The importance of water storage variations for water balance studies of the Baltic Sea

Abstract

The closing of the water and energy cycle of the Baltic Sea is one of the main aims of BALTEX (Baltic Sea Experiment), which particularily focuses on the exploration, modelling and quantification of the various processes determining the space and time variability of the energy and water budget. On the long-term mean the water budget of the Baltic Sea is determined by river runoff, net precipitation (precipitation minus evaporation) and the in- and outflows through the Baltic Sea entrance area, assuming that the mean sea level remains constant, i.e. the ability of the Baltic Sea to store a huge amount of water is averaged out over the long-term period. For shorter periods, the water storage which can be expressed by the mean sea level plays an important role on the water budget. The objective of the present study is to investigate the variability of the water storage of the Baltic Sea and relate its fluctuations to the different components of the water balance equation. The anomaly of the mean sea level of the Baltic Sea shows a well pronounced seasonal cycle, with negative values between the end of February to the end of June (minimum in the middle of May), and positive anomalies from July to mid-February (maximum in January). There is a high correlation between the mean sea level expressed by the Landsort tide gauge and the local atmospheric conditions over the Baltic Sea. The annual course of the total water balance is controlled by the local atmospheric conditions with the net fresh-water inflow only controls the general outflow conditions. Sea level, precipitation and river runoff have been obtained from observations provided by the SMHI. For the in- and outflow through the entrance area of the Baltic Sea and evaporation over the open ocean, coupled sea ice-ocean model simulations for a 10-years period have been utilized.