Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Upwelling in the Baltic Sea - A Numerical Model Case <strong>Study</strong>
Andreas Lehmann and Hans-Harald Hinrichsen
Institute for Marine Sciences Kiel, alehmann@ifm-geomar.de
1. Introduction
Up- and downwelling are typical phenomena in the Baltic
Sea. Because of the complex coastline and many islands,
wind from any direction causes up- and downwelling near
the coast. The extent of upwelling is scaled by the internal
Rossby radius which is about 2-10 km in the Baltic Sea.
During summer and autumn when the sea surface is warm,
upwelling can be observed as a local temperature drop of
several degrees by infrared satellite measurements
(Gidhagen, 1987). Cold water from below the thermocline is
lifted upwards and eventually reaches the surface, where it
replaces a well-mixed and considerably warmer upper layer.
Upwelling is forced by sudden storms or strong wind events
form different directions, with typical time scales ranging
from a few days up to week. Satellite data indicate that the
horizontal scales of coastal upwelling are of the order of 100
km alongshore and some 10-20 km in the direction out from
the coast. Sometimes upwelled water is spread several tens
of kilometers out into the basin, forming filaments of cold
water (Gidhagen, 1987).
Different upwelling events occurring in 1997 have been
analyzed and compared. Satellite images have been used to
identify strong upwelling along the coast and to estimate the
proper temporal range as well as the extent of the affected
area. The different upwelling events have been further
analyzed by utilizing modeling results of a coupled sea iceocean
model run for 1997. From the numerical model
simulation the upwelling process can be analyzed in detail
and the corresponding volume transports along and offshore
the coast can be determined, thus quantifying coastal
upwelling. To distinguish the effects of upwelling from
other processes, a numerical model case study has been
performed. Different experiments have been carried out
where a sudden constant wind affects the Baltic Sea for one
week. Then the wind was switched off and the experiments
lasted three more weeks.
2. Numerical model case study
To separate the upwelling process from other processes 11
experiments were carried out, where the Baltic Sea response
to different constant wind forcing was analyzed. Figure 1
shows the anomaly of the sea level along the coast of the
Baltic Sea. The course starts in the Mecklenburg Bight and
ends in the southern Kattegat. For the first week constant
wind of 10 m/s from the north was specified. After seven
days, the wind was switched off and the experiment lasted
three more weeks. Because of the sudden onset of the wind
seiches were excited. In case of upwelling the sea level
drops along the coast (coast of Baltic Countries), for
downwelling an increase of sea surface height occurs.
Figure 1. Anomaly of sea level along the coast of the
Baltic Sea.
When the wind vanishes the sea level returns to zero
elevation.
A quantification of alongshore and offshore transports
have been performed for the different experiments.
Generally in the Central Baltic Sea, highest alongshore
transport for the near surface waters result when winds are
mainly directed off- or onshore, while alongshore
transport below the near surface layers is lower in
magnitude and often is into the opposite direction. If
winds are more orientated along the coasts, the alongshore
transports have the tendency to be vertically more uniform
distributed and are associated with upwelling events if the
coast is located to the left hand side of the wind direction
and vice versa. Strongest off- and onshore currents in the
surface layers are mainly due to Ekman transports with the
main wind component parallel to the coast. Generally, the
flow below the surface layers are directed into opposite
direction to compensate the surface currents.
References
Gidhagen, L., Coastal upwelling in the Baltic Sea -
Satellite and in situ Measurements of sea surface
temperatures indicating coastal upwelling, Estuarine ,
Coastal Shelf Science, 24., 449-462, 1987.