ICES STATUTORY MEETING
ICES C.M.1993/C:51
Session V
TIIE MAJOR ßALTIC INFLOW IN JANUARY 1993
by
Wolfgang Matthäus and Hans-Ulrich Lass
Institut für Ostseeforschung Warnemünde,
Germany
•
and
Rainer Tiesel
Deutscher Wetterdienst, Wetteramt Rostock,
Germany
ABSTRACT
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Inflow processes to the Baltic Sea cUlminating in strong inflows
of highly saline and oxygenated water are typical but relatively
rare phenomena. Since the mid-seventies, the frequency and
intensity of major inflows have changed. Only a few events have
occurred since then, and no inflow has been recorded between
February 1983 and the end of 1992. That caused significant
changes in the stratification and favoured both the drastic decrease in oxygen concentration and the extreme increase in
hydrogen sulphide in the central Baltic Sea.
In January 1993, an effective major inflow occurred after 16
years of stagnation in the central Baltic deep water. The German
Baltic Sea Research Institute in Warnemünde observed the overflow of the water in the Darss sill area, and the propagation of
the highly saline water into the central Baltic basins during
spring and summer.
The meteorological conditions in the precursory and inflow
periods are described, and the different steps of the overflow
are discussed. The propagation of the penetrated water body is
traced and the turnover in the Bornholm and eastern Gotland
Basins is illustrated.
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1. Introduction
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Strong influxes of highly saline and oxygenated water into the
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Baltic Sea - termed major Baltic inf~ows - are attracting in•
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creas1ng attent10n dur1ng the past decade part1cularly because
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of both the absence of effect1ve
events
S1nce the m1d-sevent1es
and the drastic changes in the hYdro~raphic conditions in the
central Baltic deep waters (Matthäus! Franck 1992).- They are
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attr1buted to the extreme exchange pr?cesses between North Sea
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and Baltic and they always represent the final stage of longer
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1nflow events.
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The water exchange between the Nort~ Sea and the Baltic is
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forced by both sea level and dens1ty, d1fferences between the
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Kattegat and the Arkona Basin (HeIa 1944; wyrtki 1954; Jacobsen
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1980; Lass et al. 1987). Whereas the dens1ty d1fference 1S
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rather constant the sea level difference is strongly correlated
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with zonal winds (cf. e.g. Lass and Schwabe 1990). Hence, the
exchange varies at the time s~ale of ttie atmospheric circulation
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over northern Europe. Moreover, the narrow and shallow transition area between the two seas (Fig.
prevents a continuous
flow of salty North Sea water into :the Baltic. Furthermore,
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circulation iri the Baltic deep water is restricted horizontally
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by the bottom topography and vertically by permanent stratification. These factors cause periods of lstagnation in the Baltic
which are marked by increasing phosphate
and nitrate concen,
trat ions and decreasing salinity and loxygen concentrations in
the deep water, and sometimes cUlmi~ate
in the formation of
,
considerable hydrogen sulphide concentrations in deep basins
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(Fonselius 1981; Matthäus 1990).
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The inflow of saline water through the Danish straits and the
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Belt Sea occurs very intermittently. Mostly, inflowing water has
a small volume orfand a low salinity~ Mixing with the ambient
water lowers its density along its path from the sills into the
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central baS1ns. Th1S 1mpl1es the 1nc~m1ng water usually to be
unable to displace the stagnant bottom water and to improve
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significantly the living conditions inlthe deep basins. This can
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only be done by maj or inflows consisting of large volumes of
highly saline and oxygenated water originating from the North
Sea.
Major inflows are typical, but relatively rare, phenomena in the
Baltic Sea (Dickson 1973; B6rngen et ale 1990; Matthäus and
Franck 1992). A total of 92 events of various intensity have
been identified during the present century which are observed
more or less regularly up to the mid-seventies. The last inflow
with considerable effect in the Baltic deep water occurred in
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December 1975/January 1976 (Lass, Schwabe 1990). Since the midseventies, the frequency and intensity of maj or inflows have
changed. Only a few events have occurred since then and no
inflows have been recorded between Fehruary 1983 and the end of
1992 (Franck, Matthäus 1992).
In January 1993, an effective major inflow took place after 16
years of stagnation in the central Baltic deep water. Oceanographers of the Baltic Sea Research Institute in Warnemünde/Germany
observed the overflow by both current meters and temperatureconductivity recorders (SEACAT) moored at the Darss sill (Stat
•
001 in Fig. 1). Moreover, aseries of CTD casts was taken during
the main inflow period along a section crossing the Darss sill
into the Arkona Basin. Later the propagation and the effects of
the inflowed water has been recorded by several oceanographic
cruises in the Baltic proper.
