On the thermohaline variability of the Baltic Sea

Abstract

A coupled ice–ocean model is utilized to investigate the transports of heat, salt and water in the Baltic Sea for the years 1986, 1988, 1993 and 1994. The oceanic component of the coupled system is a three-dimensional baroclinic model of the Baltic Sea including the Belt Sea and the Skagerrak/Kattegat area. The model has a horizontal resolution of ∼5 km and 28 vertical levels specified. The ice model is based on the Hamburg Sea Ice model, with the same horizontal resolution. The coupled system is driven by atmospheric data provided by the Swedish Meteorological and Hydrological Institute (SMHI; Norrköping, Sweden) and river runoff taken from a monthly mean runoff database. The thermohaline variability of the Baltic Sea strongly depends on the fluctuations of the atmospheric forcing conditions. Therefore, high demands on the spatial and temporal resolution of the meteorological forcing are required. Besides heat and radiation fluxes, precipitation and evaporation rates have to be taken into account. From the coupled runs, the different components determining the energy and water cycle of the Baltic Sea are identified and estimates of the water, heat and salt transports are given for the different years. Furthermore, the thermohaline variability is investigated with respect to the relevant forcing mechanisms including atmospheric, as well as fresh water fluxes. Besides the heat and water fluxes of the Baltic Sea and the water mass exchange with the North Sea, internal fluxes of heat, salt and volume between the different subbasins of the Baltic Sea are presented. Sensitivity studies on the variation of the net fresh water flux indicate that uncertainties in precipitation and/or river runoff can have a strong impact on the inflow of highly saline water from the North Sea, thus, influencing the thermohaline circulation of the Baltic Sea.