Trends in the amplitude of Baltic Sea level annual cycle

Baltic Sea tide gauge data and climatic data sets are statistically analysed to investigate the centennial trends in the amplitude of the annual cycle of Baltic sea level. In almost all gauge stations analysed, an increase of the amplitude (winter-spring sea level) is detected. These trends are not large compared to the decadal variations of the annual cycle, but they are statistically significant. The magnitude of the trends is almost spatially uniform, with exception of the Skagerrak area. Since interannual and decadal variability of sea level displays a clear spatial pattern, the mechanism responsible for the trends in the annual cycle seem to be not regional, but affect the Baltic Sea basin as a whole. Several hypotheses are proposed to explain these centennial trends on the winter-minus-spring sea level: wind (through the SLP field), the barometric effect, temperature and precipitation. By elimination of three of the working hypothesis, seasonal Baltic precipitation remains a plausible candidate. For the other three, either the sign or magnitude of the trend makes them unlikely the sole explanation.