Energy and water balance of the Baltic Sea derived from merchant ship observations

Individual merchant ship observations from COADS (Comprehensive Ocean Atmosphere Data Set) were used to determine the energy and water budget of the Baltic Sea for the period 1980 to 1995. On a monthly time scale these ship reports provide reasonable estimates for the radiative and turbulent fluxes and the precipitation despite their concentration on narrow shipping routes, because of the large correlation lengths for monthly means. In order to take into account the effects of sea ice on evaporation and albedo, the ice-covered parts of the Baltic Sea are treated separately, where we applied a simple thermodynamic ice model using ice information from the GISST (Global sea Ice coverage and Sea Surface Temperature) data set. As the overall result we found a small surplus of rain compared to evaporation (5 mm per month) and a quasi-balanced energy budget (1 W m–2 energy loss of the sea surface).