Geoscience Reference
In-Depth Information
−
∇
•
F
A
is large, it is not changing much. F
sfc
remains positive, primarily as sensible
heat continues to leave the ocean reservoir and sea ice growth continues. The mean
winter (December through February) surface flux 55 W m
−2
includes the marginal
seas, where values are comparatively high. Surface-based measurements by Maykut
(
1982
) indicate that the upward flux over the central Arctic pack ice is closer to
21 W m
−2
.
Spring approaches and the change in energy storage of the atmosphere turns pos-
itive, allied with an increase in the longwave loss to space. This is because of the
growing input of solar radiation and the continued heat inputs by the surface flux.
However, the effect of the higher solar declination is strongly offset by the high
planetary albedo, largely owing to the extensive sea ice, snow cover, and clouds. But
as the atmosphere warms,
−
∇
•
F
A
begins to decline. By April, the positive change
in atmospheric energy storage attains its seasonal maximum owing in large part to
the much stronger inputs of solar radiation, related to both the increase in solar dec-
lination and the first hints of a reduction in the planetary albedo. The strong energy
gain of the atmosphere continues in May, but by June, when the solar input is larg-
est, the rate of gain begins to decline. This occurs in part because of a weakening
of the atmospheric circulation, and in part because of the net surface flux. By June,
the snow and sea ice are melting, drawing heat from the atmospheric column. A
large part of the solar input is also used to warm open water areas. The land surface
also gains heat. In July, the change in atmospheric storage is close to zero, as is the
TOA net radiation budget. For this month, the budget is determined by the opposing
effects of the atmospheric transport and the net surface flux.
The planetary albedo is at its minimum in July and August. This primarily relates to
the exposure of snow-free land, open ocean waters, and the removal of snow cover from
the remaining sea ice cover. These influences outweigh the effects of the summer maxi-
mum in (highly reflective) cloud cover seen over most of the Arctic (see
Chapter 2
). The
lower albedo of the sea ice cover does not result in sensible heating of the atmosphere
(when melt is occurring, the skin temperature is fixed to the freezing point). By contrast,
the stronger absorption of solar radiation by the sea ice promotes stronger ice melt,
which represents a loss of energy in the atmospheric column.
As part of their earlier study, Nakamura and Oort (
1988
) compared the energy
budgets for the north and south (Antarctic) polar caps. The annual cycles for the
two polar caps, while similar in a qualitative sense, were found to differ markedly
in the magnitude of the fluxes. Of interest is that the surface fluxes are much larger
in the Arctic. In the Arctic, the seasonal variation in F
sfc
was found to account for
about 70 percent of the amplitude of the net radiation at the top of the atmosphere
(R
top
). What this means is that the ocean-cryosphere heat reservoir is as important as
the atmospheric influx of energy from the middle latitudes in compensating for the
radiation loss at the top of the atmosphere. In the Antarctic, the seasonal variation
in the surface flux accounts for only a small fraction of the variation in R
top
. As dis-
cussed by Nakamura and Oort (
1988
), the differences in budget terms between the
two polar caps are not surprising given the radically different geography. Whereas
the north polar cap is relatively flat and primary ocean covered, the south polar cap
is a high-elevation, glacier-covered region.
