Fourth Study Conference on BALTEX Scala Cinema Gudhjem

4. Discussion
The discrepancy between the simulated changes in
precipitation lies partly in the simulated changes in SSTs.
The global driving model from the Hadley Centre simulates
a very large increase in SST over the Baltic Sea. On average
it is 6 O C in the summer in the A2 simulation as compared to
the control experiment. This very strong warming leads to
an increase in the evaporation over the sea and thereby to an
intensified hydrological cycle. The RCMs that use these
high SSTs consequently simulate increased precipitation
particularly over the Baltic Sea (Figure 1). In the RCAO on
the other hand, with its coupling between ocean and
atmosphere, the SSTs are calculated to increase more
moderately since heat is exported downwards into the ocean.
It is found that the difference in SST between the uncoupled
(RCA) and coupled (RCAO) model is small in summers
when there is a relatively strong north-south pressure
gradient over the Atlantic. Not surprisingly the influence of
the Baltic Sea on the regional climate is small when the
large scale atmospheric circulation is efficient in
transporting air masses across the Baltic Sea. On contrary,
during summers with more stagnant flow conditions the
influence of the Baltic Sea on the regional climate can be
much larger. Figure 3 illustrates the large difference in SSTs
between the uncoupled and coupled models in the latter type
of conditions.
Figure 3. Summertime (JJA) SST in the Baltic Sea and
Kattegat during years with a strong north-south pressure
gradient over the Atlantic. Shown is RCA (left), RCAO
(middle), and difference (right).
Finally, it should be noted that the differences in SSTs
between the different models can not fully explain the
difference in precipitation over the Baltic Sea. It can be seen
from Figure 2 that even if the models use the same SSTs the
response of the temperature at the 2m level can be very
different, at most it is about 1 O C between the models using
the HadAM3H SSTs. Such large differences in temperature,
and also other climate variables, must be generated by
differences in model formulation.
5. Acknowledgements
This work was carried out within the SWECLIM
programme. Financial support from the Foundation for
Strategic Environmental Research (Mistra) and the Swedish
Meteorological and Hydrological Institute (SMHI) is
gratefully acknowledged. It is also a part of the European
PRUDENCE project (project EVK2-CT2001-00132 in the
EU 5 th Framework program for Energy, environment and
sustainable environment. The HadAM3H data used as
boundary conditions for the RCAO-simulations were
provided by the Hadley Centre of the Meteorological Office
(U.K.), the ECHAM4/OPYC3 data by the Max Planck
Institute for Meteorology in Hamburg (Germany) and the
- 168 -
Danish Climate Centre of the Danish Meteorological
Institute.
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