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Simulating cold palaeo climate conditions in Europe with a regional climate
model
Gustav Strandberg, Jenny Brandefelt, Erik Kjellström and Benjamin Smith
Gustav Strandberg, SMHI, SE-601 76 Norrköping, Sweden, gustav.strandberg@smhi.se
Jenny Brandefelt, Dept. of Mech. KTH, SE-100 44 Stockholm, Sweden
Erik Kjellström, , SMHI, SE-601 76 Norrköping, Sweden
Ben Smith, Dept. of Physical Geography and Ecosystems Analysis, Lund University, SE-223 62 Lund, Sweden
1. Introduction
A fully coupled atmosphere-ocean general circulation model
(AOGCM) is used to simulate Last Glacial Maximum
(LGM, ~21ka BP) and conditions representative of a stadial
during Marine Isotope Stage 3 (MIS 3, ~44ka BP). Both
periods are much colder than today. Selected periods of
these runs are dynamically downscaled with a regional
climate model (RCM) operating on 50 km horizontal
resolution over Europe. The high resolution of the RCM
simulation is important for a better representation of the
boundary conditions (e.g. the topography associated with the
ice sheets covering Scandinavia during these periods).
Simulating past climates can be a way to evaluate the
models, firstly to see if it is able to reproduce a climate very
different from today at all, and also by comparing model
results with climate reconstructions of the periods.
temperature below 0˚C. Precipitation is lower compared
with RP conditions in northern Europe, partly because of
changed topography, and partly because of reduced
evaporation from the Atlantic, which to a large degree is
ice covered in the winter (not shown). However,
precipitation in LGM compared with RP is larger on the
edge of the ice sheet northwest of Fennoscandia and the
British Isles as a result of the orographic forcing. In
southern Europe, the Iberian Peninsula and the southern
Alps and Italy gets more precipitation than in the RP
climate due to changed atmospheric circulation in winter
(southward shift of the North Atlantic storm track).
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Figure 1. Land (green) and ice extent (blue) in RCA3
in Europe in LGM (left) and MIS 3 (right). Grid boxes
with a land fraction lower than 20% are not filled.
2. Method
The AOGCM (CCSM3) was run for more than 1000 model
years to get a climate in equilibrium for each period. Time
slices of 50 years were selected for downscaling with the
RCM (RCA3). The results from the RCM were used in a
vegetation model (LPJ-GUESS) to get vegetation that is in
accordance with the simulated climate. The new vegetation
is then used in a second RCA3 run. LGM is characterised by
a large ice sheet. MIS 3 is also a cold period, but with a
smaller ice sheet (Fig. 1). The CO 2 levels are set to 185
(200) ppm for LGM (MIS 3). Because of the large ice sheets
the sea surface in both periods is much lower than today,
exposing new land areas. See Kjellström et al. (2009) for
further details on the simulations.
3. Results
LGM is much colder than the recent past (RP, 1961-1990).
RCA3 simulates temperatures of the coldest month at least
25˚C colder than RP conditions in Scandinavia and around
5˚C colder around the Mediterranean Sea (Fig. 2). In
summer Scandinavia is around 15˚C colder than the RP and
the area around the Mediterranean Sea about 5-10˚C colder
(Fig. 3). Europe north of Paris has an annual average
Figure 2. Mean temperature of the coldest month in
LGM as compared to recent past climate.
Corresponding temperatures as given by proxy
based reconstructions are denoted in the filled
circles. Units are ˚C.
The simulation is colder than SST proxies over the North
Atlantic, possibly indicating a too cold climate in the
region and also potentially in northern Europe. Compared
to proxy-based data from southern Europe the regional
climate model show a good agreement for all seasons
except summer when it is colder by some 2-5˚C than the
reconstructions show (Fig. 2). A comparison to proxy data
of precipitation in southern Europe for LGM conditions
shows that the geographical distribution of simulated
changes in precipitation is similar to the proxy data.
The simulation of MIS 3 is the first long run with a fully
coupled AOGCM. CCSM3 simulates a cold, but not as
cold as LGM, climate compared to RP conditions. This is
manifested in the downscaling experiment with RCA3 as
exemplified by the 0°C isotherm for annual mean
temperature that goes south of Ireland, through England
and the southern parts of Denmark, just south of Sweden
and then eastwards (not shown). All Europe but the
Mediterranean area has average winter temperatures below
zero (Fig. 4). In summer only the ice covered areas are