Geoscience Reference
In-Depth Information
Table 3.1
Monthly and annual mean components of the atmospheric energy budget of the
north polar cap domain
Month
Fluxes and Storage Changes (W m
−2
)
∂A
E
/∂t
a
R
top
−
∇
•
F
A
F
sfcc
b
Plan. Albedo
c
Res
.
January
−2
−175
[−173]
108
56
-
−9
February
5
−171
[−169]
112
51
-
−13
March
12
−143
[−145]
110
40
0.71
−5
April
25
−88
[−92]
92
17
0.66
−4
May
21
−27
[−37]
66
−18
0.64
0
June
18
23
[18]
89
−70
0.54
24
July
1
11
[21]
94
−85
0.45
19
August
−17
−66
[−58]
98
−39
0.48
10
September
−27
−145
[−140]
106
17
0.55
5
October
−22
−183
[−180]
114
53
-
6
November
−11
−184
[−180]
105
55
-
−13
December
−3
−178
[−174]
111
58
-
−6
Mean
0
−110
[−109]
100
11
-
−1
a
Values in brackets are from CERES for the 2000-2005 period, from Porter et al. (
2010
)
b
Planetary albedo from the AVHRR Polar Pathfinder Project
c
Energy budget residual in ERA-40 calculated as
R
top
−
∇
•
F
A
+
F
sfc
-
∂A
E
/∂t
Sources:
From Serreze et al. (
2007
) and Porter et al. (
2010
).
most negative in autumn when the solar flux is decreasing rapidly. However, it is
clear from the R
top
values in
Table 3.1
, that - except for June and July - the system
actually departs quite radically from radiative balance, with longwave loss to space
exceeding the shortwave gain. The obvious explanation is that we have ignored
−
∇
•
F
A
and F
sfc
. We have furthermore ignored seasonality in the planetary albedo.
Consider autumn. The TOA net solar flux is small and becoming smaller, result-
ing in a decrease in the energy content of the atmosphere. The decline in the solar
flux and hence the cooling of the atmospheric column is greater in higher as com-
pared to middle latitudes. Hence the zonal mean temperature gradient in the atmo-
sphere increases (temperatures decreasing to the north). This means that
−
∇
•
F
A
increases, adding energy to the system to slow the cooling rate. As the atmosphere
continues to cool, the net surface flux turns positive. The ocean loses sensible heat
to the atmosphere. With continued cooling, sea ice forms, also releasing heat into
the atmosphere to slow the cooling rate. Hence, although the atmosphere is cooling
strongly in autumn, the braking effects of
−
∇
•
F
A
and F
sfc
mean that it cools at a
slower rate than expected from the decline in the solar flux. While over land, the soil
column is cooling and lakes and rivers are freezing, these effects on F
sfc
for the polar
cap are minor compared with the ocean heat losses (Serreze et al.,
2007
).
Autumn turns to winter. The atmosphere continues to lose energy, but as the win-
ter solstice approaches, the rate slows. In part, this is because there is little change
in the net solar flux (there is little solar radiation from October through February).
The atmosphere is cold, but not becoming a great deal colder, so that although
