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Due to the ephemeral nature of the atmospheric conditions over the Baltic Sea, the flow field is highly variable, and thus, changes in the resulting circulation and upwelling are difficult to observe. However, three‐dimensional models, forced by realistic atmospheric conditions and river runoff, have reached such a state of accuracy that the highly fluctuating current field and the associated evolution of the temperature and salinity field can be described. In this work, effects of remote and local atmospheric forcing on circulation and upwelling in the Baltic Sea are investigated. Changes in the characteristics of the large‐scale atmospheric wind field over the central and eastern North Atlantic can be described by the North Atlantic Oscillation (NAO). The NAO is related to the strength and geographical position of weather systems as they cross the North Atlantic and thus has a direct impact on the climate in Europe. To relate the local wind field over the Baltic Sea to the large‐scale atmospheric circulation, we defined a Baltic Sea Index (BSI), which is the difference of normalised sea level pressures between Oslo in Norway and Szczecin in Poland. The NAO is significantly related to the BSI. Furthermore, the BSI is highly correlated with the storage variation of the Baltic Sea and the volume exchange through the Danish Sounds. Based on three‐dimensional model calculations, it is shown that different phases of the NAO during winter result in major changes of horizontal transports in the deep basins of the Baltic Sea and in upwelling along the coasts as well as in the interior of the basins. During NAO+phases, strong Ekman currents are produced with increased up‐ and downwelling along the coasts and associated coastal jets, whereas during NAO−phases, Ekman drift and upwelling are strongly reduced, and the flow field can almost entirely be described by the barotropic stream function. The general nature of the mean circulation in the deep basins of the Baltic Sea, obtained from a 10‐yr model run, can be described by the depth integrated vorticity balance derived from the transport equation for variable depth.

Due to the ephemeral nature of the atmospheric conditions over the Baltic Sea, the flow fieldis highly variable, and thus, changes in the resulting circulation and upwelling are difficult toobserve. However, three-dimensional models, forced by realistic atmospheric conditions andriver runoff, have reached such a state of accuracy that the highly fluctuating current field andthe associated evolution of the temperature and salinity field can be described. In this work, effects of remote and local atmospheric forcing on circulation and upwelling in the Baltic Seaare investigated. Changes in the characteristics of the large-scale atmospheric wind field overthe central and eastern North Atlantic can be described by the North Atlantic Oscillation(NAO). The NAO is related to the strength and geographical position of weather systems asthey cross the North Atlantic and thus has a direct impact on the climate in Europe. To relatethe local wind field over the Baltic Sea to the large-scale atmospheric circulation, we defined aBaltic Sea Index (BSI), which is the difference of normalised sea level pressures between Osloin Norway and Szczecin in Poland. The NAO is significantly related to the BSI. Furthermore, the BSI is highly correlated with the storage variation of the Baltic Sea and the volume exchangethrough the Danish Sounds. Based on three-dimensional model calculations, it is shown thatdifferent phases of the NAO during winter result in major changes of horizontal transports inthe deep basins of the Baltic Sea and in upwelling along the coasts as well as in the interior ofthe basins. During NAO+phases, strong Ekman currents are produced with increased up-anddownwelling along the coasts and associated coastal jets, whereas during NAO-phases, Ekmandrift and upwelling are strongly reduced, and the flow field can almost entirely be described bythe barotropic stream function. The general nature of the mean circulation in the deep basinsof the Baltic Sea, obtained from a 10-yr model run, can be described by the depth integratedvorticity balance derived from the transport equation for variable depth.

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.

The wind driven and thermohaline circulation of the Baltic Sea and the exchange with the North Sea are investigated by using a coupled sea ice-ocean model of the Baltic Sea. Four consecutive years, namely 1992, 1993, 1994 and 1995 including the latest major Baltic inflow in January 1993 have been simulated. Atmospheric forcing and river runoff was prescribed from SMHI data bases. Data assimilation was utilized by merging hydrographic field measurements with model data. From 6 hourly analysis data 4-year averages of the 3-dimensional current and mass field have been calculated. Some aspects regarding the mean circulation, its stability and the thermohaline circulation especially where the turnover is happening are discussed from model results analysis.

A combined 3-D physical oceanographic model and a field sampling program was performed in July and August 1994 to investigate the potential drift of larval Baltic cod from the center of spawning effort in the Bornholm Basin, Baltic Sea. The goal of this exercise was to predict the drift trajectories of cod larvae in the Bornholm Basin, thereby aiding in the development of future sampling programs as well as the identification of processes influencing larval retention/dispersion in the Bornholm Basin. Distributions of variables (T, S and larval distribution) were obtained utilizing a three-dimensional eddy-resolving baroclinic model of the Baltic Sea based on the Bryan-Cox-Semtner code. Larval drift was simulated by the incorporation of a passive tracer into the model utilized to represent individual cod larvae. Additionally, simulated Lagrangian drift trajectories are presented. For model purposes, initial fields of temperature, salinity and cod larvae concentration for the Bornholm Basin were constructed by objective analysis using observations taken during a research survey in early July, 1994. Outside the Bornholm Basin generalized hydrographic features of the Baltic Sea were utilized with the baroclinic model forced by wind data for the whole Baltic taken from the Europa-Modell (EM) of the German weather service, Offenbach. Verification of simulations was performed by comparison with field measurements of hydrographic variables and ADCP derived current measurements taken during the surveys. In general, most of the hydrographic features observed during the second research cruise are correctly simulated, with variations mainly attributed to the prescribed initial conditions outside the Bornholm Basin. Results from larval sampling during the second cruise could not entirely confirm the modeled larval distributions due to the low numbers of larvae captured. However, the modeled results based on the agreement of the flow fields and hydrographic properties with observed features suggest that predictions of larval distributions can be made with a high degree of confidence if appropriate larval behaviours are included in the simulations.