Despite the larger inflows in October and November 2001; the stagnation period which started in 1995 continues in the central Baltic Sea deep basins (Fig. 4). In the Eastern Gotland Basin; the anoxic water covered the layer between 125 m and the bottom whereby the hydrogen sulphide concentrations in the bottom-near layer were comparable to concentrations measured at the end of the long-lasting stagnation period in 1992. Also in the Landsort Deep; hydrogen sulphide is present all year round from 100 m depth down to the bottom.
Results and assessment
Relevance of the indicator for describing developments in the environment
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 in order to rise the fill factor 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 currently last such major inflow took place in 1993. Since then, the oxygen level of Baltic deep waters has dramatically decreased and is still in what is called 'stagnation'.
There was another inflow starting in September 1997 with rather warm waters but low oxygen levels. It could not significantly improve the anoxic conditions in the deep layers, but its exceptional warmth is still measured today.
In 2001, the average wind conditions in the western Baltic showed an exceptional frequency of southerly wind directions, which do not favour the water exchange between Kattegat and Baltic. The sea level at Swedish Landsort is considered an excellent indicator of the Baltic fill factor and can be used to characterize inflow events (Fig. 1a).
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Figure 1: Above: Sea level at Landsort as a measure of the Baltic Sea fill factor (after data of SMHI). Below: Strength of the north-west wind (positive) at the weather station Arkona (after data of DWD). The bold curve appeared by filtering with an exponential 10-days memory.
Beside three minor inflows in May, August/September and December a larger inflow from October 26th to November 8th can be seen from the sea level rise. During the last event it went up by 58 cm, which corresponds to an increase of the water volume by about 223 km³ within 12 days, or 19 km³/day. The sea level remained high until November 23rd giving the salty waters piled up in the Western Baltic the opportunity to drift eastward before being washed out again. Effects could be observed in the Bornholm Basin already in December where the oxygen content in the deep water increased up to 4.68 ml/l. The water reached deployed recording devices in the Eastern Gotland Basin at 200 m depth on January 3rd 2002, however, without long-lasting effects.
The strength of north-easterly winds at the Arkona weather station can be used as an indicator as well. After filtering with a 10-days memory, the resulting time series exhibits a high correlation with the sea level measured at Landsort a few hours later. Fig. 1b shows this curve together with the south-east component of the wind speed. The particular events of sudden level changes can well be assigned to related patterns in the filtered wind speed.
Current meters mounted at the Darß Sill monitoring mast, between Rügen and Mön islands (marked as DS in Fig.4), continuously record temperature, salinity, and the water current velocity at several depths (Fig.2). End of October, the sudden rise of salinity to the surface is clearly visible, followed by a slower relaxation to normal conditions in November. The accumulated inflow between October 25th and November 11th , can be estimated to be about 200 km3, whereby assumed 150 km3 were passing the Danish Belts / Darss Sill and measured 50km3 were flowing through the Öresund (Fig. 3).
Figure 2: Temperature and salinity recordings at the Darss Sill monitoring mast in 2001
Figure 3: Above: Temperature and salinity in Öresund between October 1st and December 31st 2001. Below: Accumulated inflow into the Baltic Sea through Öresund for the same period. (both after data from SMHI - http://www.smhi.se/hfacoord/BOOS/y2001/inflow20014.html).
Despite this larger inflow, the stagnation period which started in 1995 continues in the central Baltic Sea deep basins (Fig. 4). In the Eastern Gotland Basin, the anoxic water covered the layer between 125 m and the bottom whereby the hydrogen sulphide concentrations in the bottom-near layer were comparable to concentrations measured at the end of the long-lasting stagnation period in 1992. Also in the Landsort Deep, hydrogen sulphide is present all year round from 100 m depth down to the bottom.
Figure 4: Location of stations and areas of oxygen deficiency and hydrogen sulphide concentrations in the near-bttom layer of the Baltic Sea in 2001. Histograms show the maximum oxygen and hydrogen sulphide concentrations of this layer. The figure contains additionally the 70 m - depth line resp. 20 m – depth line (small picture).
The author are grateful to Barry Broman, Swedish Meteorological and Hydrological Institute (SMHI) Norrköping, for making Öresund measurements and sea level data from Landsort available. We would like to thank Toralf Heene, Baltic Sea Research Institute (IOW) Warnemünde, for processing temperature and salinity data from the Darss Sill mast.
The stagnation period which started in 1995 continued in the central Baltic Sea deep basins in 2001.
See also the Indicator Report:
Hydrography and oxygen in the deep basins