Robust trends in Baltic sea level from satellite altimetry observations

Regional sea-level change in the semi-enclosed Baltic Sea is strongly influenced by atmospheric forcing and wind-driven redistribution of water masses, leading to significant spatial variability in absolute sea level trends across the different sub-basins. This study focusses on absolute sea level trends in the Baltic Sea using satellite gridded sea level anomalies (0.0625º) from the European Seas Gridded L4 product provided by the E.U. Copernicus Marine Service (https://doi.org/10.48670/moi-00141). The daily time series (from January 1993 to the end of December 2023) are first deseasoned by removing the average annual cycle at each point. Then robust linear trends are estimated at each grid point by computing median slopes. In contrast to ordinary least-squares slopes characterising linear trends in the mean, these median slopes are calculated by minimising the mean absolute deviation of a linear trend model from the observations instead of the mean quadratic deviation, which makes them more robust to outliers and sensitive to the typical tendency of changes rather than to large deviations. Uncertainty is computed assuming non-independence by the Huber sandwich robust estimator for the covariance matrix.

The derived median slopes are in general higher than ordinary linear trends in the mean, except in the northern and easternmost areas of the Baltic. In the Bay of Bothnia ordinary linear trends and median trends are very similar, while in the eastern end of the Gulf of Finland median trends are similar or even slightly lower than ordinary linear trends. In the remaining areas, median trends are significantly larger than ordinary linear trends, the largest difference occurring in the Bothnian Sea. Coastal areas exhibit trends that differ from those in the adjacent basins. In the Gulf of Finland, median trends are higher than ordinary linear trends along the Finnish coast, whereas along the Roslagen coast (northern Stockholm Archipelago) the two slope estimates are in good agreement. Along the southern coastline of the Bothnian Sea, median sea-level trends reach the highest values, exceeding 6 mm/year.

The present study is financed within the scope of the Recovery and Resilience Mechanism (MRR) of the European Union (EU), framed in the Next Generation EU, for the period 2021 - 2026, within project NewSpacePortugal, with reference 11.