Jökull - 01.01.2013, Side 97

Mass balance of Mýrdalsjökull ice cap
Breiðamerkurjökull outlet glacier (east of Öræfajökull
in Southeast-Iceland) and on the transect on the Hofs-
jökull ice cap (Figure 3). At Hofsjökull, a threefold
increase in winter mass balance is observed in the el-
evation interval considered, while the Breiðamerkur-
jökull data display an increase from south to north
(with elevation), opposite to what is seen on Mýrdals-
jökull. Both the measured winter balance and annual
balance decrease from south towards north and west
within the caldera plateau of Mýrdalsjökull (cf. Fig-
ures 1 and 3), which is equivalent to a decrease with
elevation of the survey sites. Summer mass balance
values are similar on all three glaciers and increase
(become less negative) on average linearly with ele-
vation. This demonstrates the strong effect that winter
precipitation is known to have on the annual balance
of ice caps in Iceland.
OBSERVED PRECIPITATION AND
MASS BALANCE
Rain gauges are prone to undercatch in strong winds,
in particular during snowfall, while indirect obser-
vations of orographic precipitation, such as runoff
and glacier mass balance measurements, are in fact
considered to yield the most accurate information on
precipitation in complex terrain (e.g. Rögnvaldsson
et al., 2007, and references therein). However, while
winter precipitation is well represented by the mass
balance data from the uppermost parts of Mýrdals-
jökull, summer precipitation is not, as it is generally
lost through the snow pack. Hence, traditional rain
gauge data must be investigated and here long time-
series of observed precipitation are available from
two coastal weather stations, Vík in Mýrdalur and
Vatnsskarðshólar. The stations are located to the south
of the ice cap (cf. Figure 1) and most of the precipi-
tation falls in southerly and easterly winds, in relation
with approaching extratropical lows and atmospheric
fronts. Annually, far greater precipitation is observed
at Vík than at Vatnsskarðshólar. This is the result of
two different topographic effects on the atmospheric
flow: a) Vatnsskarðshólar is approx. 6 km to the west
of Mt. Reynisfjall (340 m a.s.l.) and other smaller
scale topography, and is hence located in a partial rain
shadow. For that reason it may receive less precipita-
tion, compared to Vík on the other side of Mt. Reynis-
fjall. At Vík, the land is flat and there is open sea to
the south and east, and steep mountains to the north
and west, possibly leading to some enhancement of
precipitaton in a small scale atmospheric blocking.
Coastal convergence may also be of importance here.
b) Both the local, as well as the large scale topogra-
phy, lead to winds on average being weaker at Vík
than at Vatnsskarðshólar, and hence there is a smaller
undercatch of the rain gauge at Vík.
Figure 4. Scatterogram displaying the observed win-
ter mass balance [mwe] at three survey sites on Mýr-
dalsjökull and the observed precipitation [m] dur-
ing winter (Sept.–April) at weather stations at Vík
in Mýrdalur (Vik) and at Vatnsskarðshólar (Vat) in
South-Iceland during winters 2007–2011. Lines
with slopes of 2, 3 and 4 visually aid in indicat-
ing possible correlation between precipitation at in-
dividual stations and balance at individual survey
sites. – Dreifiritið sýnir mælda vetrarafkomu [mwe]
á þremur stöðum á Mýrdalsjökli og mælda úrkomu
[m] á veðurstöðvum á Vík í Mýrdal (Vik) og á
Vatnsskarðshólum (Vat) veturna 2007–2011. Línum
með hallatölu 2, 3 og 4 er ætlað að bregða ljósi á
mögulega fylgni milli úrkomu á veðurstöð og afkomu
á mælistað á jöklinum.
JÖKULL No. 63, 2013 97