Correcting tide gauge series due to land uplift and differences between national height systems of the baltic sea countries

Different types of tide gauges (TG) are used to monitor sea level dynamics around the Baltic Sea. They are usually connected to national levelling network and several of them are linked into regional networks (e.g. Baltic Operational Oceanographic System). The sea level readings are used for nautical navigation, modelling and forecasting of sea level changes. Long-term and historical sea level series are also useful in studying regional land uplift/subsidence or calibrating satellite altimetry data. Both tasks require precise knowledge of interconnections between height systems of countries surrounding the same sea. Presently, however, six different height reference systems are in official use in the Baltic Sea countries. Even though all these systems are based on mean sea level (MSL) observations averaged over different time-periods, but different reference TG and tidal systems have been adopted for national height systems. The differences of national height systems between the countries around the Baltic Sea can reach up to 20 cm. Overlooking this yields undesirable systematic biases between regional TG data. Additionally, the entire Fennoscandia is affected by apparent land uplift at the velocity rate up to +9 mm/year, primarily due to the viscoelastic response of the solid Earth resulting from the de-glaciation of the Pleistocene ice-sheets. Over a time span this causes notable distortions of height system realisations even within a country. Therefore, the land uplift corrections should be also taken into account in sea level series, which are used for modelling and forecasting of sea level changes. A case study in West-Estonian Archipelago involves a recently developed land uplift model EST2013LU, which is based on four repeated high-precision levelling data from 1933 to 2010. Also connections between levelling network and TG series are analysed. The results reveal that discrepancies due to obsoleteness of the heights in the national height system may cause di- crepancies in series of nearby located tide gauges up to 7 cm.