Geoscience Reference
In-Depth Information
Figure 9.2.
Schematic of the Bitz et al.
(
1996
) single-column atmosphere-sea-ice-
upper ocean climate model. F
sw
and F
olr
are
incoming shortwave and outgoing longwave
radiation, respectively. D is the atmospheric
energy flux convergence, F
w
is the ocean
heat flux, T
a
is the surface air temperature,
T
s
is the sea ice/snow temperature, T
1
is the
temperature at the top of sea ice layer 1, T
b
is
the temperature of the ocean mixed layer and
the bottom temperature of the sea ice, h is the
thickness of sea ice, and h
s
is the thickness of
snow (from Bitz et al.,
1996
, by permission
of AMS).
scattering of solar radiation. Clouds are modeled as a single layer. Along with hor-
izontal energy transports, the model includes vertical convective transport of latent
and sensible heat, and the radiative effects of ice crystal precipitation.
One of the interesting results from this study is that the volume of perennial sea
ice is simulated to vary predominantly on decadal time scales. As assessed from
sensitivity studies, variations in ice volume are most sensitive to perturbations in
atmospheric forcing not in winter, the period of ice growth, but in late spring, at
the onset of melt, pointing to the importance of ice-albedo feedbacks. The model
calculations suggest that natural variability in the thermodynamic forcing on Arctic
Ocean sea ice volume could (by variations in sea ice export from the Arctic Ocean)
lead to freshening of the North Atlantic comparable that associated with the Great
Salinity Anomaly (see
Chapter 7
).
The ISCCP-D and APP-x surface radiation flux fields outlined in
Chapter 5
repre-
sent the application of a single-column model to a two dimensional grid. Another
example of this general strategy is the study of C. Oelke et al. (
2003
). They simu-
lated the active layer of permafrost over the Arctic terrestrial drainage at a 25 × 25
km resolution using a heat conduction model with phase change developed by L.
Goodrich (
1982
). Soil is divided into layers with different thermal properties for
frozen and thawed soils. Information on the spatial distribution of soil bulk density,
and the relative compositions of clay, silt, and sand that influence thermal conduc-
tivity are taken from existing maps. Soil water content varies with each layer. Initial
soil temperatures were chosen according to the permafrost classification of the grid
cells based in the International Permafrost Association map (
Figure 2.15
).
The primary forcings for the model are daily near-surface air temperature (SAT)
and snow thickness. SAT was based on fields from the NCEP/NCAR reanalysis,
adjusted to address better the effects of topography. Snow thickness was obtained by
adjusting SSM/I snow water equivalent retrievals by climatological snow densities
