Abstract

1
poster 93
Importance of surges on the stability and size of
Vatnajökull ice cap, Iceland
Guðfinna Aðalgeirsdóttir1, Hilmar Gudmundsson2
and Helgi Björnsson1
tolly_at_raunvis.hi.is
1 Science Institute, University of Iceland,
Dunhaga 3, IS-107 Reykjavík, Iceland
2 British Antarctic Survey, Natural Environment
Research Council, Cambridge CB3 0ET, U.K.
Abstract Computations with a flow model of Vatn
ajökull ice cap, Iceland are presented. The flow
model is forced with a regression model for the
mass-balance distribution which includes mass
balance-elevation feed back. Model parameters
are kept constant, except the Equilibrium Line
Altitude (ELA) is varied between model runs. It
is shown that present size and shape of
Vatnajökull can not be modelled with a constant
ELA. Depending on the ELA the ice cap either
grows unimpeded, or it settles to steady states
that are considerably smaller than the present
ice cap. These model results on Vatnajökull
confirm theoretical predictions that small ice
caps or glaciers on steep bedrock slopes are
stable. If the ice cap grows it becomes
sensitive to small changes and when it has
reached a critical size it can grow to an Ice-Age
size. Vatnajökull is presently close to this
critical size. The largest steady state obtained
with the model is different in extent from
present ice cap. The outlet glaciers that vanish
in the model have frequent surges occurring on
them. During a surge significant amount of ice
is transported from the accumulation area to the
ablation area in a short period of time. This
irregular flow behavior is shown to influence the
stability and the overall size of the ice cap.
Figure 4. Bedrock and surface geometry that is
used as boundary and initial condition,
respectively. The bed was measured with
radio-echo sounding methods (Björnsson, 1988)
and the surface with GPS surveying methods.
Figure 5. Geometry of the Vatnajökull ice cap at
different times. From left to right initial
condition (present size and shape of the ice
cap), fast growing ice cap at 4000 years with
DELA40 (corresponds to green cross in Fig. 3),
largest possible steady state at 25000 years with
DELA55.5 m(corresponds to pink cross in Fig. 3)
and separate small ice caps computed with DELA
120 m (corresponds to purple cross in Fig. 3).
Acknowledgments This work was supported by the
National Research Council of Iceland (Grant No.
10501), the National Power Company of Iceland and
the Nordic Project Climate Water and Energy. We
thank Finnur Pálsson for preparing the data and
the Iceland Glaciological Society (JÖRFÍ) for
support during field work.