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A coupled regional climate model as a tool for understanding and
improving feedback processes in Arctic climate simulations
Wolfgang Dorn, Klaus Dethloff and Annette Rinke
Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, 14473 Potsdam, Germany;
Wolfgang.Dorn@awi.de
1. Motivation
A realistic representation of sea ice in coupled climate
models is an essential precondition for reliable simulations
of the Arctic climate. Intercomparison studies of coupled
models have shown that there are still large deviations in the
simulation of Arctic sea ice among the models (e.g., Holland
and Bitz, 2003). The representation of the processes at the
interface between atmosphere and sea ice is often
oversimplified due to poor knowledge of the underlying
physics of feedback processes between the climate
subsystems. While such feedbacks are completely absent in
stand-alone models for the subsystems, their accurate
simulation plays a key role in the performance of coupled
regional and global models.
2. The coupled regional climate model HIRHAM-
NAOSIM
The coupled model is a composite of the regional
atmospheric climate model HIRHAM (Christensen et al.,
1996; Dethloff et al., 1996) and the high-resolution version
of the North Atlantic/Arctic Ocean sea-ice model NAOSIM
(Karcher et al., 2003; Kauker et al., 2003). Both model
components were well adapted for Arctic climate
simulations and successfully applied for a wide range of
Arctic climate studies.
Figure 1. Geographical locations of the coupled
model's atmosphere domain (HIRHAM) and its oceanice
domain (NAOSIM). The domain of coupling is
given by the overlap area and covers the whole Arctic
Ocean, including all marginal seas, the Nordic Seas,
and parts of the northern North Atlantic.
The coupled model system, first introduced by Rinke et al.
(2003), was described in detail by Dorn et al. (2007). The
integration domains of the two model components are
shown in Figure 1. The horizontal resolutions currently
used in the coupled model system are 0.5° (~50 km) in
HIRHAM and 0.25° (~25 km) in NAOSIM. In the
vertical, the atmosphere is subdivided into 19 unevenly
spaced levels in hybrid sigma-pressure coordinates and the
ocean into 30 unevenly spaced z-coordinate levels. In the
near future, a sophisticated land surface model (LSM)
with 6 soil layers will be incorporated into the coupled
model system too.
3. Improvement in the model representation of
feedback processes
Feedback processes, in which sea ice is involved, like the
ice-albedo feedback, play an important role in the Arctic
climate system and may be regarded as crucial factors in
the polar amplification of climate change. In order to
improve the simulation of the ice-albedo feedback, more
sophisticated schemes for the ice growth/melting, the
snow and ice albedo as well as the snow cover fraction on
ice have been implemented into HIRHAM-NAOSIM and
tested in a series of sensitivity experiments (see Dorn et
al., 2008b).
It is found that the simulation of Arctic summer sea ice
responds very sensitively to the parameterization of the
snow and ice albedo but also to a sub-grid-scale separation
of the heat fluxes within the ice growth scheme. The
parameterization of the snow cover fraction on ice plays
an important role in the onset of summertime ice melt.
This has crucial impact on summer ice decay when more
sophisticated schemes for ice growth and ice albedo are
used. It is shown that in case of using a harmonized
combination of more sophisticated parameterizations the
simulation of the summer minimum in ice extent can be
considerably improved due to a better timing of the snow
and ice ablating periods.
4. Linkage between atmospheric and sea-ice
variability
The patterns of maximum amplitude of interannual
variability of the Arctic summer sea-ice cover have been
analyzed in HIRHAM-NAOSIM simulations for the 1980s
and 1990s and compared with SSM/I satellite-derived seaice
concentrations. It is found that natural variability of the
summer sea-ice cover is almost exclusively restricted to its
peripheral zone. Furthermore, summers with low sea-ice
extent are associated with anomalously high atmospheric
pressure over the western Arctic Ocean.
The presence of a high pressure area over the Arctic
Ocean is usually related to lower cloud cover and more
anticyclonic and divergent ice motion accompanied by a
more pronounced transpolar drift and stronger sea-ice
export through the Fram Strait (see Figure 2). These
conditions represent favorable factors for low sea-ice
extent at the end of the summer. Dorn et al. (2008a) noted
that large-scale variations in the atmospheric circulation
are likely to represent the main driver for sea-ice
variability, also with respect to the strong decline of the
Arctic summer sea-ice cover in recent years.