Water and heat exchange processes over the Baltic sea

Sammanfattning: Before we can reach understanding of the global climate and predict climate changes accurately we must be able to understand water and energy budgets. The atmosphere-water exchange processes are of great importance. In this study regional scale numerical models have been used in combination with several kinds of measurements to investigate the transport mechanisms of heat and water over the Baltic Sea.Precipitation is a difficult parameter to determine due to the large variability in time and space. Present estimates over the Baltic Sea using different methods differ with 10-20 % as averages over extended periods. More accurate measurements are needed to give more consistent estimates.A systematic overestimation of latent heat fluxes compared to measurementswas found in two models, one regional scale atmospheric model and one process-oriented ocean model. Errors in near surface and surface parameters in the models can partly explain this overestimation, but the parameterisation of turbulent fluxes in the models is incomplete.Swell is present about 40% of the time over the Baltic Sea and strongly influences the boundary layer. Swell results in decoupling between the surface and the atmosphere, with very low surface momentum flux as a result. The sensible heat flux is not as strongly influenced. Vertical gradients of wind and temperature also change during swell conditions. Stable stratification is also relatively common over the Baltic Sea and the present methods of calculating fluxes during stable stratification gives a clear overestimation.The ocean model is used for an 18-year simulation (1981 to 1998) to get long-term estimates of the net exchange of heat and water between the atmosphere and the ocean surface. It is shown that the long-term net heat exchange is small and directed from the atmosphere to the ocean. The net water transport is also directed from the atmosphere to the ocean and is in the order of 1.5 x 103 m3s-1 as a long time average.