Geoscience Reference
In-Depth Information
to isolate the air within it from external sources of advection (Overland, Miletta
Adams, and Bond,
1997
).
Temperatures rise to the melting point over the coasts by the end of May and
over most of the central Arctic Ocean by mid-June. The melt season over the central
Arctic Ocean is about 60 days long, compared to about 100 days over the coastal
sector from 90°E eastward to 120°W. July temperatures at the coast average about
5°C with a sharp rise inland over a distance of 10-20 km. Like spring, the lowest
temperatures are found over the Canadian Arctic Archipelago.
The preceding discussion must be viewed with the understanding that over the
past several decades, annual mean SATs over the Arctic as a whole have risen more
strongly than for the Northern Hemisphere as a whole, with the warming most pro-
nounced during autumn and winter over parts of the Arctic Ocean (
Figure 1.5
).
Although it is generally accepted that this strong warming (termed “Arctic ampli-
fication”) in part manifests forcing by increased concentrations of atmospheric
greenhouse gases, it appears that a suite of processes are at work, including (1)
reductions in sea ice extent, leading to stronger summer heat gain in the ocean
mixed layer, with this heat then released back to the atmosphere in autumn and win-
ter; (2) changes in atmospheric circulation leading to an increased atmospheric heat
flux convergence; (3) changes in cloud cover and water vapor; and (4) increased
concentrations of black carbon aerosols and soot on the snow cover (Serreze and
Barry,
2011
). These issues are expanded on in subsequent chapters.
A notable aspect of the Arctic atmosphere is the presence of strong low-level tem-
perature inversions during winter (situations in which temperature increases rather
than decreases with height). These surface or near-surface based features generally
extend to about 1,200 m during January-March, with a temperature difference from
the inversion base to top of typically 11°-12°C (Serreze, Kahl, and Schnell,
1992b
).
As examined in
Chapter 5
, the inversion layer tends to be maintained by an approx-
imate radiative equilibrium associated with the different longwave emissivities of
the surface (an approximate blackbody) and of the temperature maximum layer
aloft (a selective emitter), with northward heat advection countering the outward
radiation loss to space (Overland and Guest,
1991
). Especially strong winter inver-
sions are found inland in eastern Siberia and northwest Canada-Alaska in the deep
intermontane basins and valleys that characterize these regions. From May through
September, the Arctic is characterized by weaker, elevated inversions.
2.3.4
Humidity
Because of the low temperatures in the Arctic, especially during winter, the amount
of moisture in the atmosphere is quite limited. Near the surface, mean specific
humidity (the mass of water vapor per unit mass of air, including the water vapor)
for winter as averaged for the region north of 70°N is only about 1 g kg
−1
compared
to about 3-4 g kg
−1
in summer. The total water column vapor (or precipitable water,
the equivalent liquid depth of water vapor in an atmospheric column) between the
surface and 300 hPa, averaged for 70-90°N, ranges from about 2.5 mm in January
