Jökull - 01.07.2003, Blaðsíða 40

Achim A. Baylich
blown free of snow, whereas leeward areas, gullies,
channels, and concave slope surfaces are character-
ized by snow accumulations and snow beds. In Lat-
njavagge the predominant wind direction in winter is
generally N-NW, causing snow beds to form in almost
exactly the same places year after year (Molau, pers.
comm. 2000; 2003). These variations in snow cover
are of considerable geomorphologic significance, be-
cause they lead to a spatial differentiation of the types
and intensities of processes operating in autumn, win-
ter and during snow melt. Whereas the snow covered
areas are to a large extent protected against variations
of the air temperature and display almost no geomor-
phological activity until snow melt, frost and wind can
act on exposed, convex slope surfaces free of snow,
and on rock walls and rock ledges.
In the Austfirðir Mountains the high autumn and
winter wind speeds coincide with a large number of
daily freeze-thaw events causing needle ice (see be-
low). Apart from the progressive destruction of the
vegetation cover by turf exfoliation (Troll 1973; Ger-
rard 1991), the deflation of fine material here results
in the development of stone pavements, due to the rel-
ative accumulation of coarse components. In both en-
vironments fine material blowing away from convex
slope surfaces is accumulated in the snow in neigh-
bouring leeward areas, in gullies, channels and con-
cave slope areas, remaining there until snow melt. On
the east- facing slope in Latnjavagge and in gullies of
the Austdalur slope systems, characterized by accu-
mulations of wind-blown snow, geomorphologically
effective ground avalanches occur during snow melt
in early summer. Furthermore, in slope areas cov-
ered by wind-blown snow during autumn and winter,
slope wash processes induced by snow melting have
a higher intensity and last distinctly longer than in
slope areas where snow is blown away in autumn and
winter. During summer, after several successive days
without precipitation, vegetation-free regolith dries up
and deflation results. In both environments strong
gusts can trigger secondary rockfalls from rock walls
and rock ledges.
In Austdalur, N- and E-winds lead to an increas-
ing supply of moistness, and cause considerable at-
mospheric salt supplies (approx. 19 tons km
yr in the coastal area (Beylich 1999a). In Latnjavagge,
the annual atmospheric salts supply is about 3.4 tons
km
yr (Beylich et al. 2003; compare with Dar-
mody et al. 2000) .
GEOMORPHOLOGICALLY RELEVANT AS-
PECTS OF THE TEMPERATURE REGIMES
Latnjavagge
The Latnjajaure Field Station (981 m a.s.l.) is char-
acterized by an annual mean temperature of -2.3ÆC
(1993–2001). The range between the highest recorded
annual mean temperature of -1.95ÆC in 1997 and
the lowest of -2.90ÆC in 1995 is 0.95ÆC. July is
the warmest month of the year (8.0ÆC). The coldest
month is February (-10.1ÆC). The study area has an
ET climate according to Köppen (1936). The monthly
mean temperatures are above freezing point between
June and September. Early summer snow melt nor-
mally starts at the end of May/beginning of June.
Rapid snow melt in May/June can trigger geomor-
phologically effective slush flows (see also Nyberg
1985; Gude and Scherer 1999). Stable freezing tem-
peratures with little daily fluctuation at 10 cm above
ground and autumn snow accumulation usually oc-
cur from September-October onwards. A stable snow
cover >10 cm in areas without strong snow redistri-
bution by wind is normally recorded from October
onwards. At Latnjajaure Field Station frost events
may occur all through the year. The average an-
nual number of frost days is 267 (Figure 4). The
months of July and August are the only months nor-
mally free of frost. From the end of September to
the beginning of October until the end of May to the
beginning of June there is a phase of nearly perma-
nent winter frost. The 267 frost days consist of 188
ice days and 79 freeze-thaw days (Figure 4). The
months of November to April predominantly have ice
days. Only in July and August are there no ice days.
Most of the freeze-thaw days occur in May-June and
September-October. These are the interseasonal pe-
riods between the several months lasting winter frost
phase and the summer months of July and August be-
ing largely free of frost. The minimum air tempera-
tures reached on freeze-thaw days are mostly between
0ÆC and -10ÆC (Figure 5). Due to the frost sensitive-
38 JÖKULL No. 52, 2003