This intensive inflow of substantial quantities of highly saline
and oxygenated water ended the most significant and serious
stagnation period ever observed in the Baltic Sea (Matthäus
1990). A drastic decrease in temperature, salinity and oxygen
concentration was
recorded
in the Baltic deep water of
the
eastern central basin during the 1977 to 1992-stagnation period.
The period started with-the highest temperatures (7.4 Oe) and
led to the formation ofthe highest concentrations of hydrogen
sulphide (6 - 7 mg/dm3 )
ever measured in the near bottom levels.
The salinity and density values observed at the end of the
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period were the lowest ever recorded.!
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In the following, the meteorological donditions in the precursoi
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ry and inflow periods caused the major Baltic inflow are analy.
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zed, and the different steps of the Ioverflow across the Darss
sill are discussed. Moreover, the propagation of the penetrated
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water body l.S traced,
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and the turn?ver
in the Bornholm and
eastern Gotland Basins is illustrated.
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2. Meteorological conditions
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At the end of December 1992, the circulation conditions were
governed by a high pressure area passing through from the North
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Sea to the central part of Germany and resulting in weak, mostly
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westerly
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wl.nds. Thl.s Hl.gh moved towards Poland on the 31 Decem1
ber, joined to the strong High over northeasternEurope" and
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moved to eastern Poland and Belorussl.a, whl.le strengthenl.ng. Between the 31 December and 5 January, the transition area between
North Sea and Baltic was affected by winds of southerly to
southeasterly directions (velocities ~ - 12 m/s, d~ring squalls
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locally up to 20 rn/Si cf. Fl.g. 2B) caused by the east European
high pressure area and Atlantic low e~tensions moving from Great
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Britain to Norway.
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During the 5th of January, the large scale weather situation
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changed l.n the 500 hPa level to a very strong west Sl.tuatl.on.
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According to the passing ~hrough of the first Atlantic low
extension the ground wind shifted fro~ SE to SW and increased to
12 m/s, during squalls to 20 m/s.
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Without any significant variation
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th~t
very strong west weather
situation continued during nearly 3 weeks between 6 and 25
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January, 1993 (Figs. 2C and 3). During this period, the transition area was directly affected by a~ upper storm field and was
partly also situated northerly of theijet stream. Caused by both
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the partly down to the ground acting upper storm fields,
the
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strong meridional air temperature gradients and the cyclonality
of the west wind field, hurricanes occurred and passed through
on 14th, 22nd and 24th of January. At the meteorological station
Arkona (northern,Rügen Island) e.g. extreme squalls of 45 m/s
from 250 on 14 Jariuary, of 39 m/s fram 260 on 22 January and of
34 m/s from 250 0 on 24 January are recorded.
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During 3 weeks, a calming down of the situation did not occur
between the hurricane depressions due to other cyclonic eddies
passing through (cf. Fig. 3). Westerly winds of 12 - 20 m/s (up
to 30 m/s during squalls) predominated (mean velocity 14.5 m/s).
starting on the 26 January, the strang west wind situation in
the 500 hPa level changed to weaker narthwesterly arid northerly
winds. A high pressure area maved from the North Sea towards the
transition area up to the 29 January. The weakened wind shifted
from westerly to northerly directions and weak winds between NE
and E predominated on 29 January (Fig. 2D).
3.
..
The Darss sill overflow
Inflow processes cUlminating into major Baltic inflows consist
of two fundamental parts: the precursory and the inflow periods •
The precursory period covers the time from the minimum Baltic
sea level preceding the major inflow to the start of that event.
It is characterized by the inflow of water with relatively low
salinity across the Darss sill whereas highly saline water can
already pass through the Drogden sill. The inflow period is
characterized by the influx of highly saline water across both
sills up to the maximum Baltic sea level during this event.
The barotropic current in the Great Belt (Fig. 4), at the Darss
sill (Fig. 5) and in the Sound changed immediately from outflow
to inflow with the onset of the west wirid and remained this
direction except for short periods until 28 January. The corresponding mass flow through the Danish straits together with the
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river discharge raised the sea level at Landsort by about 90 cm
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(cf. Matthäus 1993).
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The inflow of highly saline water through'the Sound across.the
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Drogden S1.ll
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started 1.mmed1.ately after the onset of current
reversal and discharged water of hig~ salinity into the Arkona
Basin (H. Dahlin, pers. comm.). There ~t covered the bottom with
a thin layer (cf. Fig. 6A). The inflo~ of saline water into the
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Arkona Basin via the Great Belt was delayed by one to two weeks.
