapolation of the sea level signal required when tide gauge and altimeter data are merged. The second error source is the instrumental noise and the random noise generated by the littoral contamination on altimeter measurements and from the crustal motions that are contaminating the tide gauge signal. At local scale, it is estimated that 85 percent of the error on linear trends does not exceed 2 mm yr
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.