Satellite altimetry and in-situ observations: estimating relative and absolute sea level rise in th Northern Adriatic Sea (Venice and Trieste)

Climate-related disasters are highly demanding in terms of life land economic losses. Among them, storms are the costliest type of disaster: their impact on coastal zones strongly depend on the relative sea level, and on its changes, that thus deserve special attention particularly where people, industrial settlements and cultural heritage coexist. In the Mediterranean basin the most exposed area to this risk is the Adriatic Sea, and in particular Venice, whose high vulnerability has determined the need to realize one of the most impressive hydraulic engineering works: the MOSE barriers. In this context, the long term implications of rising sea levels have not been exhaustively investigated yet, and the contributions from the various mechanisms impacting on sea level have not been clearly quantified. The European Space Agency (ESA) Climate Change Initiative (CCI) project on "Sea Level" has produced an improved set of satellite-based sea level gridded products by reprocessing altimeter data over 1993-2015. Recently, CTOH within the project has released a new version of the dataset which includes along track high resolution (i.e., around 300 m spaced) Jason-1 and Jason-2 data. Our contribution aims at assessing the quality of the current Sea Level CCI products in the Adriatic Sea - in particular around the cities of Venice and Trieste, how close to the coast the actual Sea Level CCI products can be considered reliable, and what are the main differences between gridded and along track data. Near Trieste we also can rely on Jason-1 and Jason-2 track data reprocessed at 20 Hz. For a better understanding of the role played by subsidence on the local sea level rise, the latest CCI gridded products and the new along-track high resolution observations at the nearest suitable measurement points are compared with tide gauge data, thanks to the very short distance between the tide gauges and the coast in those two cities (4 km in Venice and 0 km in Trieste). From in-situ measurements a relative sea level rise trend of about +6.3 mm/yr has been determined in Venice, while for Trieste we got +4.7 mm/yr. The difference between in-situ data and altimetry observations supplies an estimate of the subsidence contribution in both cities [Vignudelli et al., 2019]. A partial validation of these estimates has been made against GPS-derived values, in order to distinguish the contributions of subsidence and eustatism, and in view of the possible closure of the sea level budget in those two cities. Subsidence data, when available, are derived from local permanent GPS stations. We investigate also the role of the varying surface pressure in the relative sea level rise local budget: pressure data are taken from affordable time series of meteorological data at Venice and Trieste, as well as from the ECMWF ERA INTERIM reanalysis project. This work will contribute to identify problems and challenges to extend the sea level climate record to the coastal zone with quality comparable to the open ocean, and also to assess the suitability of altimeter-derived absolute sea levels as a tool to estimate subsidence from tide gauge measurement in places where permanent GPS receivers are not available.