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The salinity at the Darss sill
exeede~
15 PSU at the 13 January,
remained constant for some days and increased to more than 17
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PSU at the 18 January (Fig. 7).. The climax
of the inflow occurI
red at the Darss sill between 26 and 28 January where water with
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temperatures of 3.6
oe
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and salinitiesiof 22 PSU was observed to
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flow into the Arkona Basin. The salinity in 7 m depth decreased
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at the Darss S1.ll start1.ng w1.th the onset of the outflow w1.th1.n
a week to 8 -
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10 PSU whereas the salinity in the lower layers
decreased to the same values' within about
two weeks. The salini,
ty of 8 -
10 PSU is the characteri~tic range of the surface
salinity in the Arkona Basin.
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During the main inflow period, huge!amounts of highly saline
water crossed the Darss sill into the Arkona Basin and accumulated there within a week. The 15 PSU Ilisohaline was lifted from
about 38 m depth to 10 m and the 20 PSU isohaline was displaced
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from 42 m to 32 m (F1.gs; 6B and e). I~ the bottom layer below 40
m depth, water with salinities between 22 - 24 PSU and temperatures of about 4 oe was observed (Fig.l 8). This water originates
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from the Sound. The 15 PSU isohaline lowered until mid-February
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to a depth of 30 - 35 m indicating a flushing time scale ofthe
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Arkona Basin of about 2 - 3 weeks (c~. Fig~ 9B).
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4.
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The propagation into the centra~ Ba~tic basins
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The salt water discharged into the Arkona
Basin was partly mixed
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intci the surface water and flowed back into the Belt Sea and the
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Sound during the following outflow phase. Another part was mixed
southward into the water of the shallow areas east of Rügen
Island. The remaining part flowed through the Bornholm Channel
into the Bornholm Basin and replaced the old bottom water. The
salinity in the bottom water increased from about 15 to 20 PSU
and the oxygen concentration raised from about 1 to 7.5 cm3 /dm3
between October 1992 and March 1993 (cf. Figs. 10A and D). The
intermediate oxygen minimum became weaker and shallower between
February and March indicating an ongoing m1x1ng between the old
bottom water and the inflowed salt water.
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The Bornholm Basin below 60 m must be considered as storage
basin for the inflowing highly saline water. The main quantity
of the water entered in January 1993 was stored in that basin.
The stagnant lower saline bottom water stored there before was
lifted above the sill depth to the Stolpe Channel (60 m) by the
inflow of saline water and moved under the action of gravity
into the eastern Gotland Basin since the beginning of March
(Figs. lOB and E). Water bodies with Ts-properties of the Bornholm Basin intermediate water were observed in the stolpe Channel in March to May. The maximum salinity of this water changed
within a week in the Stolpe Channel indicating that the outflow
of deep water from the Bornholm Basin is rather intermittent.
The outflow became weaker in May .
In the beginning of April, first indications of the bottom water
renewal in the eastern Gotland Basin were observed and the deep
water of the Gotland Deep between 200 m and bottom was renewed
in the middle of May (Figs. lOB, c, E, F). At the end of June,
salinity and oxygen content increased to 11.7 PSU and·1.5 - 2
cm30ddm3 in the near-bottom layer of the Gotland Deep. The H2Scontairiing water was lifted up and ranged between about 130 m
and 195 m depth. Weak effects of the inflow could also be identifiect in the northern Gotland Basin.
As early as in May the decrease in salinity and oxygen concentration started again in the Bornholm Basin deep water. Recent
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cruises have shown that according to the intensity of the inflow
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at the sills the effects are limit'ed in the central Baltic
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basins.
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5. Conclusions
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During the recent event, a total of about 310 km3 and from it 125
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km3 of highly saline and oxygenated water entered the Baltic Sea.
The highly saline water was characteiized by a mean salinity of
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about 19 PSU, a mean temperature of 3.5 °C, a mean density
t
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anomaly of 15.2 kg/m3 and a mean oxygen concentratl.onof 8.2
cm3 /dm3 •
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Compared to all inflows identified ~uring the present century'
(Matthäus, Franck 1992; 'Franck, Matthäus 1992), the recent
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inflow must be considered as a moderate one. According to the
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measurements Sl.nce May, we must state that the effects ,of the
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inflow in the central Baltic basins ~eem to be limited. Nevertheless, the inflow led hitherto both toa certain increase in
salinity and, at least temporarily, !to oxic conditions in the
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central Baltie deep water. This will! improve the living.conditions of fish and benthic organisms and positively influence the
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spawning behaviour and larval development of eod in the Baltic
Sea, at least in the Bornholm and Gdansk Basins.
