Geoscience Reference
In-Depth Information
horizontal divergence of the latent heat flux as sea ice; and (3) the net
surface heat flux through which the atmosphere and ocean communicate.
If the underlying column represents land, the situation is simpler. Here,
the time changes in energy storage, primarily contained in the latent heat
content of snow and ice and sensible heat in the soil column, is essentially
balanced by the net surface flux.
We first consider conditions averaged over a simple polar cap domain,
extending from 70
o
N to the pole. For long-term annual means, the change
in atmospheric energy storage is approximately zero. This is not true,
however, when we consider the annual cycle. The atmosphere gains
energy during spring, but there is a loss in autumn. This is manifested
in annual cycles of atmospheric temperature, humidity, and the height
of atmospheric pressure surfaces. In winter, incoming solar radiation
is negligible and there is a strong net radiation deficit at the top of the
atmosphere driven by the longwave radiative losses. The net radiation
deficit is mostly balanced by large poleward atmospheric energy
transports, but also by a significant heat input from the underlying column
to the atmosphere by the upward net surface flux. This is primarily
represented by the growth of sea ice and the release of sensible heat
stored in the ocean; for the polar cap as a whole, energy exchanges over
land play a secondary role. The incoming solar flux is largest in summer,
allied with a seasonal maximum in longwave loss to space. However, the
net radiation balance at the top of the atmosphere is near zero in July.
Compared to winter, the atmospheric transports are smaller. For July, this
transport is approximately that which is needed to balance the downward
(into the underlying column) net surface flux, largely associated with
the melt of sea ice and sensible heat gain by the ocean. When attention
is focused to an Arctic Ocean domain, we find that the ocean gains heat
through both the horizontal convergence of oceanic heat transport and the
export of sea ice out of the Arctic. Together, these support a small annual
mean upward net surface heat flux, representing a heat source to the
overlying atmosphere.
3.1
The Arctic and the Global Energy Budget
3.1.1
The Radiation Balance
Considered as a whole and for long-term annual means, and assuming a station-
ary climate, the earth is in a state of radiation balance, meaning that the net solar
radiation at the top of the atmosphere (TOA) is balanced by the longwave radiation
emitted to space:
R
top
= (1-A) SπR
2
- σT
e
4
4πR
2
= 0
(3.1)
