Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Retention or dispersion of larvae from the spawning ground has been identified as one of the key processes influencing recruitment success in fish stocks. An exercise combining 3-D hydrodynamic model simulations and field data on spatial distributions of juvenile Baltic cod was utilised to investigate the potential drift of larvae from the centre of main spawning effort in the Bornholm Basin, Baltic Sea. In the simulations cod larvae were represented as Lagrangian drifters. Habitats in which larvae and juvenile cod potentially dwell and where juveniles settle were identified to ascertain the importance of predicting transport. The transport of Baltic cod larvae was investigated by detailed drift model simulations for the years 1986 to 1999. The results yielded a clear dependency on wind-induced drift of larval cod, which is mainly controlled by the local atmospheric conditions over the Baltic Sea. Seasonally averaged distributions of drifters were compared with actual distributions of 0-group cod, as determined from bottom and pelagic trawl surveys conducted in autumn of the years 1993 to 2000 in and around the Bornholm Basin. The results suggest that juveniles caught in different areas can be assigned to different times of the spawning season. Because of seasonal differences in the circulation patterns, the southern coastal environment is on average most important for early and late spawners, whereas larvae hatching in mid-summer were on average transported towards the north or to a higher degree remained in the spawning ground.

Beschreibung: Review is given about the main results of the oceanographic component of the BALTEX research programme (one of the six continental scale experiments within GEWEX-WCRP to study water and energy cycles in the regional climate system) and related programmes/projects over the last 10 years. Working closely together with two other components – regional meteorology and hydrology of the Baltic Sea drainage basin – oceanographic research has considerably improved the understanding of and ability to model the Baltic Sea marine system. In the Baltic Sea physics seven different broad topics are identified where knowledge has significantly improved. These are reviewed together with a discussion of gaps in knowledge. The focus is on the water and energy cycles of the Baltic Sea, but various aspects of forcing and validation data and modelling are also discussed. The major advances achieved through BALTEX and related programmes are: • Meteorological, hydrological, ocean and ice data are now available for the research community. • Progress in understanding of the strong impact of large-scale atmospheric circulation on Baltic Sea circulation, water mass exchange, sea ice evolution, and changes in the ocean conditions of the Baltic Sea. • Progress in understanding of the importance of strait flows in the exchange of water into and within the Baltic Sea. • Progress in understanding of intra-basin processes. • Ocean models introduced into Baltic Sea water and energy studies. • Development of turbulence models and 3D ocean circulation models for application to the Baltic Sea. • Improved Baltic Sea ice modelling and increased understanding of the need for coupled atmosphere–ice–ocean-land models.

Beschreibung: Variations in oxygen conditions below the permanent halocline influence the ecosystem of the Baltic Sea through a number of mechanisms. In this study, we examine the effects of physical forcing on variations in the volume of deep oxygenated water suitable for reproductive success of central Baltic cod. Recent research has identified the importance of inflows of saline and oxygenated North Sea water into the Baltic Sea for the recruitment of Baltic cod. However, other processes have been suggested to modify this reproduction volume including variations in timing and volume of terrestrial runoff, variability of the solubility of oxygen due to variations in sea surface temperature as well as the influence of variations in wind stress. In order to examine the latter three mechanisms, we have performed simulations utilizing the Kiel Baltic Sea model for a period of a weak to moderate inflow of North Sea water into the Baltic, modifying wind stress, freshwater runoff and thermal inputs. The model is started from three-dimensional fields of temperature, salinity and oxygen obtained from a previous model run and forced by realistic atmospheric conditions. Results of this realistic reference run were compared to runs with modified meteorological forcing conditions and river runoff. From these simulations, it is apparent that processes other than major Baltic inflows have the potential to alter the reproduction volume of Baltic cod. Low near-surface air temperatures in the North Sea, the Skagerrak/Kattegat area and in the western Baltic influence the water mass properties (high oxygen solubility). Eastward oriented transports of these well-oxygenated highly saline water masses may have a significant positive impact on the Baltic cod reproduction volume in the Bornholm Basin. Finally, we analysed how large scale and local atmospheric forcing conditions are related to the identified major processes affecting the reproduction volume.