Water exchange between the Baltic Sea and the North Sea and conditions in the deep basins

	Authors: Rainer Feistel and Günther Nausch, IOW

Key message

After the major Baltic inflow in January 2003 which had renewed most of the deep water in the Baltic Sea, no new major inflow has taken place. The near-bottom water in the Bornholm and eastern Gotland Basin returned back to anoxic conditions in the middle of 2004. However, as already in summer 2002, a warm water inflow in summer 2003 resulted in an intense deep water substitution process.

Background

Deep water renewal processes in the Baltic Sea depend on specific meteorological circumstances, which force substantial amounts of seawater, enriched with salt and oxygen, from the Kattegat through the Danish Straits into the Western Baltic. From there, it slowly moves as a thin bottom layer into the central Baltic basins, replacing aged water masses there. To make this happen, easterly winds have to blow continuously for about 10 days to lower the Baltic fill factor, followed by a sudden turn to westerly gale winds, which again need to last for about 10 days or longer in order to cause the fill factor rising to its maximum.

Before about 1980, such events were relatively frequent and could be observed on average once a year. In the last two decades, however, they became rather scarce; the last two major inflows took place in 1993 and 2003.

Results and assessment

The major Baltic inflow (MBI) in 2003 terminated a long-lasting stagnation period which lasted since 1995. It transported approximately 200 km³ cold and oxygenated water into the Baltic Sea, importing a total of 2 Gt (2 x 10¹² kg) salt.

The 2003 inflow takes rank 25 in the list of the 97 MBI’s since 1897.

The effect of the January inflow was enhanced by preceding smaller events in autumn 2002. Exceptional weather conditions in late summer 2002 caused the transport of extremely warm waters into layers at and below the permanent pycnocline. Although carrying only small amounts of oxygen, they were capable of completely ventilating the Gdansk Basin by November 2002. These warm water masses were reaching the eastern Gotland Basin in March 2003 well before the intensive ventilation of the cold January inflow which was recorded at the end of April.

In May 2003 an oxygen content of 3.96 ml/l could be measured in the near-bottom layer of the central station in that basin (Station 271 = BMP J1). Similar high concentrations were detected only twice before, during the 1930s and in May 1994 (Figure 1).

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Figure 1. Location of stations (black squares=MARNET-stations) and areas with oxygen deficiency and hydrogen sulphide in the near bottom layer of the Baltic Sea in May 2003, in a stage of progressing deep water ventilation. Histograms show the maximum oxygen and hydrogen sulphide concentrations of this layer. The figure contains additionally the 70 m resp. 20 m (small picture) isobaths.

Until May 2004, the arrival of the inflow could be detected successively also in the northern and western Gotland Basin, and most of the deep basins were free of hydrogen sulphide.

However, because no new MBI has taken place after January 2003, the oxygen situation started to deteriorate soon. The system returned back to anoxic conditions in the middle of 2004 in the deep waters of the Bornholm and eastern Gotland Basin as well (Figures 2 and 3). The most recent situation is given in Figure 4 indicating probably the beginning of a new stagnation period.

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Figure 2. Development of the oxygen situation at the central station in the Bornholm Basin (213 = BMP K1) between January 2003 and July 2004 indicating the MBI and the subsequent deterioration of oxygen conditions in the deep water

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Figure 3. Development of the oxygen situation at the central station in the eastern Gotland Basin (271 = BMP J1) between January 2003 and July 2004 indicating the MBI and the subsequent deterioration of oxygen conditions in the deep water

Click image to enlarge!

Figure 4. Location of stations (black squares = MARNET-stations) and areas with oxygen deficiency and hydrogen sulphide in the near bottom layer of the Baltic Sea in August 2004. Histograms show the maximum oxygen and hydrogen sulphide concentrations of this layer. The figure contains additionally the 70 m resp. 20 m (small picture) isobaths.

The cold MBI of January 2003 resulted in low bottom temperatures (3.6 °C in November 2003 in the eastern Gotland Basin). Quite surprisingly, temperature there began rising rapidly again and reached as much as 6.8 °C in March 2004. The only plausible explanation for this intense deep water substitution process is a near bottom inflow of warm and salty water across the Darss Sill in summer 2003 during a general outflow situation.

An intense warm water inflow was already observed in summer 2002, may be indicating a new phenomenon for the Baltic Sea. Expectations caused by observations in August 2004 that the process is repeated a third time in summer 2004 apparently did not come true, however.

References

Feistel, R., Nausch, G, Mohrholz, V., Lysiak-Pastuszak, E., Seifert, T., Matthäus,W., Krüger, S. and Sehested Hansen, I. 2003: Warm waters of summer 2002 in the deep Baltic.-  Oceanologia 45 (4), 571-592. 

Feistel, R., Nausch, G., Matthäus, W. and Hagen, E. 2003: Temporal and spatial evolution of Baltic deep water renewal in spring 2003. - Oceanologia 45 (4), 623-642.

Feistel, R., Nausch, G., Heene, T., Piechura, J. and Hagen, E.: Evidence for a warm water inflow to the Baltic Proper in summer 2003. – submitted to: Oceanologia, 2004.

Additional information

Cruise reports, oxygen deficiency maps: http://www.io-warnemuende.de/research/en_datbild.html

MARNET Darss Sill records: http://www.io-warnemuende.de/projects/monitoring/en_home.html

BSH MARNET: http://www.bsh.de/en/Marine%20data/Observations/MARNET%20monitoring%20network/index.jsp

Marine Science reports: http://www.io-warnemuende.de/research/mebe.html

BALTIC atlas: http://www.io-warnemuende.de/projects/baltic/index.html

 Acknowledgments 

The German part of Baltic Monitoring Programme (COMBINE) and stations of the German Marine Monitoring Network (MARNET) in the Baltic Sea (Darss Sill mast, Arkona Basin buoy) are conducted by IOW on behalf of the Bundesamt für Seeschifffahrt and Hydrographie (BSH), financed by the German Bundesministerium für Verkehr (BMV). The authors thank Jan Szaron, Oceanographic Laboratory of SMHI, Gothenburg, for providing us with hydrographic-hydrochemical observations from the Swedish Ocean Archive SHARK, obtained within the framework of the Swedish monitoring programme.

Summary

The inflow of January 2003 renewed most of the deep water in the Baltic Sea. However, oxygen situation is already deteriorating again.