# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 3 Study Area  
The Baltic Sea is a semi-enclosed microtidal sea with geographical limits of 53\({}^{\circ}\)N-66\({}^{\circ}\)N and 10\({}^{\circ}\)E-30\({}^{\circ}\)E and roughly a 54 m mean depth, with a 393,000 km\({}^{2}\) total surface area. The sea is linked to the open ocean solely through narrow Danish Straits [61] (cf. Section 4.2). The sea-level dynamics of the Baltic Sea are affected by several meteorological and ocean-graphical components, including the variation in temperature, salinity, precipitation, and evaporation (in a decadal time frame), as well as extreme sea levels (in a short-time frame) as a result of storm surges due to the interaction of strong winds, waves, and pressure [62]. It features a dual stratification, marked by a seasonal thermocline in the summer and a persistent, robust halocline throughout the year. The halocline, at approximately 60-80 m, divides the brackish surface water with a salinity of about 7 psu from the deeper water, with 12 psu salinity. The surface salinities vary from 32 psu in the Kattegat to 1-2 psu in the northern parts [63]. Sequential (year to year) annual average sea-level variations in this region could exceed \(\pm 8\) cm, whereas the maximum difference within the entire study period (1995-2022) could reach 25 cm, as presented in Figure 11. This large annual mean variation in sea level is challenging for short-term (decadal) sea-level trend estimations. The ASL of the Baltic Sea is also influenced by (i) expansion in the world ocean sea level (due to thermal seawater expansion and the melting of glaciers), which also propagates into the semi-enclosed Baltic Sea; (ii) low salinity, which causes an additional steric component, elevating the ASL specifically in the innermost parts of the sea, where the lowest densities occur; and (iii) local atmospheric forces, including wind and air pressure [33].  
Over the past 50 years, previous studies have noted that the Baltic ASL increase is larger than the GMSL [63,64]. Also, the ongoing viscoelastic response of the Earth to the last deglaciation contributes to the sea-level variation in the Baltic Sea together with other effects such as North Atlantic and GMSL changes, wind, and waves affecting erosion and sediment transport [64]. Also, the spatial variations in the Baltic Sea level rates are mainly due to the (i) occasions of massive water entrance from the North Sea, (ii) intensified westerly winds, (iii) the poleward shift of low-pressure systems, and (iv) other minor contributions derived from local changes in baroclinicity, causing a basin-internal redistribution of water [64].  
The changes in sea-level extremes depend on many variables, including storm surges [65], coastal upwellings [66], and wave-breaking processes [67], which can cause a devastating impact on small islands and their inhabitants. Hence, the cumulative effect of sea-level rises could lead to serious consequences over the Baltic Sea regions, where numerous archipelagos and small islands exist. Recent studies exploring future climate scenarios show an anticipated rise (around 6 mm/yr) in Baltic Sea levels (e.g., [29,30]). In contrast to the current study, many of these research works utilize gridded SA data or SLA for determining sea-level trends.