Seasonal fluctuations in Baltic sea level determined from satellite altimetry

International audienceThis study used gridded daily satellite altimetry observations to investigate the spatial variability in the Baltic seasonal sea-level cycle and to assess the main factors that affected its interannual changes during the 1993-2018 period. In addition to the widely studied annual (Sa) and semiannual (Ssa) harmonics, the characteristics of higher frequency oscillations (terannual (three times a year or Sta) and quarterannual (four times a year or Sqa)) were investigated. We demonstrated that the Sa, Ssa, and Sqa harmonics can be reliably identified from gridded daily satellite altimetry data for all regions of the Baltic Sea. The Sta harmonic was reliably identified in all regions except the southeast part of the Baltic Proper and Danish Straits. Low amplitudes and large uncertainties caused by noise in the SLA time series impeded the Sta evaluation in these two regions. In the Baltic Sea, annual oscillations of sea level propagate from the southwest to northeast at a speed of 0.15-0.36 m/s, which is close to the theoretical phase speed of the baroclinic Kelvin wave. The Ssa, Sta, and Sqa oscillations did not demonstrate a statistically reliable spatial variability in the phase. In many regions of the Baltic Sea for the 1993-2018 period, a statistically significant (at the 95% level) increase of +1.3- + 2.2 mm/yr was observed for the amplitudes of the Sa harmonic, while for the amplitudes of the Ssa harmonic, a statistically significant decrease was detected. No statistically significant changes were observed in the amplitudes of the Sta and Sqa harmonics. Wind, atmospheric pressure, and water exchange with the North Sea correlated well with interannual changes in the sea-level seasonal cycle. The air temperature correlated well only with the Sa component. Precipitation, evaporation, and river input were not correlated with seasonal or interannual changes in the sea-level cycle