Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 92 -
On Variability of the Riverine Waters in the Gulf of Gdansk - a Model
<strong>Study</strong>
Andrzej Jankowski
Institute of Oceanology of PAS., Powstańców Warszawy 55, 81-712 Sopot, Poland, e-mail: jankowsk@iopan.gda.pl
1. Introduction
A three−dimensional baroclinic σ-coordinate model was
used to investigate the development and evolution of
buoyant river plumes in the Gulf of Gdansk (Gdansk Basin).
Situated in the southern part of the Gdansk Basin, the Gulf
of Gdansk, is one of the Baltic's open gulfs. Its
hydrological regime is formed by atmospheric forcing and,
due to wide and deep connection with the open sea, under
influence of the Baltic proper (Majewski 1990). Its surface
waters are influenced by riverine inflows among which the
greater significance has those of the Vistula River (with
mean annual discharge about 1000 m3/s Cyberski 1997),
the biggest river in the region. The mixed river waters can
be observed at the open boundary of the Gulf of Gdansk or
even futher after the strong flood events. The inflow of
riverine waters substantially modifies the environmental
conditions in the surface water of the Gulf by reducing its
salinity as well as contaminating it with their pollution load
(Andrulewicz 1996, Cyberska and Krzyminski 1988, 1996).
The purpose of this study is to obtain more detailed
information on fate of riverine water as it mixes and moves
around with the currents and winds in the Gulf of Gdansk
and to test the capabilities of the model to simulate the
characteristic features of the spreading of the riverine waters
during summer flood event.
2. Model
The model is based on the Princeton Ocean Model (POM) -
code of Blumberg and Mellor 1987 adapted to the Baltic Sea
conditions (Jankowski 2002a). POM is based on a standard
formulation of the conservation equations for momentum
and mass, utilizing the hydrostatic and the Boussinesq
approximation. The model uses Smagorinsky (1963)
parameterization for the horizontal mixing (turbulent
exchange). To calculate the vertical eddy viscosity and
diffusion coefficients a second turbulence closure scheme
(Mellor and Yamada 1974, 1982) is applied. The model
domain comprises the whole Baltic Sea with the Gulf of
Bothnia, the Gulf of Finland and the Gulf of Riga as well as
the Danish Straits and Kattegat and Skagerrak. At the open
boundary in the Skagerrak simplified radiation-type
boundary conditions are applied. The bottom topography of
the Baltic Sea used in the model is based on data from
Seifert and Kayser (1995). A “C” numerical grid (Mesinger
and Arakawa 1976) is applied. With a horizontal resolution
of ≈ 5 km and with 24 σ-levels in vertical, the model
enables variability as well as mesoscale features of the order
of 10 km of currents and thermohaline fields in the Baltic
Sea to be investigated.
To date, the model has been investigated to simulate major
features of the coastal area of the Southern Baltic
(Jankowski, 2002a, 2002b).
3. Model experiments
The climatological forcings were coupled to the model by
means of the so-called method of ’relaxation towards
climatology’ ( cf. Lehmann 1995). Wind field is estimated
from atmospheric surface pressure charts and surface heat
fluxes at the sea surface are calculated with standard bulk
formulae.
In order to simulate the characteristic features of wind-
and river inflow-induced variability of hydrodynamic
conditions in the Gdansk Basin, the prognostic hindcast
calculations were performed for summer period of 1980
(summer flood event in the Vistula catchment). Model
runs, starting with climatological montly means of
temperature and salinity were carried out for the period of
1 st July to 31 st August 1980. For this simulation the model
was forced by realistic forcing estimated on 3-hourly
atmospheric data (pressure, air temperature, relative
humidity and the wind - field) taken from (BED, 2000)
and by climatological forcings.
A comparison of computed and measured temperature and
salinity shows that the model reproduces the vertical
structure of seawater temperature and salinity in a good
agreement to the in situ observations.
Besides the realistic meteorological forcing the spatially
uniform winds from 8 directions and without wind were
also considered. The winds from 8 directions: SE, S, SW,
W, NW, N, NE and E, of constant speed over the whole
Baltic area. The wind stress was assumed to be in a range
from 0.025 to 0.1 N/m2.
4. Results and comments
The sea water salinity was used as a natural tracer for
visualization of spreading of riverine waters in the Gulf of
Gdansk. The numerical experiments visualize the
patterns of weak saline surface water (riverine plume)
propagation on their way from the Vistula mouth toward
the open boundary of the Gulf of Gdansk.
The results show that local hydrodynamics near the river
mouth, and consequently the spreading of the river plume,
are highly dependent on the driving river discharge and
wind field characteristics.
In the absence of wind forcing the modeled river plumes
typically consist of an offshore bulge and a alongshore
currents in the counterclockwise direction.
In the presence of wind forcing the favorable conditions
for offshore removal of coastal low-salinity waters include
favorable wind stress and circulation pattern highly
dependent on open sea hydrodynamic conditions.
The results of simulations with space uniform winds
showed that the surface water plume can be particularly
sensitive to the wind stress because it is thin. Its extent
is influenced by the wind stress and varied depending on
the orientation of the wind stress.
The values of area of mixing zone (salinity < 7 PSU) vary
from 1100-1200 km˛ (wind from E and NE) to 400-500
km2 (winds from W and SW). Under stronger winds the
area of mixing zone decrease almost twice. These findings
and general response of surface waters to river run-off
and winds are in agreement with in situ measurements(e.g.