Estimating Relative and Absolute Sea Level Rise and Vertical Land Movement Rates in the Adriatic Sea with in situ observations and the ESA SL_CCI altimetry dataset

Considerable efforts are made by space agencies and scientists to develop consistent and long-term satellitebased datasets: the European Space Agency (ESA) Climate Change Initiative (CCI) initiative Sea Level Project (SLCCI) is being producing climate-oriented altimeter sea level products optimized for the coastal zone, a monthly-mean gridded product covering the global ocean at 0.25x0.25 degrees (1993-2015). Based on this heritage, the operational production of climateoriented altimeter sea level products has been taken over by the European Copernicus Climate Change Service (C3S) with a daily-mean product gridded at 0.125x0.125 degrees covering the global ocean 1993-present. In parallel, refined products are expected in the second phase of the SLCCI project. We made a comparison of the SLCCI satellite altimetry dataset with sea level time series at selected tide gauges in the Mediterranean Sea, focusing on Venice and Trieste. There the coast is densely covered by civil settlements and industrial areas with a strongly rooted seaside tourism, and tides and storm-related surges reach higher levels than in most of the Mediterranean Sea, causing damages and casualties as in the recent storm of November 12th, 2019: the second higher water registered in Venice since 1872. Moreover, in the Venice area the ground displacements exhibit clear negative trends which deepen the effects of the absolute sea level rise. Several authors have pointed out the synergy between satellite altimetry and tide gauges to corroborate evidences of ground displacements. Here we exploit the long satellite-altimetry dataset duration and the high quality of sea-level time series at selected tide gauges of the Mediterranean Sea, to estimate the ground displacement rates. While in Venice, in the period 1993-2015, a relative sea level rise trend of about +6.17±1.51 mm y-1 has been determined from tide gauge at Acqua Alta Platform, 14 km offshore, in Trieste the tide gauge registered a trend of +4.10±1.38 mm y-1. Similarly, the altimetry product reports at the closest grid points absolute sea level rise rates of +4.02±1.27 mm y-1 (Venice) and +1.15±1.35 mm y-1 (Trieste). The estimated vertical land displacement rates, following the direct approach [Cazenave et al. 2009], resulted -1.79±0.72 mm y-1 (Venice), and -2.95±0.75 mm y-1 (Trieste). The estimated fitting slopes [Vignudelli et al., 2019] are the object of our investigation using the generalized least mean square procedure with constraints [Menke, 1989]. A partial validation of the resulting estimates has been made against Global Positioning System-derived (GPS) time series at selected stations. This work will contribute to identify problems and challenges to extend the sea level climate record to the coastal zone with quality comparable to open ocean, and also to assess the suitability of altimeter-derived absolute sea levels as a tool to estimate subsidence where permanent GPS receivers are not available. The Northern Adriatic is a laboratory to assess this tool, in particular considering the prospect of coming refined global products that are being generated within the ESA SLCCI extension (CCI+) project.