Water Exchange between the Baltic Sea and the North Sea, and conditions in the Deep Basins

	Editors: Günther Nausch, Rainer Feistel and Volker Mohrholz, IOW

Key message

The inflow activity from the Kattegat into the Baltic Sea in the past decade exhibited 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 low inflow activities with only a slight intensification since 2006. Except for the southern Baltic, the stagnation period lasting since 2004/2005 resulted in a continuous increase of hydrogen sulphide concentrations in the central basins. A baroclinic inflow in summer 2006, followed by small barotropic inflows in 2007 and 2009 caused again very high temperatures observed in the central Baltic deepwater. The warming trend continued during the following years.

After nearly a decade without a major Baltic inflow, a relative large inflow was detected in the western Baltic during winter 2011/12 which ventilated the Bornholm Basin and could be traced until the southern part of the eastern Gotland Basin but was not able to renew the deep water there.

Results and assessment

In the Baltic Sea, two types of lateral deep water renewal processes can be distinguished, the “classical” barotropic Major Baltic Inflows (MBIs) and the “new” baroclinic inflows (Matthäus et al. 2008). MBIs which occur 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 (Matthäus et al. 2008). In the subsequent decades, however, they became rather scarce; the last three major inflows took place in 1993, 1997 and 2003, and a smaller one in 2001.

After 8 years, a relatively large inflow from the Kattegat took place in November/December 2011. In a series of medium and small inflow pulses through the Sound and the Belt, about one billion tons (1.0 Gt) of salt were imported. However, the recent MBI remains clearly behind the earlier MBI’s in winter 2003 (with 2.0 Gt) and 1993 (with 3.4 Gt).

In 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 September 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 carried very warm water into this basin. Thereafter, the water 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. Subsequently, the near-bottom temperatures increased again and exceeded the long-term mean as a consequence of the baroclinic inflow of August 2003, and were rising even further in 2007 due to the baroclinic inflows of 2006 (Nausch et al. 2007, 2008, Fig. 1) and 2009/2010 (Nausch et al. 2010, 2011). In particular, the baroclinic inflow of August/September 2006 carried very warm water into the Bornholm Deep, residing there from November 2006 onward. Apparently it was lifted over the Slupsk Sill in January 2007 and reached the Gotland Basin in April 2007, when extremely high values of 7.1°C were measured there in the near-bottom layer. Despite its drastic temperature signal (Fig. 1), this was a rather smooth substitution process without significant signals in salinity. In September/October and November/December 2009, two minor barotropic inflows occurred that resulted in a salinity signal of 22 g/kg in the near-bottom layer of the MARNET Arkona Basin buoy in December, the highest value ever recorded there since it went on service in September 2003 (Nausch et al. 2010). Their thermal signals arrived at the Gotland Deep in February and March 2010, see Fig. 1. Since the beginning of 2011, the deep water of the eastern Gotland Basin between 170 m and 220 m had a constant temperature of 6.43 °C. Such a constancy of a year-long temperature record was never observed before. In the other deeps temperature is decreasing.

The major Baltic inflow from January 2003 was the last strong inflow event into the Baltic Sea. The deep basins were subsequently influenced by a warm summer inflow in 2003. The salinity development in the Gotland Basin reflects these inflow processes (Fig. 2). In the deep water, steep increases after inflows are followed by slow decreases in the stagnation periods afterwards. It was particularly the short inflow of August 2003 which elevated the salinity again to levels typical for the 1960s and 1970s. The surface salinity is following this trend delayed by a decade. The effects of those events in 2003 are phasing out; a new stagnation period had started already in 2004 and continued until 2012 in all deep basins of the central Baltic Sea, in contrast to the Bornholm Deep and the Slupsk Channel.

Changes of the near-bottom distribution of dissolved oxygen resp. hydrogen sulphide reflect these processes and are displayed here for the years 2009 – 2012 (Fig. 3). In 2003, the MBI of January had ventilated the Bornholm, Gdansk and Eastern Gotland Basins 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, with a short interruption in 2007, 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. In the Gotland and Farö Deeps, hydrogen sulphide concentrations were among the highest recorded so far:

Also the vertical extension of the layer containing hydrogen sulphide is increasing. In May 2012 in the Gotland and Farö Deeps, H₂S was found from about 100 m depth to the bottom. At the Landsort Deep station the water column between 100 m and the bottom (roughly, at 430 m) was anoxic.

In the Bornholm Basin, in contrast, the baroclinic inflows of 2006 and the small barotropic ones between 2007 and 2010 repeatedly oxygenated the deep water in those years. Thus, in 2008-2010 no hydrogen sulphide was found in the Bornholm Basin, while in the second half of 2011 anoxic conditions temporarily restored directly at the bottom but were terminated soon after by the MBI of November /December 2011. This favorable situation is supporting recruitment of the eastern cod. East of Bornholm, the cod population of 400 000 tons has returned to values found 20 years ago (ICES, 2011).

A surprising finding was the complete ventilation of the formerly increasingly anoxic water column at the Karlsö Deep in April 2008 when small concentrations of oxygen (0.5 ml/l) and nitrate (1 µmol/l) were measured near the bottom. A similar but weaker event was also observed in January 2010. Since then, the bottom water remains anoxic. The stratification of the water column is not as stable as in other regions and apparently permitted partly winter convection.

References

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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

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Matthäus, W., Nehring, D., Feistel, R., Nausch, G., Mohrholz, V., Lass, H.U., 2008, The inflow of highly saline water into the Baltic Sea. In Feistel, R., Nausch, G., Wasmund, N. (Eds.), State and Evolution of the Baltic Sea, 1952 – 2005. A Detailed 50-Year Survey of Meteorology and Climate, Physics, Chemistry, Biology, and Marine Environment. John Wiley & Sons, Inc., Hoboken, 265-309

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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)

Since 2003, a first major inflow event was observed in winter 2011/12 which ventilated the Bornholm Basin but was too weak to renew the central basins.

Figures

Fig. 1. Temperature series August 1997- May 2012 of the EGB mooring near the Gotland Deep at 174, 204 and 219 m depth, referred to as the “Hagen curve”. The bathymetric depth at the anchor position is H = 224 m. The temperature signals caused by the warm and cold inflow events appear as sudden leaps with subsequently fading fluctuations over typically one year relaxation time. Note the exceptional thermal quiescence in 2011.

Fig. 2. Salinity in the Gotland Deep at the surface, at 90 m, 120 m and 200 m depth from 1952 to 2011. Monthly mean time series till 2005 are available from the BALTIC atlas (Feistel et al. 2008). After the surface salinity minimum of 2002, values are gradually rising again as a result of the high deep-water salinity after the 1993 low (Feistel et al., 2006; Matthäus et al., 2008; Reissmann et al., 2009; Holliday et al., 2011).

Fig. 3: Areas with oxygen deficiency and hydrogen sulphide in the near bottom layer of the Baltic Sea in May in the years 2009 – 2012. 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.

For reference purposes, please cite this Baltic Sea Environment Fact Sheets as follows:
Günther Nausch, Rainer Feistel and Volker Mohrholz (2012). Water Exchange between the Baltic Sea and the North Sea, and conditions in the Deep Basins. HELCOM Baltic Sea Environment Fact Sheets 2012. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated: 3 September 2012