ICES STATUTORY MEETING 1994
lCES C.M. 1994/Q:2
Dynamics of the transition area between
Baltic Sea and North Sea
by
W.Fennel and T.Seifert
Institut für Ostseeforschung Warnemünde
•
Abstract
Aseries of experiments with a regional numerical model of the
Baltic Sea is used to study the dynamical control of the water
exchange by the features of the transition area between Baltic
Sea and North Sea. The model is based on the free surface version of the Bryan-Cox code adapted to the Baltic Sea. The model
is initialised with the mean summer stratification and forced by
buoyancy and wind.
Current patterns in the Belt Sea and transports through the
Belts and Sound are discussed. The flow through the sound gives
a substantial contribution to the water exchange. In particular,
east-west sequences of the wind forcing can generate transports
of the same order of magnitude through the Great Belt and the
Sound.
1. Introduction
The understanding of the processes controlling the water exchange between Baltic Sea and North Sea is an important research
sUbject which has attracted many scientists around the Baltic
for several decades. The sea water exchange of the Baltic is
highly variable, the ratio of the standard variation to the long
term mean being of the order of 10, JACOBSEN (1980). The long
1
,
#
"
".
term net flow can be estimated from Knudsen's theorem based on
the freshwater supply (river runoff and rain minus evaporation
over the Baltic) and the salinity differences of in- and outflowing waters.
The freshwater supply is basically an external forcing while
the salinity differences depend on oceanographic processes at
various temporal arid spatial scales in the Baltic Sea. Knudsen's
theorem utilizes the conservation of mass and salinity. The
ladder is approximatly confirmed by the more or less constant
mean distribution of the long term haline stratif!cation. However, the theorem cannot explain the underlying mechanism.
•
The key processes which control the water exchange are the sea
level differences between Kattegat and Baltic, the water mass
transformation in the Baltic, and the transfer properties of the
transition area.
The water mass
transformation
is
controlled by
small
scale
mixing and effective stirring of waters by mesoscale patterns
such as,
.
e.g.,
.
eddies arid filaments.
Thc driving forces ar~
winds, heat fluxes and sea level differences (barotropic pressure gradients) with time scales of the weather patterns (a few
days) and the seasonal cycle ~ The formation of the brackish
water in the surface layer of the Baltic implies an upward flux
of salto. A substantial amount of salt leaves
the Baltic through
,
the Sound and Great Belt with tlie brackish water. The same
amount of salt has to be transported into the Baltic by the
bottom flows.
The transfer properties of the transition area control dynamically the propagation of water masses and salt through this
area. The transfer is retarded by dynamical processes which are
affected by geometry of the irregular coastlines and the bottom
topography in the transition area.
STIGEBRANDT (1983, 1984) constructed a barotropic nonlinear
tranfer function based on thc sea level differerices between
Kattegat and Baltic. The dynamical ingredients of the model are
mainly rotational hydraulic control, friction and geometry. Full
transfer of a sea level difference between Kattegat and Baltic
is achieved for time scales of the order of one year. The tran~
2
fer is reduced to 50% for time scales in the range of 1 - 2
months, depending on the amplitude owing to the nonlinearity of
the tranfer function.
For forcing periods of the order of one
week only 10% of a sea level difference is communicated to the
Baltic.
LASS (1988) considered the geostrophically controlled barotropic
water flow through the Belt Sea utilizing the theory of GARRETT
and TOULANY (1982). The dynamical ingredients of the model are
the geostrophic adjustment established by Kelvin waves and the
geometry. LASS found that the for time scales less than five
days the exchange is very low. The sea level in the Baltic is
affected by the sea level in the Kattegatt only for time scales
•
exceeding 10 days.
However, the barotropically-driven motion of water must not
necessarily lead to an effective exchange of water. It is possible that the same water may just oscillate back and forth in
the transition area.
These conceptual models are useful to discuss the dynamics of
major saltwater inflows, where the water are vertically mixed
and the flow through the transition area is governed by barotropic processes. The occurrence of major inflows depends on
strong western winds, which act on the system with a preconditioned mean sea level in the Baltic. Such events are of outstanding importance for the ecosystem of the Baltic. However,
they are rather rare and occur irregulary with time difference
of three to 15 years.
The exchange properties with time scales less than 10 days and
with weIl developed stratification are more difficult to study.
In this case the mesoscale processes in the transition area have
also to be considered. This can be achieved by the use of advanced numerical models. The present paper aims at a better
understandig of the·role of mesosca1e processes for the transfer
properties of the transition area by means of numerical experiments. section 2 gives abrief description of the dynamical
properties of the transition area. In section 3 we discribe a
series of numerical experiments and a summary with conclusions
is given in
section
4.
