Water exchange between the Baltic Sea and the North Sea, and conditions in the deep Basins

	Editor(s): 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 in 2004 which continues until now.

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 three major inflows took place in 1993, 1997 and 2003, and a minor one in 2001.

In 2003, the very contrasting thermal signatures of both inflow types provided (and still provide) natural ‘tracers’ and allowed a clear insight into the dynamics of deep water propagation through the main basins of the western and central Baltic (Figure 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. Meanwhile, near-bottom temperatures have exceeded the long-term mean again:

The major Baltic inflow from January 2003 was the last strong inflow event into the Baltic Sea. The deep basins were additionally influenced by a warm summer inflow in 2003. The salinity development in the deep water at 200m in the Gotland Basin reflects these inflow processes (Figure 2). Steep increases after inflows are followed by slow decreases in the stagnation periods afterwards. It was particularly 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. The effects of these events are phasing out, a new stagnation period had started already in 2004 and continued in 2006 in all deep basins of the central Baltic Sea. Thus, salinity in the bottom layer decreased further compared to July 2005:

Since 2003, a slight increase of the surface salinity in the central and western Baltic is observed as a consequence of the deep water processes.

Changes of the near-bottom distributions of dissolved oxygen resp. hydrogen sulphide reflect these processes and are displayed for the years 2003 – 2006  in Figure 3. 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. Since then a deterioration of the oxygen situation in the deep water was observed. Hydrogen sulphide concentrations (expressed as negative oxygen equivalents) in the near-bottom layer increased continuously:

Recent investigations in July 2006 have shown that the vertical extension of the layer containing hydrogen sulphide has increased, too. At station  271 (Gotland Deep), hydrogen sulphide was found between around 137 m and the bottom, and at station 286 (Farö Deep) the layer between 120 m and the bottom was anoxic. The situation was especially extreme at station 284 (Landsort Deep). Already in 70 m water depth, a hydrogen sulphide concentration of – 0.23 ml/l was measured. The whole water column below that depth was anoxic.

References

Feistel R., Nausch G., Hagen E., 2003, 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., 2003, 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., 2003, 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., 2004, 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., 2004, 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., 2006, Unusual Baltic Inflow Activity in 2002-2003 and varying Deep-Water Properties, Oceanologia 48(S), 21-35. http://www.iopan.gda.pl/oceanologia/48_S.html#A2

Meier, H.E.M., Feistel, R., Piechura, J., Arneborg, L., Burchard, H., Fiekas, V., Golenko, N., Kuzmina, N., Mohrholz, V.,Christian Nohr, Paka, V. T., Sellschopp, J., Stips, A., Zhurbas, V., 2006, Ventilation of the Baltic Sea deep water: A brief review of present knowledge from observations and models. Oceanologia 48(S), 2006, 133-164               http://www.iopan.gda.pl/oceanologia/48_S.html#A8

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)

No important inflow events occurred since 2003. Thus, a new stagnation period prevails in the deeper basins since 2004/2005.

Figures

Figure 1. Temperature series 1997-2005 of the EGB mooring near the Gotland Deep 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. Click image to enlarge.

Figure 2. Salinity at 200 m at the Gotland Deep station 271 between 1968 and 2005. Click image to enlarge.

Figure 3: Areas with oxygen deficiency and hydrogen sulphide in the near bottom layer of the Baltic Sea in May in the years 2003 - 2006. 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. Click images to enlarge.

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

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2006. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
 

Last updated 31.8.2006.