## 2 Materials and Methods
### Oceanographic Factors
Changes in water levels in the Baltic Sea on a scale of months to years are caused by, inter alia, the thermal expansion of water and the effects of atmospheric circulation, barometric pressure, and sea currents. Local changes in water levels may be human-madeand result from, e.g., the training of rivers, hydraulic engineering work, or the exploitation of natural resources. Nevertheless, on a scale of decades, water levels in the Baltic Sea have been continuously rising--a trend directly related to climate changes. Melting glaciers and continental ice sheets of Greenland and Antarctica feed huge masses of water to oceans, contributing to a global increase in sea levels. Isostatic changes, i.e., local rises or falls in land levels, are another factor affecting the sea levels. In the north of the Baltic Sea (e.g., in Sweden and Finland), the processes lead to a relative fall of the sea level, whereas on the southern coast of the Baltic (Poland), they result in its rise [96]. Ocean tides occur twice a day. In the Baltic Sea, their amplitudes are relatively small, and their maximum range is estimated at 24 cm (Figure 4). Daily tides have the greatest effect, especially in the eastern Baltic (the Gulf of Finland and the Bothinian Bay) [97]. Their total impact on the development of the LNG infrastructure is negligible [98]. The latest research emphasizes significant fluctuation of tidal patterns, especially in the Eastern Gotland Basin, where, previously, tidal currents had a steady drift and set towards the north and long-term cycles of 100-150 days [99]. An analysis of data from the last five decades shows new tidal patterns in the Baltic Sea. It has been found that drifts of surface and bottom currents are on the rise, presumably related to the positive phase of the North-Atlantic Oscillation (NAO). It has also been noted that sub-surface tidal currents (at a depth of 20, 40, and 60 m) are more stable (0.4-0.7 m/s) compared to surface currents (0.2-0.5 m/s). On average, the drift of sub-surface currents at a depth of 20 m is 0.1 m/s slower than that of surface currents. This shows that surface currents are more ephemeral and more dependent on the prevailing weather conditions, whereas sub-surface currents do not change considerably [100]. A factor characterized by considerable daily fluctuations that strongly affect the overall hydrological and meteorological conditions is wind waves. These are waves generated by wind acting on the water's surface, where energy from movement in the atmosphere is transferred to the surface of the sea [101; 102; 103; 104; 105]. The process takes place in the function of time and space, and its intensity as well as the height of waves depend on a number of factors. Therefore, for each wind speed, there is a boundary wind duration and boundary run-up length, and, once they are reached, the height of waves has attained its maximum, and waves stop increasing in height [106; 107]. The height of wave in oceanographic physics is defined as the mean height of 1/3 of the highest waves occurring in a group of waves in a certain location, observed in time [108; 109]. Mean extreme heights of the significant wave in the southern part of the Baltic Sea do not  
Figure 4: Yearly mean sea level [66]. exceed 5 m, whereas its maximum height observed in the period under analysis is 7.3 m (January 2015). The highest mean values of wave height are observed in winter and are correlated with higher wind speeds measured during the occurrence of a maximum wave (Table 6). Wave size depends mainly on the wind speed that generates the waves--the stronger the wind, the higher the waves may become--as well as the duration of wind above water from a constant (\(\pm 15^{\circ}\)) direction--the longer the wind of a certain speed blows above water, the higher the waves generated (Table 7) [110]. The direction of wind-generated wave propagation is closely related to the wind's direction, i.e., a wind-generated wave moves in the direction from which the wind blows.