3
•
2. Dynamics of
th~
Transition area
The transition area between Baitic and North Sea consists of the
three Danish straits, the Sound, the Great Belt and the Little
Belt and of the Beltsea. The mean depth is about 22 m. The
bottom flow is hindered by sills. The shallowest part of the
Darss sill is 18 m and the saddle depth of the Drogden sill in
the Sound is only 8 m. The coastlines are rather irregular and
there
are
several
bights which provide buffer volumes.
topography is shown in Fig. 1. JACOBSEN (1980)
The
found that the
ratio of the sum of in- and outflow through the Sound, the Great
Belt, and the Little Belt is approximately 3:7:1.
Most of the year the water in the transition area is stratified.
Current measurements on the Darss sill show that in- and outflow
•
close to the bottom are of the same likelihood while at the
surface outflow clearly dominates, FRANCKE (1983). An example of
stick plots of the currents in"four levels over the Darss sill
for May 1986 is displayed in Fig. 2. The currents show a high
variability with time scales of a few days. Evidence of a similar high variability in the Fehmarnbelt has been reported by
LANGE (1975).
Observations showed that the mesoscale dynamics in the Beltsea
is complex with coastal jets, eddies and topographically guided
patterns, FENNEL and STURM (1992). Those features were confirmed
by satellite images of the surface temperature, SIEGEL et ale
(1994). The driving forces are mainly the local winds and the
sea level differences (barotropic pressure gradients), while
baroclinic pressure gradients are results of geostrophic adjustment rather than
forcing functions.
3. Numerical experiments
We used a high resolution regional model of the western Baltic
based on the free surface version of the BRYAN-COX-SEMTNER code,
see KILLWORTH et al. (1989). The area modelled is from the
western edge of the Skagerrak to the eastern border of the
Bornholm basin and is closed by straight walls, see Fig. 1. The
horizontal grid size is one nautical mile and the vertical
4
...
resolution is two meters in the firs~ 12 layers. The thickness
of the lower layers increases with depth and varies from 4 m to
50 m. The total numbers of layers varies from 25 in the Skagerrak to a minimum of 3 layers in the shallow coastal regions. The
high resolution bottom topography of SEIFERT and KAYSER (1994)
was used.
The model was initialised with the mean summer stratification of
the western Baltic and has been forced with homogenous wind with
a sinusoidal time behaviour acting on a fluid at rest, Fig. 3.
The wind directions chosen are west-east, southwest-northeast,
and north-south. The wind reaches its maximum value after the
first and the third day and goes through zero after two and four
days. We corisider the cases of a Gaussian shaped wind patch over
the transition area and a large scale wind acting over the whole
model area, Fig. 1.
We start with the west-east sequence of a wind patch. The surface currents (3 m) are governed by Ekman transport and the generation of coastal jets at the parts of the coast where the wind
is alongshore, see Fig. 4a. We observe an inflow through the
Belts and outflow through the Sound. The sea level, Fig. 4b,
shows a windstau (wind force balanced by the sea level gradient), with lifted sea level in the eastern part and lowered
level in the western part. The transports, Fig. 4c, confirm the
in- and outflow pattern indicated by the surface currents.
After 48 h the wind has ceased and the pressure gradient, which
is no longer balanced by the wind acts as a barotropic forcing.
Eddy patterns are generated by the interaction of the flow with
topography and the coastal jets weakens, Fig. 5a. The eddies are
more pronounced "in the transports as in the surface currents.
The sea level tends towards its equilibrium, but the transports
are approximately in geostrophical balance with the water level,
Fig. 5b and Fig. 5c. The outflow through the Sound remains but
the flow through the Fehmarbelt goes partly into the Kiel Bight
and partly into the Great Belt. This implies, that the communication between the sub-basin delay the exchange with the Kattegate
5
•
Later the eastern wind
start~
and the Ekman transport dominates
the surface fiow again. The eddy-patterns are destroyed after
about 6 hours and coastal jets witti reversed directions are
generated. After 72 hours the surface currents, the sea level
and the transports (not shown) are practically the same as after
24 hours, except that the signs are reversed.
After 84 hours the eastern wind weakens and the barotopic pressure gradient,
which was built up by the windstau competes
partly with the local wind.
