Jökull - 01.01.2013, Side 96

H. Ágústsson et al.
on comparable elevations on Hofsjökull. This is rea-
sonable, given the location of Mýrdalsjökull close to
the coast and thus, earlier warming and onset of sum-
mer melting, and hence an earlier start of densification
of the winter snow there. Furthermore, due to its lo-
cation, Mýrdalsjökull will receive far larger amounts
of rain during winter than Hofsjökull. The rain will
refreeze within the snow pack (except during early
autumn) and contribute towards a higher density, in
particular for M1 which will tend to receive higher ra-
tios of rain vs. snow compared to M2 and M3 (wind
carries falling snow aloft further onto the plateau than
rain). The temperature of the winter layer at Mýr-
dalsjökull has normally been very close to the melting
point during spring measurements, whereas a week
earlier the winter cold wave is still present on Hofs-
jökull (Figure 2c). For further comparison, we plot
the average annual-layer density of Hofsjökull points
from the elevation interval 1250–1600 m a.s.l. on its
northern slopes versus the day of year of the measure-
ments in Figure 2d. The same graph also includes
the averages from the Mýrdalsjökull points. Although
based on few data points, this comparison confirms
qualitatively the difference in density increase during
spring on the two ice caps, as seen in Figure 2b.
The measured mass balance on Mýrdalsjökull is
shown in Table 2 and Figure 3. M0 was only sur-
veyed in 2001, when the winter mass balance was 5.8
mwe and summer mass balance -0.9 mwe. The great-
est winter balance (5.1–7.8 mwe) is observed at M1,
with a maximum thickness of nearly 13 m of winter
snow in 2007 but as little as 8.4 m in 2010. Summer
balance at M1 (-2.1 – -0.9 mwe) is most often simi-
lar to that observed at M2, where the most negative
summer balance has been measured (-3.1 mwe). At
site M2 the winter balance is on average near 5 mwe.
The lowest winter balance is observed at M3, varying
between 3.4 and 5.1 mwe, but some of the precipita-
tion may be lost by snowdrift. M3 is located at the
highest elevation of the sites, and consequently expe-
riences on average the least negative summer balance
(-2.4 – -0.9 mwe). The annual mass balance within
the accumulation area on the plateau shows large tem-
poral and spatial variability (2.1–5.9 mwe). The mea-
sured winter balance at the plataeu of Mýrdalsjökull is
significantly greater than at similar elevations on the
Figure 3. Observed winter balance (a), summer balance (b) and annual balance (c) as a function of elevation
[m] of sites on Mýrdalsjökull (2001, 2007–2011) as well as mean values for the same years from three sites
on Breiðamerkurjökull in Southeast-Iceland and four sites on Sátujökull on the northern part of Hofsjökull in
central Iceland. – Mæld vetrarafkoma (a), sumarafkoma (b) og ársafkoma (c) [mwe] sem fall af hæð mælistaðar
á Mýrdalsjökli (2001, 2007–2011), auk meðaltala mælinga sömu ára á þremur stöðum á Breiðamerkurjökli og
fjögurra á Sátujökli á norðanverðum Hofsjökli.
96 JÖKULL No. 63, 2013