Long-term variations in sea ice extent can influence trends in maximum sea level in the northeastern Baltic Sea

Abstract

Compared to the relative mean sea level changes at Estonian tide gauges, the maximum sea levels have increased faster, by 10–40 cm over the past hundred years. It has been argued that the west-facing coast is sensitive to long-term changes in the westerly wind component and storminess. In addition to those, we test the hypothesis that the decrease in Baltic Sea ice cover can also be a reason for higher surges. To study trends, we use long-term sea level and meteorological data from Estonian coastal stations and ice records in the Gulf of Riga. The ERA5 forced WRF-FVCOM modelling suite is used to quantify the effect of the lack of ice cover on sea level patterns in the Gulf of Riga and on the maxima at Pärnu during three storms. According to the linear trend, air temperatures on the Estonian coast have increased by about 2.5 °C between 1950 and 2023, and the number of ice days has decreased by 62 % at Kihnu and by 42 % at Pärnu. The sea level trend-changing mechanism can apply in two ways. Firstly, some mid-winter extremely high storm surges have occurred in ice free conditions, especially in more recent decades. And secondly, when some strong winter storms did not cause remarkably high sea level events in the ice-covered Gulf of Riga, but the FVCOM indicated the potential for storm surges of at least 60 cm higher than took place. Such events could have had a notable impact in the first half of the 20th century, when seasonal ice cover on the Baltic Sea was more extensive than it is nowadays. The combination of a decreasing ice trend and an increasing probability of undamped storm surges can contribute to a steepening of sea level maxima and higher quantile trends in centennial time series.