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Acknowledgements
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The authors would like to thank the Monitoring Group of the
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Baltic Sea Research Institute Warnemünde and the erews of the
research vessels "A. v. Humboldt" and' "Prof. Albrecht Penck" for
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the assistence in the field works. Aeknowlegement is given to
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the Federal Maritime and Hydrographie Ageney Hamburg for sup.
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portl.ng our work by the outputs from the operational model
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running in the Agency, and to the Swedish Meteorologieal and
Hydrological Institute Norrköping (SMHI) for information exj
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change concerning the start and progress of the reeent event. We
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are also grateful to Barry Broman (SMHI) for providing with the
Landsort sea
level data and to the Great Belt Link Company
Copenhagen for making available current and salinity observations in the Great Belt area.
References
Börngen, M., P. Hupfer, and M. Olberg, 1990: Occurrence and
absence of strong salt influxes into the Baltic Sea. Beitr.
Meereskunde Berlin, 61, 11 - 19.
Dickson, R. R., 1973: The prediction of major Baltic inflows.
Dt. Hydrogr. Z., 26, 97- 105.
Fonselius, S., 1981: Oxygen and hydrogen sulphide conditions
in the Baltic Sea. Mar. PoIl. Bull., 12, 187 - 194.
Franck, H., and W. Matthäus, 1992: The absence of effective
major inflows and the present changes in the hydrographie
conditions of the central Baltic deep water • . Proc. 12th
Symp. Baltic Marine Biologists, Helsing~r 1991. In:
Intern. Syrop. Series, Olsen & Olsen, Fredensborg, 53 - 60.
Hela, I., 1944: Über die Schwankungen des Wasserstandes in der
Ostsee. Merentutk. Julk., 134, 108 pp.
Lass, H. U., R. Schwabe, W. Matthäus, and E. Francke, 1987: On
the dynamics of water exchange between Baltic and North
Sea. Beitr. Meereskunde Berlin, 56, 27 - 49.
•
Lass, H. U., and R. Schwabe, 1990: An analysis of the salt
water inflow into the Baltic in 1975 to 1976. Dt. Hydrogr.
Z., 43, 97 - 125.
Matthäus, W., 1993: Major inflows of highly saline water into
the Baltic Sea - a review~ ICES Statutory Meeting 1993, Paper ICES C.M. 1993/C:52.
Matthäus, W., and H. Franck, 1992: Characteristics of major
Baltic inflows - a statistical analysis. Cont. Shelf
Res., 12, 1375 - 1400.
Matthäus, W., 1990: Langzeittrends und Veränderungen
ozeanologischer Parameter während der gegenwärtigen
Stagnationsperiode im Tiefenwasser der zentralen Ostsee.
Fischerei-Forsch. Rostock, 28, 3, 25 - 34.
Wyrtki, K., 1954: Der große Salzeinbruch in die Ostsee im
November und Dezember 1951. Kieler Meeresforsch., 10, 19 25~
Baltic
Sea
Area:
Volume:
373000 km 2
20900 km 3
Drainage area: 1.62 Mill km 2
Darss Sill:
Depth
18 m
Cross section 0.8 km 2
Fig. 1. The Baltic Sea and the situation of the sills in the transition
area to the North Sea
•
40
A
.lan 1993
-
- --
~
.... - - ,
1·5 Jan
,
, ,
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.-
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10
....
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-,
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...
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-----
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,,
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o
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------ -
10
\
\
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.-
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/
-10
/
-20
-30
0.2 <x< S
< 10
I-----------------~
\
\
" ----
... ,
20
...
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...
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.... ""
.- .... ---
20
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~
...
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_-_
...
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....
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.1111
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27·31 Jan
,
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/~--- ... , '. '.
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....
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----...... -- ....
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6·26 Jan
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....
.- .... ---~
, - - .... ...