The surface currents show first
Indications of eddy formation, Fig. 6a. The outflow through the
Belts and the inflow through the Sound is weIl developed; The
sea level differences are decrasing, Fig; 6b. In the Fehmarnbelt
both the surface flow and the transports show in- and outflow
pattern, Fig. 6c, which did riot occur during the phase of increasing
winds~
After four days, 96 hours, the dynamical patterns have practically the same structure as after 48 hours;
except that the
signs are reversed •.
These features are corifirmed by the transports througti several
test sections as displayed in Fig. 7a, which show a symmetrical
behaviour.
A comparison of the transports with those of the
large scale wind, depicted in Fig. 7b, shows differences in the
magnitudes and a completely different phase of the flow through
the Sound. The higher amplitudes are easy to understand because
in the case of the large scale forcing more energy is supplied
into the system and ttie barotropic pressure gradients are stronger than in the case of the wind patch.
The different phase behaviour of the transport through the Sound
has an interesting dynamical reason. In the large scale case the
wirid forcing drives also coastai jets in the Arkona basin. In
particular a strong jet develops off the souttiern coast of
Sweden. This coastal jet forces the waters at the southern mouth
of the Sound to flow eastward rather than into the Sound. This
is shown in Fig. 8a,b, where the transports driven by the wind
patch and the large scale wind, respectively, are displayed for
the model time of 36 hours. The patterns of the sea level are in
geostrophic balance with the transports. These structures are
6
relatively stable even if the wind d~creases. This effect, which
is ultimately a consequence of the vorticity conservation,
delays the communication between Baltic and Kattegat.
Experiments with the southwest-northeast wind patch give the
transports through the sectioris depicted in Fig. 9a. The trans.
,
ports through the Danish straits are now dominated by the flow
through the Little Belt and through the Sound with opposite
phases, while the transport through the Great Belt is relatively
small~ During the southwest wind phase Baitic waters flow out
through the Sound and Kattegat waters come in through the Little
Belt, while during the riortheastern winds the situation reverses, i.e., the Baltic waters flow out through the Little Beit
arid Kattegat waters come in through the Sound.
For the case of
large scale wind we obtain a different pattern
as shown in Fig. 9b. In this case the transports through the
Danish straits are approximately in phase and the flow through
the Great Belt gives a substantial contribution. However, the
transports throu~h .the sections Fehmarnbelt; WarnemUnde, and
Arkona behave quite different. The large scale wind forces a
substantial transport from the Beltsea irito the Arkona basin.
This creates a large barotropic pressure gradient within
12 hours which drives a reversed transport from the Arkona basin
back into the Beltsea before the wind reaches its maximum. After
the reversal of the wind the effect of the pressure gradient and
the wind combine and the water piles up in the Beltsea.
The surface currents develop in a qualitativelY slmilar manner
as iri the west-east-case. The patterns are governed by Ekman
transports, coastal jets at the parts of the coast with along~
shore wind and eddy formation if the wind weakens. The currents
in the Beltsea are rather similar both for a wind patch and a
large scale wind. The differences are due to the domiriating role
of the barotropic pressure gradient which implies that the
formation of eddies starts earlier in the case of the large
scale wind. However, strong differences are found in the Danish
straits. An exampie of the surface current for 48 hours is shown
in Fig. lOa,b.
Finally we consider the transports through the Danish straits
a
,
'
!
; . ,
.
7
I
for the case of a north-south wind sequence, which are shown in
Fig. 11a,b for a wind patch and a large scale wind. In the large
scale case the transports follow the sinusoidal behaviour of the
wind except that the transports change their sign about
six hours earlier as the wind. This is clearly due to the role
of the barotropic pressure gradient. The flow through the Great
Belt gives the largest contribution. In the case of a wind patch
the signals are smaller and the flow through the Little Belt is
out of phase. This is caused by the much weaker transport from
the Beltsea to the Arkona basin. The water piles up in the Kiel
bight and leaves mainly through the Fehmarnbelt and partly
through the Little Belt.
As in the other experiments the surface currents are governed by
the Ekman transport, coastal jets at the parts of the coast with
alongshore wind and eddy formation if the wind weakens~
4. Conclusions
For time scales less than 10 days simple barotropic models of
the transfer function of the transition area between Baltic Sea
and North Sea predict practically no exchange of water. However,
the typical time scales of weather patterns are smaller than
10 days. Since the total amount of salt in the Baltic is more or
less conserved there must be a certain exchange also at smaller
time scales. This requires studies which include and resolve the
mesocale dynamics in the transition area.
