Fróðskaparrit - 01.01.1998, Síða 175

SEINHOLOSENA VEÐURLAGIÐ ELVDITIL JARÐFORMANDI VIRKSEMI í FØROYUM [ g ]
Assuming the meteorological Tórshavn
observations to be roughly representative
for regional conditions within the Faroe Is-
lands, these data might provide a basis for
an attempt to evaluate the likelihood of pe-
riodical late Holocene perennial snow sur-
vival and associated geomorphic effects in
the highlands. Figure 6 shows a plot of the
mean summer (June-August) air tempera-
ture versus the annual precipitation for all
Tórshavn data (filled triangles), together
with analogue meteorological data repre-
senting conditions at the equilibrium line at
various modern glaciers world-wide (grey
circles), where mass balance measurements
are being carried out, using data from
Ohmura et al. (1992). From a geomorphic
point of view, glacier equilibrium lines are
important as any incipient glaciation, in
general, requires the existence of an equi-
librium line at the terrain surface. The rate
of cold-climate bedrock weathering is also
known to reach a maximum for altitudes
near the ELA, wherefore this represents a
geomorphic zone of high importance for
understanding long-term, high-relief land-
scape evolution (Humlum, unpubl.).
Average annual accumulation on a glaci-
er approximates closely to the annual pre-
cipitation at the equilibrium line, while ab-
lation is most usefully predicted by the
mean summer air temperature (Sutherland,
1984). As is seen from Figure 6, there is a
close, non-linear, empirical relationship be-
tween precipitation and the mean summer
air temperature at the equilibrium line
(Ohmura et al., 1992). This positive corre-
lation between glacier mass-balance input
(accumulation) and summer air tempera-
ture (a substitute for ablation) is known to
apply both spatially and temporally within
many regions with modern glaciers (see,
e.g., Oerlemans, 1982; Sutherland, 1984,
Muszynski and Birchfield, 1985; Brom-
wich, 1988; Fortuin and Oerlemans, 1990).
The variation of the AD 1867-1995
Tórshavn meteorological observations sig-
nals a considerable spread of associated
mass balance characteristics for Faroese
snow accumulations during the instrumen-
tal period (Fig. 6). Some of the best years as
seen from a positive mass balance point of
view (reflecting good possibilities for snow
survival) are represented by AD 1868,
1869,1885, 1888, 1902and 1907, whilees-
pecially bad years are represented by AD
1880, 1889, 1933, 1934, 1941 and 1960.
From Fig. 2 is seen that the years with good
regional conditions for survival of snow in
general are coinciding with cold summers,
but that a number of even colder summers
in the early AD 1920s are not represented
within this group owing to low annual pre-
cipitation. Likewise, the years signalling
poor regional possibilities for survival of
snow are generally associated with warm
summers, even though some years with hot
summers are not represented due to high
annual precipitation. In particular the peri-
od AD 1930-47 appears to have provided
extraordinary bad conditions for snow sur-
vival in the Faroese highlands.
Figure 6 also illustrates an attempt to es-
timate climatic conditions in the Faroese
highlands (800 m a.s.l.) during the instru-
mental period. By assuming a mean annual
vertical lapse rate of 0.0065°Cm‘l, the tem-
perature adjusted plot (squares) in Fig. 6