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-----
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Arkona
Wind speed rn/s
January 1993
B
< 11 >11 m/s
Arkona
Wind. E-componenl m/s
January 1993
11
Fig. 2. Wind characteristics at the meteorological
station Arkona (Rügen Island) during different
periods in January 1993
16
21
26
31
Fig. 3. Wind speed and E-component (negative
values correspond to wind from west) at
the meteorological station Arkona (Rügen
Island) in January 1993
100.,.--------------------,
January 1993
'.5
r-
7 m
.5
12 m
'CI)
Greal eell rasl Chan",1
Currenl, North Campon.nt /
Dolly vertlcal meons
cm/s
SBr Cop.nhog.n
- 100 -k........:.-rrrrrn"TT"T"TT"T.,.......1TT"""rT"T"'rTTTTT"T.,.......TTrrTT"lCT"T"T,..,..,.TTTTTT-~
IOecl992
15
-...
0
0
C.
.5
E
IJonl993
25
30
15
c:
Q)
c:
E
17 m
0
100 ....------------------~
uI
W
0
Z
.5
0
"
19.5m
t.,. I.
i 14
,.
.
'. 't . . . ~l~· l ':
27.12.
Greal eell Wesl Channel
Currenl, Norlh Camponenl /
DoUy vertlcal moons
- I 00
cm/s
ser Cop.nhogon
-k...""':'-rrrrrn"TT"T"TT"T"TTT""rr-rrrnTTTT"TTTTT"rTT"lr-rrr"TTTTTT"TT"T"rT"T"'rTTT"l
IOecl992
15
25
IJonl993
15
30
Fig. 4. N-component of the currents in the
Great Belt West and East Channels
(data from SBF Copenhagen)
PI {r
.
10.1.
1" .
24.1.
•....
-,
7.2.
21.2.
7.3,
Fig. 5. NE-component of the currents in different depths at
the Darss Sill (Pos. 001, cf. Fig. 1)
•
A
50
Arkana Basin
m
20 I 21 January 1993
70
0
B
10
20
30 .
Fehmarnbelt
40
50
Arkana Basin
m
26 January 1993
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70 --'-
0
c
10
20
30
Fehmarnbelt
40
50
m
Arkana Basin
27 January 1993
70 - - ' - - - - - - - - - - - - - - - - - - - - - - - ' - '
Fig. 6. Longitudinal transects of salinity (in PSU)
crossing the Darss Sill into the Arkona Basin
during the inflow event in January 1993
.------------------------------
,
30
Greot Bell
25
20
15
10
Salinity / PSU
Vertical means
5
lDec1992
15
25
lJan1993
15
30
Fig. 7. Vertical means of the salinity in the
Great Belt (data from SBF Copenhagen)
and the Darss Sill (Pos. 001, cf. Fig. 1)
4.5
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o
_
r-------..!
4.0
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...
Q)
I
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.... 3.5
ca
...
Q,
E
•Belt
fit •.,-.
;r
. ../
Q)
3.0
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•
"S'..
Sound
water
1-----I
I.
Sea
water
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•
Q)
I-
Arkana Basin
21 - 27 Jan 1993
2.5
8
10
12
14
16
Salinity
18
20
22
24
I PS U
Fig. 8. TS-diagram of the observations in the
central Arkana Basin
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,
°l:-----~-~~~----~~-~--~.,..,
A
....... ~ .
...
:.
\\
.
10
.
,,~
..
~
10/~--=;-~=~";
. :i~10~..
~ .
20
.
~"
",~.
• •' ,
JO
•
Fehmarn·
Darss
Sill
belt
40
50
Arkana Basin
m
70 -'-
February 1991
....LJ
O.,-~-~---~--~-------------,
10
20
40
Arkana Basin
50
m
February 1993
70
0
10
20
--
030
Fehmarn·
belt
.," ..
..... '..... -:-.
.......... .. .... :.-,.f'':
"~~';=
.... .
~.....
:15 -"," "'-10~
.~~' ···.~1~
40
50
Arkana Basin
m
March 1993
70
Fig. 9. Longitudinal transects of salinity (in PS U)
between Fehmarnbelt and Arkona Basin
during a year without major inflow (A), in
February (B) and March 1993 (C)
0
0
::~
....
A
/i
100
ICO
150
150
200
200
Fabruary .1993
F.bruary 1993
250
250
. -..
.,
0
:\
B
100
100
ISO
150
200
200·
m
March/AprU 1993
March/AprU 1993
250
250
..
0
..
-_._- .-- -_.
Ce
50
50
100
."'........
• asln
150
E
50
SO
100
0
SO
50
0
..
.-......
Stol~
c.........
ISO
200
200
M.y 1993
250
250
Fig.10. Propagation of saline water into the central Baltic iIIustrated
by salinity (A • C. in PSU) and oxygen distributions (0 • F.
in cm'/dm 3 ) along a transeet between Bornholm Basin and
Färö Deep
.
-