The mesoscale response patterns of the transistion area can
roughly be characterized as foliows: After the onset of winds
the dominating features are Ekman transports, coastal jets and
windstau. For diminishing winds the barotropic pressure gradient
is no longer balanced by the wind and starts to act as an external forcing. The currents driven by the pressure gradient interact with the topography and generate eddy patterns.
The bights in the transition area can act as buffer volumes
which have to be filled before the in- or outflow through the
Great Belt increases. Geostrophically adjusted transports delay
the equalizing of the sea level and, therefore, delay the commuB
nication of sea level differences through the transition area.
These are two examples of processes which cause retarding effects of the transfer function.
The properties of the in- and outflows through the Danish
straits depend strongly on the wind patterns. In particular,
the experiment with a southwest-northeast wind patch showed substantial transports through the Sound and the Little Belt while
the transports
through the Great Belt was much weaker.
The
transports through the Great Belt play a dominating role in the
case of north-south winds. Thus the high varability of the water
exchange reported by JACOBSEN (1980) can only be understood if
the external forces and the dynamics of the transistion area are
taken into account.
The presented study is a first step towards a better understanding of the transfer properties for time scales below 10 days
based on advanced circulation models.
Further investigations
with a closer linkage of field observations and improved model
versions are planned.
9
.
References
FENNEL,W. and M.STURM (1992): Dynamics of the western Baltic.
Jour. Mar. Systems, 3, 183-205
FRANCKE,E. (1983) Ergebnisse langzeitiger Strömungsmessungen in
der Deckschicht des Seegebietes der Darßer Schwelle. Beitr.
Meeresk., Heft 48, 23-45.
GARRETT,C.J.R. and B.TOULANY (1982): Sea level variability due
to meteorological forcing in the northeast Gulf of st.
Lawrence. Jour. Geophys. Res., 87, 1968-1978.
KILLWORTH, P. 0., D. STAINFORTH, D. J. WEBB, and S. M. PATERSON
1989: A free surface Bryan-cox-semtner model. Institute of
Oceanogr. Sciences, Deacon Lab., Rep. No. 270.
LANGE,W. (1975) Zu den Ursachen langperiodischer Strömungsänderungen im Fehmarnbelt. Kieler Meeresfo. 26, 65-81.
LASS,H.U.
(1988): A theoretical study of the barotropic water
exchange between the North Sea and the Baltic and the sea
level variations of the Baltic. Beitr. Meeresk., 58, 19-33.
JACOBSEN,T.S. (1980): Sea water exchange of the Baltic: Measurements and methods. Copenhagen; National Agency of environmental protection, pp106.
SEIFERT, T. and B. KAYSER (1994): A high resolution grid topography of the Baltic Sea. Technical Report, IOW, Warnemünde
(in preparation) .
SIEGEL, M.GERTH, H. RUDLOFF, and G. TSCHERSICH (1994): Dynamical
features in the western Baltic Sea investigated by NOAAAVHRR- data. Deut. Hydrogr.Zeitschr. (in press).
STIGEBRANDT,A. (1983): A model of the exchange of water and salt
between the Baltic and the ·Skagerrak.
Jour.
Phys.
Ocea-
nogr.,13, 411-427.
STIGEBRANDT,A.
(1984):
Analysis of an 89-year-long sea level
record from the Kattegat with special reference to the
barotropically driven water exchange between the Baltic and
the sea. Tellus, 36A, 401-408.
10
.
Figure captions
Fig. 1
The area of the regional model. The dashed square indicates the
transition area between Baltic Sea and Kattegat. The six thick
lines mark test sections for later use. The dashed circle indicates the shape of a Gaussian wind patch by means of the line of
50% wind magnitude.
Fig. 2
A time series of currents on the Darss sill in four level for
May 1986.
Fig. 3
Time behaviour of the wind amplitude used for the numerical
experiments.
Fig. 4
Responses to a west-east wind patch after 24 h:
a) surface currents,
b) sea level,
c) transports.
Fig. 5
As Fig. 4, but for 48 h.
Fig. 6
As Fig. 4, but for 84 h.
Fig. 7
Time series of transports through the test sections, indicated
by thick lines in Fig. 1, for the west-east wind case:
a) wind patch,
b) large scale wind.
11
Fig. 8
Transports for the west-east wind case at 36 hours:
a) wind patch,
b) large scale wind.
Fig. 9
As Fig. 7, but for the southwest-northeast wind case.
Fig. 10
Surface
currents
for
the
southwest-northeast
48 hours:
a) wind patch,
b) large scale wind.
Fig. 11
As Fig. 7, but for the north-south wind case.
12
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