Geoscience Reference
In-Depth Information
in advection between the mean clear and cloudy cases, then the reduction in the
longwave radiative cooling rate will result in a warmer atmospheric profile. As the
difference in the effective emissivity between the surface and temperature maxi-
mum layer is reduced, radiative equilibrium requires that the difference in surface
temperature and that of the temperature maximum is less than for clear skies with
the shape of the profile adjusting accordingly.
The surface net radiation budget turns positive in spring in response to solar
input, helping to break down the inversion structure. The seasonal increase in cloud
cover assists in this process. Once the snow over the tundra melts, the strength and
frequency of inversions decline as the upward turbulent sensible heat flux becomes
stronger. Over Arctic lands during summer, convection with consequent release of
latent heat is not unusual. Over sea ice, summer melting of the surface means that
the upward longwave radiation flux cannot vary. One result of this is that the sur-
face cannot adjust to achieve quasi radiative equilibrium. However, the fixed sur-
face temperature (~0ºC), is often associated with shallow surface-based inversions
(Busch et al.,
1982
).
As discussed in
Chapters 1
and
2
, the strong downward trend in end-of-summer
sea ice extent has led to an increase in seasonal heat uptake in the ocean mixed
layer. This fosters large upward radiative and turbulent energy fluxes in autumn
and to a lesser extent winter, which is recognized as a major driver of observed
strong rises in air temperature, largest at the surface and larger than for the Northern
Hemisphere as a whole (termed Arctic amplification). This leads to a reduction in
autumn and winter inversion strength over the Arctic Ocean.
5.11
Radiation-Climate Feedbacks
5.11.1
The Concept of Feedbacks
The basic idea behind a feedback is that an initial perturbation to the climate sys-
tem can be either amplified (a positive feedback) or dampened (negative feedback)
through interactions with other climate variables. W. Kellogg (
1973
) developed a
simple conceptual framework to understand feedbacks, which still finds wide use.
M. Schlesinger (
1985
) provides a formalized description for use in surface energy
balance models. As outlined in the review by Curry et al. (
1996
), there are a num-
ber of radiation-climate feedbacks identified as potentially important in the Arctic:
(1) ice-albedo feedbacks; (2) cloud-radiation feedbacks; (3) water vapor feedbacks;
(4) cloud-temperature feedbacks; and (5) cloud phase and precipitation feedbacks.
The first three are very much tied into the surface energy budget and are briefly
reviewed.
5.11.2
The Ice-Albedo Feedback
This is probably the best known of climate feedbacks. If the climate warms, the
extent of snow and ice cover will decrease. The earth's surface albedo decreases,
