Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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What Causes Stagnation of the Baltic Sea Deepwater?
H.E. Markus Meier 1 and Frank Kauker 2
1 Swedish Meteorological and Hydrological Institute, Rossby Centre, SE-60176 Norrköping, Sweden, markus.meier@smhi.se
2 Alfred Wegener Institute for Polar and Marine Research, Bussestr. 24, PoBox 120161, D-27515 Bremerhaven, Germany
1. Introduction
The Baltic Sea is one of the world's largest brackish-water
sea areas, with large horizontal and vertical salinity
gradients. For the past century the average salinity amounts
to about 7.4‰ (Meier and Kauker, 2003a). A large net
freshwater supply mainly from river discharge of about
15,000 to 16,000 m 3 s -1 in combination with the hampered
water exchange through the Danish Straits (Fig.1) causes
this low salinity. Decadal variations of the average salinity
are of the order of 1‰, and no long-term trend is detectable
during 1902-1998 (Fig.2).
Figure 1. Bottom topography of the Baltic Sea including
Kattegat and Skagerrak. In addition, the location of the
section of Figure 3 is shown.
The bottom water in the deep sub-basins is ventilated mainly
by major Baltic saltwater inflows. These events occur
randomly during the winter half year at intervals of one to
several years (e.g. Fischer and Matthäus, 1996). Since the
mid-1970s, the frequency and intensity of major inflows has
decreased. They were completely absent between February
1983 and January 1993. During this phase a significant
depletion of salt and oxygen occurred, and an increase of
hydrogen sulphide were observed in the deep layer of the
Gotland Basin. In this study possible mechanisms for the
decreased frequency of major inflows are investigated.
Figure 2. 4-year running mean simulated salinity in the
Baltic Sea (in ‰): reference run (black), sensitivity
experiment with climatological monthly mean freshwater
inflow (red), and sensitivity experiment with
climatological monthly mean freshwater inflow and 4-year
high-pass filtered sea level pressure and associated surface
winds (blue).
2. Method
Hindcast simulations for the period 1902-1998 have been
performed using the Rossby Centre coupled ice-ocean
model (RCO) for the Baltic Sea with a horizontal
resolution of 2 nautical miles (Meier, 2001; Meier and
Faxén, 2002; Meier et al., 2003). Daily sea level
observations at the open boundary in Kattegat, monthly
basin-wide discharge data, and reconstructed atmospheric
surface data have been used to force RCO. The
reconstruction utilizes a statistical model to calculate daily
sea level pressure and monthly surface air temperature,
dew-point temperature, precipitation, and cloud cover
fields (Kauker and Meier, 2003). For 1980-2002 an
additional simulation for validation purposes was
performed using atmospheric forcing calculated from 3hourly
gridded observations from all synoptic stations
available at SMHI.
3. Validation
Here, only one example from the inflow event 1993 is
presented. Figure 3 shows a cross section of salinity from
Fehmarn Belt to Stolpe Channel through Arkona and
Bornholm Basin between February 14 and 17, 1993.
Simulated maximum bottom salinities at Bornholm Deep
are 19‰ whereas 19-20‰ were observed. The model
overestimates entrainment and consequently
underestimates the salinity in Bornholm Deep slightly.
Strikingly good is the correspondence between model
results and data concerning the gradients across the
halocline. The halocline is very sharp in the Arkona Basin
separating the 10 m thick bottom pool from the surface
water whereas in the Bornholm Basin the gradient is much
smoother. For details, the reader is referred to Meier et al.
(2003).