The hydrographic-hydrochemical state of the Baltic Sea in 2003

After five milder winters in succession, the winter 2002/2003 was an average cold one. It was the most severe after the winter 1995/1996 and the second severe after 1986/1987. Consequently, the maximum ice cover with 232 000 km² was 10 % higher then the long-term mean. As a result of the extreme cold December 2002 the cooling down of the water took place rapidly and the water temperature in the central and northern Baltic Sea was in January well below the average.

With the exception of October 2003, all months from March onwards were too warm. The summer months May to August were with more then 2 ^oC distinctly higher then normal. After 1997 and 2002, the summer of 2003 was the third warmest since 1890, at least in the southern Baltic Sea. In the eastern Gotland Sea and in the Gulf of Finland surface temperatures up to 25 ^oC were measured in July, in the Gulf of Bothnia temperature showed positive anomalies up to + 5 K.

The most important event for the Baltic Sea in 2003 was, however, the intensive, cold and oxygen-rich inflow of Kattegat water in January. With this major Baltic inflow approximately 200 km³ water were transported into the Baltic Sea, importing a total of about 2 Gt (2 x 1012 kg) salt. These amounts of water and salt were only half of that transported during the last major Baltic inflow in 1993. Thus, the 2003 inflow takes rank 25 in the list if strongest inflows since 1897. The effect of the January saltwater inflow was supported by smaller inflow events in March and May due to the fact that the Arkona and Bornholm Basins were already filled with dense water masses allowing a fast propagation of the inflowing water into the central basins. With this major Baltic inflow the long stagnation period was terminated which lasted since 1995.

The inflow had grasped the whole Bornholm Basin already in February displacing the warm water from the summer inflow 2002 there. These warm water masses were reaching the deep water in the eastern Gotland Basin in March well before the intensive ventilation of the cold January inflow which was recorded at the end of April. In May an oxygen content of 3.96 ml/l could be measured in the near-bottom layer at the central station in that basin. Similar high concentrations were detected only twice before, during the 1930s and in May 1994. During the second half of 2003, changes in temperature and salinity signalled the arrival of the inflow also in the western Gotland Basin despite the hydrochemical parameters reflected this only indirectly. The water renewing in the Bornholm and eastern Gotland Basin was accompanied by drastic changes in the nutrient regime in the deep water. Phosphate and ammonium concentrations decreased rapidly whereas nitrate was enriched as a result of nitrification processes.

Similar to 2002, the calm and extremely warm weather between June and August caused a near bottom inflow of warm and salty water from the Kattegat across the Darss Sill during a general outflow situation. The duration in 2003 was shorter compared to 2002. However, the effects can be tracked again up to the eastern Gotland Basin.

Dr. Günther Nausch