Quantification of Baltic sea water budget components using dynamic topography

Abstract. Accurate quantification of the Baltic Sea's dynamics and water budget components is essential for understanding both seasonal and long-term variations influenced by climate change. In this study, we utilize dynamic topography (DT), referenced to a geoid-based chart datum, to derive dynamic water volume and to improve estimates of the main water balance components, such as river runoff and water exchange through the Danish Straits. We utilize DT for the period from 2017 to mid-2021, which was corrected for vertical sea level biases and whose vertical datum thus coincides with the geoid. Our findings reveal seasonal dynamic volume variations, with a minimum in spring (78.9 ± 60 km3) and a maximum in autumn and winter (121 ± 57 and 124 ± 80 km3, respectively). Anomalies in DT highlight a specific region (northern Baltic Proper) as representing equilibrium mean DT for the entire Baltic Sea, while areas in the eastern and southern Baltic are prone to extremes. Barotropic exchange analysis shows that no major Baltic inflows occurred during the study period, with small to medium inflows averaging 1.6 km3 d−1 in autumn and winter, while outflows averaged 2.36 km3 d−1. River discharge, indirectly calculated from the water budget, peaked in summer (2.08 km3 d−1) and was lowest in autumn (1.26 km3 d−1), with hydrological models underestimating flows in these seasons. As a result, the method and results show great potential for quantification, validation, and a better understanding of the dynamics of the Baltic Sea, especially with a changing climate.