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

	Authors: Rainer Feistel, Günther Nausch and Eberhard Hagen, IOW

Key message 

The inflow activity of recent years from the Kattegat into the Baltic Sea was coined by a quite unusual sequence of events: a warm summer inflow 2002 was followed by a cold gale-forced one in January 2003, and again by a warm summer inflow 2003, all together they terminated the stagnation period in the Baltic deep water which lasted since 1995. The period afterwards was characterized by only low inflow activities. Thus a new stagnation period has started in the deep basins.

Results and assessment

• Deep water renewal processes can be divided into two types. The “classical” barotropic Major Baltic Inflows (MBIs) and the “new” baroclinic inflows. MBIs occurring in winter and spring are causing higher salinities, low temperatures and increased oxygen levels in the deep basins, while those of either type in summer and autumn increase salinity along their pathway with high temperatures, but carry only low amounts of oxygen.

• Before about 1980, MBIs 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.
• The very contrasting thermal signatures of both inflow types provided natural ‘tracers’ and allowed a clear insight into the dynamics of deep water propagation through the main basins of the western and central Baltic (Fig. 1). In the largest Baltic basin, the Eastern Gotland Basin (EGB), the barotropic inflows in autumn 1997 and October 2001 increased the temperature at about 200 m depth to more than 6.5°C but did not improve the oxygen conditions significantly. In addition, the exceptional baroclinic inflow in summer/autumn 2002 brought very warm water into this basin. Thereafter, it was immediately replaced by a very cold and dense MBI in January 2003, enhanced by some smaller events in spring. Temperature dropped down to around 4.5°C. But, already in December 2003 temperatures started to rise steeply again to values at and above 6°C, caused by a short but very intense baroclinic inflow in summer 2003.

Fig.1. Temperature series 1997-2005 of the EGB mooring near the Gotland Deep (Fig. 2) at 174, 204 and 219 m depth. Bathymetric depth at the anchor position is H=224 m. The temperature signals caused by the latest warm (red) and cold (blue) inflow events are indicated by arrows.

• Changes of the near-bottom distributions of dissolved oxygen resp. hydrogen sulphide reflect these inflow processes and are displayed for the years 2002 – 2005 in Fig. 2. In 2002, at the end of the long stagnation period since 1995, extended regions of oxygen deficiency or even substantial concentrations of hydrogen sulphide were encountered in the central Baltic below 70 m. In spring 2003, the MBI of January had ventilated the Bornholm, Gdansk and Eastern Gotland Basin with considerable amounts of oxygen. In the latter basin an oxygen content of 3.96 ml/l could be measured in the near-bottom layer. Comparable high concentrations were detected only twice before, during the 1930s and in May 1994. The anoxic situation at the Farö Deep and beyond was still unchanged or even worse at this time. The baroclinic inflow of summer 2003 pushed the oxygenated deep waters further north, ventilating the Farö Deep, but at the same time displacing the old, stagnant waters into the basins west of Gotland. It took until spring 2005, two years after the actual MBI event, that these oxygenated waters reached as far as the Karlsö Deep southwest of Gotland. At this time, the Bornholm and Eastern Gotland Basin are already returning into stagnation with increasing hydrogen sulphide concentrations (Fig. 2). Some regions, esp. around the Landsort Deep have not been ventilated completely at all through the water substitution process.

Click image to enlarge!

Fig. 2. Areas with oxygen deficiency and hydrogen sulphide in the near bottom layer of the Baltic Sea in May in the years 2002 - 2005. Histograms show the maximum oxygen and hydrogen sulphide concentrations of this layer. The figure contains additionally the 70 m isobath. The top-left corner magnifies the western Baltic Sea with the 20 m isobath.

• The salinity development in the deep water reflects the inflow processes (Fig. 3). Steep increases after inflows are followed by slow decreases in the stagnation periods afterwards. It was especially the short inflow of August 2003 which pushed the salinity from 12.5 to 12.96 psu on 18 July 2004, a 30-years high after 13.05 psu found there on 16 May 1977.

Fig. 3. Salinity at 200 m at the Gotland Deep station 271 between 1968 and 2005.

References

Feistel R., Nausch G., Hagen E., 2003a, The Baltic Inflow of Autumn 2001, Meereswiss. Ber. Warnemünde, 54, 55-68.                http://www.io-warnemuende.de/documents/mebe54_inflow01.pdf 

Feistel R., Nausch G., Matthäus W., Hagen E., 2003b, Temporal and Spatial Evolution of the Baltic Deep Water Renewal in Spring 2003, Oceanologia 45, 623-642. http://www.iopan.gda.pl/oceanologia/454feis2.pdf 

Feistel R., Nausch G., Mohrholz V., Łysiak-Pastuszak E., Seifert T., Matthäus W., Krüger S., Sehested Hansen I., 2003c, Warm Waters of Summer 2002 in the Deep Baltic, Oceanologia 45, 571-592. http://www.iopan.gda.pl/oceanologia/454feis1.pdf 

Feistel R., Nausch G., Matthäus W., Łysiak-Pastuszak E., Seifert T., Sehested Hansen I., Mohrholz V., Krüger S., Buch E., Hagen E., 2004a, Background Data to the Exceptionally Warm Inflow into the Baltic Sea in Late Summer of 2002, Meereswiss. Ber. Warnemünde, 58, 1-58. http://www.io-warnemuende.de/documents/mebe58_2004_paper.pdf 

Feistel R., Nausch G., Heene T., Piechura J., Hagen E., 2004b, Evidence for a Warm Water Inflow into the Baltic Proper in Summer 2003, Oceanologia 46, 581-598. http://www.iopan.gda.pl/oceanologia/464feist.pdf

 Feistel R., Nausch G., Hagen E., 2005, Unusual Inflow Activity 2002/3 and varying Deep-Water Properties, presented at the BSSC, Sopot, Poland.

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/Meeresdaten/Beobachtungen/MARNET-Messnetz/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 (<20 words)

 The last four are very characterized by two warm inflows in the summer 2002 and 2003 interrupted by a cold major Baltic inflow in January 2003.

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

 Last updated 25.11.2005.