Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 158

104mm/year), Northern Canada (e.g. Resolute Bay 130 mm/year) and the
central Arctic Ocean (Russian drifting stations 100 mm/year) report even
less. In other parts of the world, areas with such small amounts of precipita-
tion would be considered deserts.
Snow covers the tundra for 8-9 months of the year with a relatively shallow
snow cover, typically of 30 cm thickness. The transition in spring (June) is
fast and impressive. While snow is present, most of the solar radiation is
reflected back to space, while in contrast to this, the bare tundra absorbs some
80% of the radiation. This leads to a sudden temperature increase (Weller and
Wendler 1990 ), and the tundra becomes green and starts flowering. This
transition period is very short, but the ''summer'' last only 3 months. Further,
as can be seen from Table 5.3 , the highest monthly mean temperature is less
than 10 8C. In recent decades, the spring melt has occurred somewhat earlier
in the western sector of the Arctic. This might have been caused by the
observed temperature increase, which was well pronounced for the spring.
On the other hand, the annual and winter precipitation have decreased for this
area. This would also lead to an earlier break-up, more positive surface
energy budget, and warmer temperatures. So it is difficult to judge if the
warmer temperatures are the sole cause of the earlier break-up.
Other atmospheric influences
In general, the air in the Arctic is clean and visibility in winter frequently
exceeds 100 km. However, in spring ''Arctic haze'' can often be observed
(see Bokoye et al. 2003 for a useful summary and references). The existence
of Arctic haze has long been known, and some 50 years ago the US Air Force
investigated it with the so-called Ptarmigan flights, which originated in
Central Alaska and went over the Arctic Ocean. In the meantime, chemical
analyses showed that these are products consisting of mid-latitude emissions
from fossil fuel consumption, smelting, and industrial processes. Large
source regions are Europe and Russia, and the pollutants are transported to
the Arctic in spring, when the washout due to precipitation is minor and the
meridional temperature gradients and with it the circulation intensity are
strong. Further, sand particles from the Gobi desert have been observed in
Alaska, again indicating that the air over the Arctic is well connected with the
lower latitudes.
Permafrost, in some areas hundreds of meters in thickness, underlies the
tundra. The active layer, the layer that annually thaws, is only 30-50 cm deep.
This underlying permafrost prevents drainage of surface water, and the tundra
is spotted with lakes and the surface is wet. Hence, the term ''polar desert,''
which is sometimes used, is not appropriate, as desert implies the lack of
water (K¨ppen 1936 ), and no river can originate in such areas. Permafrost
temperatures have in general increased, and in areas where permafrost is